The JACKDAW database package
Challis, M.F.
University of Cambridge, Computer Laboratory
1974-10
en
Text
UCAM-CL-TR-1
ISSN 1476-2986
This report describes a general database package which has been
implemented in BCPL on an IBM 370/165 at the University of
Cambridge. One current application is the provision of an
administrative database for the Computing Service.
Entries within a database may include (in addition to primitive
fields such as ‘salary’ and ‘address’) links to other entries: each
link represents a relationship between two entries and is always
two-way.
Generality is achieved by including within each database class
definitions which define the structure of the entries within it;
these definitions may be interrogated by program.
The major part of the package presents a procedural interface
between an application program and an existing database, enabling
entries and their fields to be created, interrogated, updated and
deleted. The creation of a new database (or modification of an
existing one) by specifying the class definitions is handled by a
separate program.
The first part of the report describes the database structure and
this is followed by an illustration of the procedural interface.
Finally, some of the implementation techniques used to insure
integrity of the database are described.
Scheduling for a share of the machine
Larmouth, J.
University of Cambridge, Computer Laboratory
1974-10
en
Text
UCAM-CL-TR-2
ISSN 1476-2986
This paper describes the mechanism used to schedule jobs and control
machine use on the IBM 370/165 at Cambridge University, England. The
same algorithm is currently being used in part at the University of
Bradford and implementations are in progress or under study for a
number of other British Universities.
The system provides computer management with a simple tool for
controlling machine use. The managerial decision allocates a share
of the total machine resources to each user of the system, either
directly, or via a hierarchial allocation scheme. The system then
undertakes to vary the turnaround of user jobs to ensure that those
decisions are effective, no matter what sort of work the user is
doing.
At the user end of the system we have great flexibility in the way
in which he uses the resources he has received, allowing him to get
a rapid turnaround for those (large or small) jobs which require it,
and a slower turnaround for other jobs. Provided he does not work at
a rate exceeding that appropriate to his share of the machine, he
can request, for every job he submits, the ‘deadline’ by which he
wants it running, and the system will usually succeed in running his
job at about the requested time – rarely later, and only
occasionally sooner.
Every job in the machine has its own ‘deadline’, and the machine is
not underloaded. Within limits, each user can request his jobs back
when he wants them, and the system keeps his use to within the share
of the machine he has been given. The approach is believed to be an
original one and to have a number of advantages over more
conventional scheduling and controlling algorithms.
A replacement for the OS/360 disc space management
routines
Stoneley, A.J.M.
University of Cambridge, Computer Laboratory
1975-04
en
Text
UCAM-CL-TR-3
ISSN 1476-2986
In the interest of efficiency, the IBM disc space management
routines (Dadsm) have been completely replaced in the Cambridge
370/165.
A large reduction in the disc traffic has been achieved by keeping
the lists of free tracks in a more compact form and by keeping lists
of free VTOC blocks. The real time taken in a typical transaction
has been reduced by a factor of twenty.
By writing the code in a more appropriate form than the original,
the size has been decreased by a factor of five, thus making it more
reasonable to keep it permanently resident. The cpu requirement has
decreased from 5% to 0.5% of the total time during normal service.
The new system is very much safer than the old in the fact of total
system crashes. The old system gave little attention to the
consequences of being stopped in mid-flight, and it was common to
discover an area of disc allocated to two files. This no longer
happens.
The dynamic creation of I/O paths under
OS/360-MVT
Stoneley, A.J.M.
University of Cambridge, Computer Laboratory
1975-04
en
Text
UCAM-CL-TR-4
ISSN 1476-2986
In a large computer it is often desirable and convenient for an
ordinary program to be able to establish for itself a logical
connection to a peripheral device. This ability is normally provided
through a routine within the operating system which may be called by
any user program at any time. OS/360 lacks such a routine. For the
batch job, peripheral connections can only be made through the job
control language and this cannot be done dynamically at run-time. In
the restricted context of TSO (IBM’s terminal system) a routine for
establishing peripheral connections does exist, but it is extremely
inefficient and difficult to use.
This paper describes how a suitable routine was written and grafted
into the operating system of the Cambridge 370/165.
Parrot – A replacement for TCAM
Hazel, P.
Stoneley, A.J.M.
University of Cambridge, Computer Laboratory
1976-04
en
Text
UCAM-CL-TR-5
ISSN 1476-2986
The terminal driving software and hardware for the Cambridge TSO
(Phoenix) system is described. TCAM and the IBM communications
controller were replaced by a locally written software system and a
PDP-11 complex. This provided greater flexibility, reliability,
efficiency and a better “end-user” interface than was possible under
a standard IBM system.
System programming in a high level language
Birrell, Andrew D.
University of Cambridge, Computer Laboratory
1977-12
en
Text
UCAM-CL-TR-6
ISSN 1476-2986
This thesis is concerned with the construction of a high level
language system suitable for the implementation of a general purpose
operating system for a computer. There are three aspects to this
task: firstly a suitable high level language must be chosen or
designed; secondly, a suitable implementation of this language must
be manufactured; thirdly, the operating system itself must be
written. These three aspects inevitably overlap in time – experience
in implementing the language may cause one to review decisions taken
in the design of the language, and experience in constructing the
operating system will bring to light inadequacies, inconveniences
and inelegancies in both the implementation and design of the
language.
Most previous work in this field has been concerned with the first
of these aspects, and has adopted the approach of designing special
‘System Programming Languages’ (SPLs) or ‘Machine Oriented
Languages’ (MOLs). Various such languages have been developed,
although few have achieved the elegance and generality of
general-purpose languages such as Pascal or Algol68. Little or no
investigation has previously been made into the second of these
aspects, the implementation of the language. This aspect can have a
considerable effect on the practicability of using the resulting
language for manufacturing an operating system. The implementation,
however suitable the language makes the difference between the
language being an aid or an impediment to the system programmer. It
is with aspects of the implementation this this thesis is mainly
concerned.
Local area computer communication networks
Hopper, Andrew
University of Cambridge, Computer Laboratory
1978-04
en
Text
UCAM-CL-TR-7
ISSN 1476-2986
In this thesis a number of local area network architectures are
studied and the feasibility of a LSI design for a universal local
network chip is considered. The thesis begins with a survey of
current network technologies and a discussion of some of the
problems encountered in local network design. Existing
implementations of local networks are then discussed, and their
performance compared. Ultimately the design considerations for a
general purpose, microprogrammed, LSI network chip is discussed.
Such a circuit is able to handle a range of network architectures
and can be reconfigured to suit various traffic patterns. Finally
some of the protocol requirements of local networks are discussed,
leading to a redesign of the Cambridge ring to provide hardware
support for protocol implementation.
Evaluation of a protection system
Cook, Douglas John
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-9
ISSN 1476-2986
The CAP research project was set up in 1969 to investigate memory
protection by designing and building a computer with hardware
support for a very detailed protection system based on the use of
capabilities. The computer has been built and an operating system
written which exploits its protection facilities. It is time,
therefore, to assess how successful the project has been. A
necessary component of such an assessment is an evaluation of the
CAP’s protection system and this thesis presents the results of the
author’s research in this area.
Protection in computer systems is first introduced with a brief
description of various models of protection systems and mechanisms
for the provision of protection. There follows a description in some
detail of the CAP computer and the CAP operating system with
particular attention paid to those aspects of the design which are
relevant to the research reported. A brief introduction to
performance evaluation techniques is given followed by a discussion
of performance evaluation on the CAP computer.
The need for measuring the benefits and costs of protection is
discussed and there is a detailed critical description of the
previous research in this area. A simple model of a protection
system is presented as is a protection measure based on this model.
There is then a discussion of how the services provided by modules
in the system fit into the model and the protection measure. The
application of the protection measure to the CAP operating system is
described. The results led to suggestions for the improvement of the
protection aspects of the operating system and these are discussed
in detail. The implications of the results for operating system
design in general are also discussed.
The experiements to investiage the cost of using the protection
provided on the CAP are described next. Some performance evaluation
work was done in connection with the protection cost experiments and
this too is discussed.
Prediction oriented description of database
systems
Pezarro, Mark Theodore
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-10
ISSN 1476-2986
A descriptive model of database systems is presented. The model is
intended to provide a general framework for the description of
database systems which is not limited to any particular DBMS or even
any of the three mainstream approaches to DBMS architecture. This
generality is derived from a new analysis of file organisation
methods on which the model is based. The model concentrates on the
aspects of a database system relevant to first-order performance
prediction. These include database structure, the hardware and
software used in implementing the system, the size of the database
at various points in its lifetime, and its known or anticipated
usage. Particular attention has been devoted to arriving at a
general treatment of the details of database systems at the physical
level, including access paths and their encoding, storage devices
and their operating characteristics, and the mapping of data
representations to storage devices.
A formal language has been devised in which to write textual
descriptions of a database system in terms of the model. In addition
an experimental prediction program has been written which accepts a
description of a database system expressed in the language and
produces performance estimates for the described activity using
computational methods based on expected value formulae. Some
preliminary results obtained by comparing estimates given by the
program with measurements of an operational database system are
presented. Further experimentation that would allow a definitive
evaluation of the prediction program is outlined and a review is
made of the current limitations of the model and program with
suggestions for further research.
Automatic resolution of linguistic ambiguities
Boguraev, Branimir Konstatinov
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-11
ISSN 1476-2986
The thesis describes the design, implementation and testing of a
natural language analysis system capable of performing the task of
generating paraphrases in a highly ambiguous environment. The
emphasis is on incorporating strong semantic judgement in an
augmented transition network grammar: the system provides a
framework for examining the relationship between syntax and
semantics in the process of text analysis, especially while treating
the related phenomena of lexical and structural ambiguity.
Word-sense selection is based on global analysis of context within a
semantically well-formed unit, with primary emphasis on the verb
choice. In building structures representing text meaning, the
analyser relies not on screening through many alternative structures
– intermediate, syntactic or partial semantic – but on dynamically
constructing only the valid ones. The two tasks of sense selection
and structure building are procedurally linked by the application of
semantic routines derived from Y. Wilks’ preference semantics, which
are invoked at certain well chosen points of the syntactic
constituent analysis – this delimits the scope of their action and
provides context for a particular disambiguation technique. The
hierarchical process of sentence analysis is reflected in the
hierarchical organisation of application of these semantic routines
– this allows the efficient coordination of various disambiguation
techniques, and the reduction of syntactic backtracking,
non-determinism in the grammar, and semantic parallelism. The final
result of the analysis process is a dependency structure providing a
meaning representation of the input text with labelled components
centred on the main verb element, each characterised in terms of
semantic primitives and expressing both the meaning of a constituent
and its function in the overall textual unit. The representation
serves as an input to the generator, organised around the same
underlying principle as the analyser – the verb is central to the
clause. Currently the generator works in paraphrase mode, but is
specifically designed so that with minimum effort and virtually no
change in the program control structure and code it could be
switched over to perform translation.
The thesis discusses the rationale for the approach adopted,
comparing it with others, describes the system and its machine
implementation, and presents experimental results.
HASP “IBM 1130” multileaving remote job entry protocol with
extensions as used on the University of Cambridge IBM
370/165
Oakley, M.R.A.
Hazel, P.
University of Cambridge, Computer Laboratory
1979-09
en
Text
UCAM-CL-TR-12
ISSN 1476-2986
This document brings together most of the information required to
design, write and operate a HASP Remote Job Entry Terminal program.
Most of the document describes facilities available using any host
computer supporting the HASP protocols. The remainder of the
document describes improvements to these facilities which have been
made in order to enhance the reliability of the system, to make it
easier to run, and to provide for a wider range of peripherals than
the basic system.
Resource allocation and job scheduling
Hazel, Philip
University of Cambridge, Computer Laboratory
1980
en
Text
UCAM-CL-TR-13
ISSN 1476-2986
The mechanisms for sharing the resources of the Cambridge IBM
370/165 computer system among many individual users are described.
File store is treated separately from other resources such as
central processor and channel time. In both cases, flexible systems
that provide incentives to thrifty behaviour are used. The method of
allocating resources directly to users rather than in a hierarchical
manner via faculties and departments is described, and its social
acceptability is discussed.
Store to store swapping for TSO under OS/MVT
Powers, J.S.
University of Cambridge, Computer Laboratory
1980-06
en
Text
UCAM-CL-TR-14
ISSN 1476-2986
A system of store-to-store swapping incorporated into TSO on the
Cambridge IBM 370/165 is described. Unoccupied store in the dynamic
area is used as the first stage of a two-stage backing store for
swapping time-sharing sessions; a fixed-head disc provides the
second stage. The performance and costs of the system are evaluated.
The implementation of BCPL on a Z80 based
microcomputer
Wilson, I.D.
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-15
ISSN 1476-2986
The main aim of this project was to achieve as full an
implementation as possible of BCPL on a floppy disc based
microcomputer, running CP/M or CDOS (the two being esentially
compatible). On the face of it there seemed so many limiting
factors, that, when the project was started, it was not at all clear
which one (if any) would become a final stumbling block. As it
happened, the major problems that cropped up could be programmed
round, or altered in such a way as to make them soluble.
The main body of the work splits comfortably into three sections,
and the writer hopes that, in covering each section separately, to
be able to show how the whole project fits together into the
finished implementation.
Reliable storage in a local network
Dion, Jeremy
University of Cambridge, Computer Laboratory
1981-02
en
Text
UCAM-CL-TR-16
ISSN 1476-2986
A recent development in computer science has been the advent of
local computer networks, collections of autonomous computers in a
small geographical area connected by a high-speed communications
medium. In such a situation it is natural to specialise some of the
computers to provide useful services to others in the network. These
server machines can be economically advantageous if they provide
shared access to expensive mechanical devices such as discs.
This thesis discusses the problems involved in designing a file
server to provide a storage service in a local network. It is based
on experience gained from the design and implementation of a file
server for the Cambridge ring.
An important aspect of the design of a file server is the choice of
the service which is provided to client machines. The spectrum of
choice ranges from providing a simple remote disc with operations
such as read and write block, to a remote file system with
directories and textual names. The interface chosen for the
Cambridge file server is “universal” in that the services it
provides are intended to allow easy implementation of both virtual
memory systems and filing systems.
The second major aspect of the file server design concerns
reliability. If the server is to store important information for
clients, then it is essential that it be resistant to transient
errors such as communications or power failures. The general
problems of reliability and crash resistance are discussed in terms
of a model developed for this purpose. Different reliability
strategies used in current data base and filing systems are related
to the model, and a mechanism for providing atomic transactions in
the Cambridge file server is described in detail. An improved
mechanism which allows atomic transactions on multiple files is also
described and contrasted with the first version. The revised design
allows several file servers in a local network to cooperate in
atomic updates to arbitrary collections of files.
Three papers on parsing
Boguraev, B.K.
Spärck Jones, K.
Tait, J.I.
University of Cambridge, Computer Laboratory
1982
en
Text
UCAM-CL-TR-17
ISSN 1476-2986
This collection of three papers examines current problems in the
parsing of natural language. The first paper investigates the
parsing of compound nouns, and suggests that the existing strategies
are inadequate. Accepting that better approaches are needed, the
paper then proceeds to examine the implications for natural language
processing systems.
The second paper in the collection examines the task of recognising
conjunctions within an ATN grammar. To do this only through the
grammar specification is difficult and results in a bulky grammar.
The paper therefore presents some ideas for extending the ATN
mechanism to better deal with conjunctions.
The final paper considers ways in which semantic parsers can exploit
syntactic constraints. Two specific semantic parsers are considered:
those of Cater and Boguraev which are regarded as being
representative of two styles of parsing. The main conclusion to be
drawn is that there are significant disadvantages to semantic
parsing without complete syntactic processing of the input.
Automatic mesh generation of 2 & 3 dimensional
curvilinear manifolds
Wördenweber, Burkard
University of Cambridge, Computer Laboratory
1981-11
en
Text
UCAM-CL-TR-18
ISSN 1476-2986
Analysis and inference for English
Cater, Arthur William Sebright
University of Cambridge, Computer Laboratory
1981-09
en
Text
UCAM-CL-TR-19
ISSN 1476-2986
AD-HAC is a computer program which understands stories. Its three
principal components each deal with significant subareas of the
overall language-processing task: it has a sentence analyser, which
creates conceptual representations of the meanings of individual
sentences; an inferencer, which assimilates these into the existing
representation of a story, determining pronoun referents and
answering questions as a byproduct of this activity; and a sentence
generator, which produces english sentences conveying the meaning of
conceptual representations. The research reported here has focussed
on the analyser and the inferencer.
The analyser uses an ATN to identify low-level syntactic
constituents, such as verb groups or prepositional phrases:
‘requests’ associated with words, particularly verbs, are then
applied in a nondeterministic preference-directed framework, using
the constituents as building blocks in the analysis of phrases,
clauses and sentences: the requests fall into five distinct
processing classes. The partial analyses which result from the
application or non-application of particular requests are ordered by
preference, and the most-preferred partial analysis is persued
first, giving a predominantly left-to-right scan through the
sentence. A surprising result is that the analyser performs better
if it is permitted to keep only a small number of partial analyses.
The inferencer exploits the primitives of the conceptual
representation language, using these as the main indicator of the
appropriate set of inferences. The inferences are specified by means
of inference networks associated with the conceptual primitives.
Tests are applied to elementary propositions derived from input
sentence analyses, and select paths through the networks where
appropriate inferences are made. Inference networks are also
associated with ‘functions’ of objects, permitting higher-level than
can normally be made using the primitives alone: the resulting
system offers a synthesis of low-level inference and script-like
inference. The inferences made by the networks are also used to
determine the referents of pronouns, and to provide the answers to
questions: the program takes an identical approach to these two
tasks.
The performance of the system is illustrated by reference to texts
which have been successfully processed by AD-HAC.
On using Edinburgh LCF to prove the correctness of a parsing
algorithm
Cohn, Avra
Milner, Robin
University of Cambridge, Computer Laboratory
1982-02
en
Text
UCAM-CL-TR-20
ISSN 1476-2986
The methodology of Edinburgh LCF, a mechanized interactive proof
system is illustrated through a problem suggested by Gloess – the
proof of a simple parsing algorithm. The paper is self-contained,
giving only the relevant details of the LCF proof system. It is
shown how tactics may be composed in LCF to yield a strategy which
is appropriate for the parser problem but which is also of a
generally useful form. Also illustrated is a general mechanized
method of deriving structural induction rules within the system.
The correctness of a precedence parsing algorithm in
LCF
Cohn, A.
University of Cambridge, Computer Laboratory
1982-04
en
Text
UCAM-CL-TR-21
ISSN 1476-2986
This paper describes the proof in the LCF system of a correctness
property of a precedence parsing algorithm. The work is an extension
of a simpler parser and proof by Cohn and Milner (Cohn & Milner
1982). Relevant aspects of the LCF system are presented as needed.
In this paper, we emphasize (i) that although the current proof is
much more complex than the earlier one, mqany of the same
metalanguage strategies and aids developed for the first proof are
used in this proof, and (ii) that (in both cases) a general strategy
for doing some limited forward search is incorporated neatly into
the overall goal-oriented proof framework.
Constraints in CODD
Robson, M.
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-22
ISSN 1476-2986
The paper describes the implementation of the data structuring
concepts of domains, intra-tuple constraints and referential
constraints in the relational DBMS CODD. All of these constraints
capture some of the semantics of the database’s application.
Each class of constraint is described briefly and it is shown how
each of them is specified. The constraints are stored in the
database giving a centralised data model, which contains
descriptions of procedures as well as of statistic structures. Some
extensions to the notion of referential constraint are proposed and
it is shown how generalisation hierarchies can be expressed as sets
of referential constraints. It is shown how the stored data model is
used in enforcement of the constraints.
Two papers about the scrabble summarising system
Tait, J.I.
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-23
ISSN 1476-2986
This report contains two papers which describe parts of the Scrabble
English summarizing system. The first, “Topic identification
techniques for predictive language analyzers” has been accepted as a
short communication for the 9th International COnference on
Computational Linguistics, in Prague. The second, “General summaries
using a predictive language analyser” is an extended version of a
discussion paper which will be presented at the European Conference
on Artificial Intelligence in Paris. Both conferences will take
place during July 1982.
The [second] paper describes a computer system capable of producing
coherent summaries of English texts even when they contain sections
which the system has not understood completely. The system employs
an analysis phase which is not dissimilar to a script applier
together with a rather more sophisticated summariser than previous
systems. Some deficiencies of earlier systems are pointed out, and
ways in which the current implementation overcomes them are
discussed.
Steps towards natural language to data language translation
using general semantic information
Boguraev, B.K.
Spärck Jones, K.
University of Cambridge, Computer Laboratory
1982-03
en
Text
UCAM-CL-TR-24
ISSN 1476-2986
The aim of the work reported here is to maximise the use of general
semantic information in an AI task processor, specifically in a
system front end for converting natural language questions into
formal database queries. The paper describes the translation
component of such a front end, which is designed to work from the
question meaning representation produced by a language analyser
exploiting only general semantics and syntax, to a formal query
relying on database-specific semantics and syntax. Translation is
effected in three steps, and the paper suggests that the rich and
explicit meaning representations using semantic primitives produced
for input sentences by the analyser constitute a natural and
effective base for further processing.
A clustering technique for semantic network
processing
Alshawi, Hiyan
University of Cambridge, Computer Laboratory
1982-05
en
Text
UCAM-CL-TR-25
ISSN 1476-2986
This paper describes techniques for performing serial processing on
the type of semantic network exemplified by NETL. They make use of
an indexing scheme that can be based on semantic clustering. The
basic algorithm is aimed at performing fast intersection operations.
It is claimed that the scheme is suitable for its current
application in text processing. The semantic criteria for clustering
that have been tried are briefly described. Extensions of the scheme
are suggested for use with large networks.
Portable system software for personal computers on a
network
Knight, Brian James
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-26
ISSN 1476-2986
This dissertation is concerned with the design of the portable
operating system TRIPOS, and its use as the basis for an operating
system to run in ‘single connection’ computers – that is, computers
whose only peripheral is an interface to a local area network.
TRIPOS is a lightweight, yet powerful, multi-tasking operating
system aimed at personal minicomputers. It is designed to be
relatively straightforward to transport to new hardware, providing
an almost identical user interface and program environment on each
machine. Particular emphasis has been placed on avoiding unnecessary
complexity, in order to make it simple to understand, explain, and
adapt for special purposes. The majority of the system and utilities
are written in the language BCPL, and can be moved without change to
different computers. They run on a kernel and device drivers written
in assembly language for each particular machine. The user’s view of
the system is presented first, with samples of console dialogue, and
then its internal structure is described.
The main part of the work described concerns the building of a
portable operating system presenting user and program interfaces as
similar as possible to ordinary TRIPOS, but running in processors
connected only to a local area network – the Cambridge Ring. The
system makes use of ‘server’ computers on the ring in order to gain
access to disc storage, terminals, and printers. Several methods are
investigated for using the primitives provided by a universal
file-server to construct a filing system which can be shared by
machines of different types. Some conclusions are drawn on the
effects of distributing operating system functions in this way.
Exception handling in domain based systems
Johnson, Martyn Alan
University of Cambridge, Computer Laboratory
1981-09
en
Text
UCAM-CL-TR-27
ISSN 1476-2986
Modern operating systems allow the creation of protection domains;
these enable subsystems to cooperate whilst being protected from
each other. This creates a number of problems in the handling of
exceptions such as the expiry of time limits or the receipt of
console ‘quit’ signals. Particular problems arise when parts of the
operating system are implemented as protection domains which cannot
easily be distinguished from user programs by the underlying
protection system.
The dissertation surveys some traditional methods of dealing with
such problems, and explains why they are inadequate in a domain
based system. In addition, work done on related topics in the
operating system for the Cambridge CAP computer is described.
The major part of the research described is concerned with a class
of exception not usually recognized by operating system designers.
This arises from the observation that protection domains which
implement subsystems can retain useful state information between
invocations, and care needs to be taken to ensure that domains are
given an opportunity to keep their private data structures in a
consistent state. In particular, domains which fall into disuse need
to be notified of the fact so that they can tidy up the data
structures they manage before they are destroyed. An intuitively
simple solution to the problem is discussed, and its limitations and
implementation difficulties are noted. Refinements of the mechanism
are proposed which provide an improved treatment of the problem; and
it is suggested that the moderate run time overhead which these
revisions impose can be minimized by providing hardware or
microprogram support for the mechanism.
Poly report
Matthews, D.C.J.
University of Cambridge, Computer Laboratory
1982-08
en
Text
UCAM-CL-TR-28
ISSN 1476-2986
Poly was designed to provide a programming system with the same
flexibility as a dynamically typed language but without the run-time
oveheads. The type system, based on that of Russel allows
polymorpphic operations to be used to manipulate abstract objects,
but with all the type checking being done at compile-time. Types may
be passed explicitly or by inference as parameters to procedures,
and may be returned from procedures. Overloading of names and
generic types can be simulated by using the general procedure
mechanism. Despite the generality of the language, or perhaps
because of it, the type system is very simple, consisting of only
three classes of object. There is an exception mechanism, similar to
that of CLU, and the exceptions raised in a procedure are considered
as part of its ‘type’. The construction of abstract objects and
hiding of internal details of the representation come naturally out
of the type system.
Introduction to Poly
Matthews, D.C.J.
University of Cambridge, Computer Laboratory
1982-05
en
Text
UCAM-CL-TR-29
ISSN 1476-2986
This report is a tutorial introduction to the programming language
Poly. It describes how to write and run programs in Poly using the
VAX/UNIX implementation. Examples given include polymorphic list
functions, a double precision integer package and a subrange type
constructor.
A portable BCPL library
Wilkes, John
University of Cambridge, Computer Laboratory
1982-10
en
Text
UCAM-CL-TR-30
ISSN 1476-2986
Too often, programs written in BCPL are difficult to port from one
system to another, not because of the language, but because of
differences between ‘standard’ libraries. Almost without exception,
the definitions of these libraries are loose, woolly and inaccurate
– the proposed BCPL standards document being a prime example. The
author has developed and implemented a new BCPL library which is
explicitly designed to aid the portability of programs between
systems. In addition to being largely portable itself, it has two
other features of interest: it uses an exception handling system
instead of return codes, and it makes no distinction between system
and user defined stream handlers. This paper defines the interface
to the package.
Ponder and its type system
Fairbairn, J.
University of Cambridge, Computer Laboratory
1982-11
en
Text
UCAM-CL-TR-31
ISSN 1476-2986
This note describes the programming language “Ponder”, which is
designed according to the principles of referencial transparency and
“orthogonality” as in [vWijngaarden 75]. Ponder is designed to be
simple, being functional with normal order semantics. It is intended
for writing large programmes, and to be easily tailored to a
particular application. It has a simple but powerful polymorphic
type system.
The main objective of this note is to describe the type system of
Ponder. As with the whole of the language design, the smallest
possible number of primitives is built in to the type system. Hence
for example, unions and pairs are not built in, but can be
constructed from other primitives.
How to drive a database front end using general semantic
information
Boguraev, B.K.
Spärck Jones, K.
University of Cambridge, Computer Laboratory
1982-11
en
Text
UCAM-CL-TR-32
ISSN 1476-2986
This paper describes a front end for natural language access to
databases making extensive use of general, i.e. domain-independent,
semantic information for question interpretation. In the interests
of portability, initial syntactic and semantic processing of a
question is carried out without any reference to the database
domain, and domain-dependent operations are confined to subsequent,
comparatively straightforward, processing of the initial
interpretation. The different modules of the front end are
described, and the system’s performance is illustrated by examples.
An island parsing interpreter for Augmented Transition
Networks
Carroll, John A.
University of Cambridge, Computer Laboratory
1982-10
en
Text
UCAM-CL-TR-33
ISSN 1476-2986
This paper describes the implementation of an ‘island parsing’
interpreter for an Augmented Transition Network (ATN). The
interpreter provides more complete coverage of Woods’ original ATM
formalism than his later island parsing implementation; it is
written in LISP and has been modestly tested.
Recent developments in LCF: examples of structural
induction
Paulson, Larry
University of Cambridge, Computer Laboratory
1983-01
en
Text
UCAM-CL-TR-34
ISSN 1476-2986
Manna and Waldinger have outlined a large proof that probably
exceeds the power of current theorem-provers. The proof establishes
the unification algorithm for terms composed of variables,
constants, and other terms. Two theorems from this proof, involving
structural induction, are performed in the LCF proof assistant.
These theorems concern a function that searches for an occurrence of
one term inside another, and a function that lists the variables in
a term.
Formally, terms are regarded as abstract syntax trees. LCF
automatically builds the first-order theory, with equality, of this
recursive data structure.
The first theorem has a simple proof, induction followed by
rewriting. The second theorem requires a cases split and
substitution throughout the goal. Each theorem is proved by reducing
the initial goal to simpler and simpler subgoals. LCF provides many
standard proof strategies for attacking goals; the user can program
additional ones in LCF’s meta-language, ML. This felxibility allows
users to take ideas from such diverse fields as denotational
semantics and logic programming.
Rewriting in Cambridge LCF
Paulson, Larry
University of Cambridge, Computer Laboratory
1983-02
en
Text
UCAM-CL-TR-35
ISSN 1476-2986
Many automatic theorem-provers rely on rewriting. Using theorems as
rewrite rules helps to simplify the subgoals that arise during a
proof.
LCF is an interactive theorem-prover intended for reasoning about
computation. Its implementation of rewriting is presented in detail.
LCF provides a family of rewriting functions, and operators to
combine them. A succession of functions is described, from pattern
matching primitives to the rewriting tool that performs most
inferences in LCF proofs.
The design is highly modular. Each function performs a basic,
specific task, such as recognizing a certain form of tautology. Each
operator implements one method of building a rewriting function from
simpler ones. These pieces can be put together in numerous ways,
yielding a variety of rewriting strategies.
The approach involves programming with higher-order functions.
Rewriting functions are data values, produced by computation on
other rewriting functions. The code is in daily use at Cambridge,
demonstrating the practical use of functional programming.
The revised logic PPLAMBDA : A reference manual
Paulson, Lawrence
University of Cambridge, Computer Laboratory
1983-03
en
Text
UCAM-CL-TR-36
ISSN 1476-2986
PPLAMBDA is the logic used in the Cambridge LCF proof assistant. It
allows Natural Deduction proofs about computation, in Scott’s theory
of partial orderings. The logic’s syntax, axioms, primitive
inference rules, derived inference rules and standard lemmas are
described as are the LCF functions for building and taking apart
PPLAMBDA formulas.
PPLAMBDA’s rule of fixed-point induction admits a wide class of
inductions, particularly where flat or finite types are involved.
The user can express and prove these type properties in PPLAMBDA.
The induction rule accepts a list of theorems, stating type
properties to consider when deciding to admit an induction.
Representation and authentication on computer
networks
Girling, Christopher Gray
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-37
ISSN 1476-2986
Controlling access to objects in a conventional operating system is
a well understood problem for which solutions are currently in
existence. Such solutions utilize mechanisms which accurately and
trivially provide the identity of an accessing subject. In the
context of a collection of computers communicating with each other
over a network, provision of this mechanism is more complex. The
design of such a mechanism and its implementation on the Cambridge
Ring at Cambridge University is described.
The vehicle used to prove the identity of an object irrefutably is
called a representation and the deduction of an object’s identity is
called authentication. Methods of authentication are given which
show that the mechanism can cope with identification needs that
arise in practice (even in a network where the function asigned to
each computer is constantly changing). These generate
representations for such important components of a computer network
as people, services and addresses. The implementation of a
representation system utilizing some of these methods is described,
including the incorporation of its use into a real operating system.
The place of representations within the communication protocols that
must transport them is considered and some enhancements are
proposed. in addition, some interesting variations and extensions of
the system are explored.
Views and imprecise information in databases
Gray, Mike
University of Cambridge, Computer Laboratory
1982-11
en
Text
UCAM-CL-TR-38
ISSN 1476-2986
Providing user views of a database is an important way of achieving
data independence and ease of use of DBMSs. This dissertation
discusses one aspect of the problem of supporting views. It is shown
that a crucial factor in the support of views is the richness of the
data model used, and in particular its ability to represent certain
kinds of incomplete information. This dissertation discusses various
ways of handling incomplete information, and the operations on views
that can be supported. The implementation of an experimental system
which supports views on a relational database is described.
The first chapter describes the problem of treating views as
first-class objects, that is allowing all the usual database
operations to be performed on data in views. It is shown how this is
related to the problem of representing incomplete information in the
conceptual schema. The second chapter proposes the use of lattices
to represent incomplete information, and shows how this covers
various particular kinds of imprecise information. The third chapter
reviews other work relating to imprecise information in databases.
The fourth chapter discusses certain further implications of
representing imprecise information, and makes proposals regarding
the interpretation of keys, constraints, and the open-world
assumption in this environment. The fifth chapter discusses in
detail the relational operations that are appropriate with imprecise
data and proposes modified Join and Group-by operations. The
implementation of a system with these features is discussed. Chapter
six illustrates some of the points made by considering an example
database, and finally chapter seven concludes this dissertation with
a summary and examination of further possibilities.
Tactics and tacticals in Cambridge LCF
Paulson, Lawrence
University of Cambridge, Computer Laboratory
1983-07
en
Text
UCAM-CL-TR-39
ISSN 1476-2986
The tactics and tacticals of Cambridge LCF are described. Tactics
reason about logical connectives, substitution and rewriting;
tacticals combine tactics into more powerful tactics. LCF’s package
for managing an interactive proof is discussed. This manages the
subgoal tree, presenting the user with unsolved goals and assembling
the final proof.
While primarily a reference manual, the paper contains a brief
introduction to goal-directed proof. An example shows typical use of
the tactics and subgoal package.
The SKIM microprogrammer’s guide
Stoye, W.
University of Cambridge, Computer Laboratory
1983-10
en
Text
UCAM-CL-TR-40
ISSN 1476-2986
This paper describes the design and implementation of the SKIM
micorprocessor. The processor has a 24 bit ALU with 16 general
purpose registers. The main unique feature is a large microcode
store of up to 64K 40 bit words, with the intention that the
microcode could be used like the machine code on a conventional
processor, with operating system primitives being programmed in
microcode.
The processor has been constructed from TTL logic, with a microcode
assembler running on Phoenix. A debugger for both the hardware and
microcode programs runs on the host machine, currently a BBC
Microcomputer.
The processor architecture is discussed, with examples of microcode
programming. comparisons with other processors are made, and some of
the limitations of the present design are noted.
LCF_LSM, A system for specifying and verifying
hardware
Gordon, Mike
University of Cambridge, Computer Laboratory
1983-09
en
Text
UCAM-CL-TR-41
ISSN 1476-2986
The LCF_LSM system is designed to show that it is practical to prove
the correctness of real hardware. The system consists of a
programming environment (LCF) and a specification language (LSM).
The environment contains tools for manipulating and reasoning about
the specifications. Verification consists in proving that a
lov-level (usually structural) description is behaviourally
equivalent to a high-level functional description. Specifications
can be fully hierarchical, and at any level devices can be specified
either functionally or structurally.
As a first case study a simple microcoded computer has been
verified. This proof is described in a companion report. In this we
also illustrate the use of the system for other kinds of
manipulation besides verification. For example, we show how to
derive an implementation of a hard-wired controller from a
microprogram and its decoding and sequencing logic. The derivation
is done using machine checked inference; this ensures that the
hard-wired controller is equivalent to the microcoded one. We also
show how to code a microassembler. These examples illustrate our
belief that LCF is a good environment for implementing a wide range
of tools for manipulating hardware specifications.
This report has two aims: first to give an overview of the ideas
embodied in LCF_LSM, and second, to be a user manual for the system.
No prior knowledge of LCF is assumed.
Proving a computer correct with the LCF_LSM hardware
verification system
Gordon, Mike
University of Cambridge, Computer Laboratory
1983-09
en
Text
UCAM-CL-TR-42
ISSN 1476-2986
A machine generated correctness proof of a simple computer is
described.
At the machine code level the computer has a memory and two
registers: a 13 bit program counter and a 16-bit accumulator. There
are 8 machine instructions: halt, unconditional jump, jump when the
accumulator contains 0, add contents of a memory location to
accumulator, subtract contents of a location from accumulator, load
accumulator from memory, store contents of accumulator in memory,
and skip. The machine can be interrupted by pushing a button on its
front panel.
The implementation which we prove correct has 6 data registers, and
ALU, a memory, and a microcode controller. The controller consists
of a ROM holding 26 30-bit microinstructions, a microprogram
counter, and some combinatorial microinstruction decode logic.
Formal specifications of the target and host machines are given, and
we describe the main steps in proving that the host correctly
fetches, decodes and executes machine instructions.
The utility of LCF_LSM for general manipulaton is illustrated in two
appendices. In appendix 1 we show how to code a microassembler. In
appendix 2 we use the LCF_LSM inference rules to design a hard-wired
controller equivalent to the original microcoded one.
N.B. This report should be read in conjunction with LCF_LSM: A
system for specifying and verifying hardware. University of
Cambridge, Computer Laboratory technical report number 41.
Extending the local area network
Leslie, Ian Malcom
University of Cambridge, Computer Laboratory
1983-02
en
Text
UCAM-CL-TR-43
ISSN 1476-2986
This dissertation is concerned with the development of a large
computer network which has many properties associated with local
area computer networks, including high bandwidth and lower error
rates. The network is made up of component local area networks,
specifically Cambridge rings, which are connected either through
local ring-ring bridges or through a high capacity satellite link.
In order to take advantage of the characteristics of the resulting
network, the protocols used are the same simple protocols as those
used on a single Cambridge ring. This in turn allows many
applications, which might have been thought of as local area network
applications, to run on the larger network.
Much of this work is concerned with an interconnection strategy
which allows hosts of different component networks to communicate in
a flexible manner without building an extra internetwork layer into
protocol hierarchy. The strategy arrived at is neither a datagram
approach nor a system of concatenated error and flow controlled
virtual circuits. Rather, it is a lightweight virtual circuit
approach which preserves the order of blocks sent on a circuit, but
which makes no other guarantees about the delivery of these blocks.
An extra internetwork protocol layer is avoided by modifying the
system used on a single Cambridge ring which binds service names to
addresses so that it now binds service names to routes across the
network.
Structural induction in LCF
Paulson, Lawrence
University of Cambridge, Computer Laboratory
1983-11
en
Text
UCAM-CL-TR-44
ISSN 1476-2986
The fixed-point theory of computation can express a variety of
recursive data types, including lazy types, conventional first-order
(strict) types, mutually recursive types, and types with equational
constraints. Lazy types contain infinite objects, regarded as the
limit of a chain of finite objects. Structural induction for all
these types follows from fixed-point induction, though induction for
lazy types is only sound for a certain class of formulas.
The paper presents the derivation of structural induction for each
type, and justifies the necessary axioms by furnishing models for
them. It presents example type definitions of lazy lists, strict
lists, syntax trees for expressions and finite sets. Strict data
types are proved to be flat in their partial ordering. Primitive
recursion operators are introduced for each type, providing
theoretical insights as well as a concise notation for defining
total functions.
The research was done using LCF, an interactive theorem-prover for
the fixed-point theory. The paper documents the theory of LCF data
types, and surveys several LCF proofs involving structural
induction. In order to be self-contained, it makes little reference
to LCF details and includes a summary of the fixed point theory.
Compound noun interpretation problems
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1983-07
en
Text
UCAM-CL-TR-45
ISSN 1476-2986
This paper discusses the problems of compound noun interpretation in
the context of automatic language processing. Given that compound
processing implies identifying the senses of the words involved,
determining their bracketing, and establishing their underlying
semantic relations, the paper illustrates the need, even in
comparatively favourable cases, for inference using pragmatic
information. This has consequences for language processor
architectures and, even more, for speech processors.
Intelligent network interfaces
Garnett, Nicholas Henry
University of Cambridge, Computer Laboratory
1985-05
en
Text
UCAM-CL-TR-46
ISSN 1476-2986
Local Area Networks are now an accepted part of computing research.
The technology of the network itself and the hardware to interface
it to a computer is standard and in the cases of networks like
Ethernet and the Cambridge Ring is commercially available. The next
level up from the hardware is the software interface between the
host computer and the network. This dissertation is concerned with
one specific type of interface where the host is not itself directly
connected to the network, but must access it via a second Network
Interface Processor (NIP).
The dissertation begins by describing the design and implementation
of the two low level interfaces for the Cambridge Ring. The first of
these, the type 2, is machine independent and although based on a
simple processor offers some sophisticated facilities to its host.
The second, Spectrum, is not so sophisticated, but is customized to
interface to just one operating system. The difference between these
two approaches is discussed.
We go on to introduce the High Level Interface, which removes all
protocol and network related processing from the host machine. This
can benefit both the protocol implementation, by reducing system
overheads, and the host operating system, by freeing CPU time for
other tasks. This is particularly true in the case of time-shared
machines which rely on the network for terminal connections. The
design and implementation of such an interface are described.
The dissertation concludes by considering the possible roles of the
NIP in the areas of security, protection and reliability. Some
thoughts are also given on the design of protocols which exploit the
features of a NIP.
Automatic summarising of English texts
Tait, John Irving
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-47
ISSN 1476-2986
This thesis describes a computer program called Scrabble which can
summarise short English texts. It uses large bodies of predictions
about the likely contents of texts about particular topics to
identify the commonplace material in an input text. Pre-specified
summary templates, each associated with a different topic are used
to condense the commonplace material in the input. Filled-in summary
templates are then used to form a framework into which unexpected
material in the input may be fitted, allowing unexpected material to
appear in output summary texts in an essentially unreduced form. The
system’s summaries are in English.
The program is based on technology not dissimilar to a script
applier. However, Scrabble represents a significant advance over
previous script-based summarising systems. It is much less likely to
produce misleading summaries of an input text than some previous
systems and can operate with less information about the subject
domain of the input than others.
These improvements are achieved by the use of three main novel
ideas. First, the system incorporates a new method for identifying
the idea or topics of an input text. Second, it allows a section of
text to have more than one topic at a time, or at least a composite
topic which may be dealt with by the computer program simultaneously
applying the text predictions associated with more than one simple
topic. Third, Scrabble incorporates new mechanisms for the
incorporation of unexpected material in the input into its output
summary texts. The incorporation of such material in the output
summary is motivated by the view that it is precisely unexpected
material which is likely to form the most salient matter in the
input text.
The performance of the system is illustrated by means of a number of
example input texts and their Scrabble summaries.
A mechanism for the accumulation and application of context
in text processing
Alshawi, Hiyan
University of Cambridge, Computer Laboratory
1983-11
en
Text
UCAM-CL-TR-48
ISSN 1476-2986
The paper describes a mechanism for the representation and
application of context information for automatic natural language
processing systems. Context information is gathered gradually during
the reading of the text, and the mechanism gives a way of combining
the effect of several different types of context factors. Context
factors can be managed independently, while still allowing efficient
access to entities in focus. The mechanism is claimed to be more
general than the global focus mechanism used by Grosz for discourse
understanding. Context affects the interpretation process by
choosing the results, and restricting the processing, of a number of
important language interpretation operations, including lexical
disambiguation and reference resolution. The types of context
factors that have been implemented in an experimental system are
described, and examples of the application of context are given.
Programming language design with polymorphism
Matthews, David Charles James
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-49
ISSN 1476-2986
This dissertation describes the design and implementation of a
programming language, Poly. By treating types as values, procedures
can be written which can be applied to objects of many different
types (polymorphism).
Poly was not designed specifically to investigate polymorphism,
rather it was designed to provide a simple yet powerful alternative
to large languages like Ada. The type system came out of a desire to
treat several different programming language concepts by means of a
single parameterisation mechanism, that of procedure application.
For example, generic types are considered simply as procedures.
Polymorphism as Poly can also be used to provide the effect of
overloading without building resolution rules into the language.
Unlike the language Russell, Poly does not require that procedures
be “variable free”. However, it is still possible to statically
type-check a Poly program.
After an introduction to the principles behind modern languages, in
particular types and their relation to abstraction, there is a
survey of several languages. Adu, CLU, Russell, ML and the Cedar
Mesa Kernel illustrate different aspects of language design. Poly is
described by means of some examples and then the background to the
design is discussed. The rationale behind the type system of Poly is
considered and comparisons are made with two other polymorphic
languages, ML and Russell. The remainder of the language is
developed and some applications are discussed. There is a
description of some problems encountered while implementing poly.
Verifying the unification algorithm in LCF
Paulson, Lawrence
University of Cambridge, Computer Laboratory
1984-03
en
Text
UCAM-CL-TR-50
ISSN 1476-2986
Manna and Waldinger’s theory of substitutions and unification has
been verified using the Cambridge LCF theorem prover. A proof of the
monotonicity of substitution is presented in detail, as an example
of interaction with LCF. Translating the theory into LCF’s
domain-theoretic logic is largely straightforward. Well-founded
induction on a complex ordering is translated into nested structural
inductions. Correctness of unification is expressed using predicates
for such properties as idempotence and most-generality. The
verification is presented as a series of lemmas. The LCF proofs are
compared with the original ones, and with other approaches. It
appears difficult to find a logic that is both simple and flexible,
especially for proving termination.
Using information systems to solve recursive domain
equations effectively
Winskel, Glynn
Larsen, Kim Guldstrand
University of Cambridge, Computer Laboratory
1984-07
en
Text
UCAM-CL-TR-51
ISSN 1476-2986
This paper aims to make two main contributions. One is to show how
to use the concrete nature of Scott’s information systems to
advantage in solving recursive domain equations. The method is based
on the substructure relation between information systems. This
essentially makes a complete partial order (cpo) of information
systems. Standard domain constructions like function space can be
made continuous on this cpo so the solution of recursive domain
equations reduces to the more familiar construction of forming the
least-fixed point of a continuous function. The second contribution
again relies on the concrete nature of information systems, this
time to develop a basic theory of effectively given information
systems and through this present a simple treatment of effectively
given domains.
The design of a ring communication network
Temple, Steven
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-52
ISSN 1476-2986
This dissertation describes the design of a high speed local area
network. Local networks have been in use now for over a decade and
there is a proliferation of different systems, experimental ones
which are not widely used and commercial ones installed in hundreds
of locations. For a new network design to be of interest from the
research point of view it must have a feature or features which set
it apart from existing networks and make it an improvement over
existing systems. In the case of the network described, the research
was started to produce a network which was considerably faster than
current designs, but which retained a high degree of generality.
As the research progressed, other features were considered, such as
ways to reduce the cost of the network and the ability to carry data
traffic of many different types. The emphasis on high speed is still
present but other aspects were considered and are discussed in the
dissertation. The network has been named the Cambridge Fast Ring and
and the network hardware is currently being implemented as an
integrated circuit at the University of Cambridge Computer
Laboratory.
The aim of the dissertation is to describe the background to the
design and the decisions which were made during the design process,
as well as the design itself. The dissertation starts with a survey
of the uses of local area networks and examines some established
networks in detail. It then proceeds by examining the
characteristics of a current network installation to assess what is
required of the network in that and similar applications. The major
design considerations for a high speed network controller are then
discussed and a design is presented. Finally, the design of computer
interfaces and protocols for the network is discussed.
A new type-checker for a functional language
Fairbairn, Jon
University of Cambridge, Computer Laboratory
1984-07
en
Text
UCAM-CL-TR-53
ISSN 1476-2986
A polymorphic type checker for the functional language Ponder
[Fairbairn 82] is described. The initial sections give an overview
of the syntax of Ponder, and some of the motivation behind the
design of the type system. This is followed by a definition of the
relation of ‘generality’ between these types, and of the notion of
type-validity of Ponder programs. An algorithm to determine whether
a Ponder program is type-valid is then presented. The final sections
give examples of useful types which may be constructed within the
type system, and describe some of the areas in which it is thought
to be inadequate.
Lessons learned from LCF
Paulson, Lawrence
University of Cambridge, Computer Laboratory
1984-08
en
Text
UCAM-CL-TR-54
ISSN 1476-2986
The history and future prospects of LCF are discussed. The
introduction sketches basic concepts such as the language ML, the
logic PPLAMBDA, and backwards proof. The history discusses LCF
proofs about denotational semantics, functional programs, and
digital circuits, and describes the evolution of ideas about
structural induction, tactics, logics of computation, and the use of
ML. The biography contains thirty-five references.
Executing temporal logic programs
Moszkowski, Ben
University of Cambridge, Computer Laboratory
1984-08
en
Text
UCAM-CL-TR-55
ISSN 1476-2986
Over the last few years, temporal logic has been investigated as a
tool for reasoning about computer programs, digital circuits and
message-passing systems. In the case of programs, the general
feeling has been that temporal logic is an adjunct to existing
languages. For example, one might use temporal logic to specify and
prove properties about a program written in, say, CSP. This leads to
the annoyance of having to simultaneously use two separate
notations.
In earlier work we proposed that temporal logic itself directly
serve as the basis for a programming language. Since then we have
implemented an interpreter for such a language called Tempura. We
are developing Tempura as a tool for directly executing suitable
temporal logic specifications of digital circuits and other discrete
time systems. Since every Tempura statement is also a temporal
formula, we can use the entire temporal logic formalism for our
assertion language and semantics. Tempura has the two seemingly
contradictory properties of being a logic programming langauge and
having imperative constructs such as assignment statements.
The presentation given here first describes the syntax of a first
order temporal logic having the operators ∘ (next) and ◻ (always).
This serves as the basis for the Tempura programming language. The
lesser known temporal operator chop is subsequently introduced,
resulting in Interval Temporal Logic. We then show how to
incorporate chop and related constructs into Tempura.
A new scheme for writing functional operating
systems
Stoye, William
University of Cambridge, Computer Laboratory
1984-09
en
Text
UCAM-CL-TR-56
ISSN 1476-2986
A scheme is described for writing nondeterministic programs in a
functional language. The scheme is based on message passing between
a number of expressions being evaluated in parallel. I suggest that
it represents a significant improvement over previous methods
employing a nondeterministic merge primitive, and overcomes numerous
drawbacks in that approach. The scheme has been designed in a
practical context, and is being used to write an operating system
for SKIM, a functionally programmed machine. It is not yet well
understood in a mathematical sense.
Constructing recursion operators in intuitionistic type
theory
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1984-10
en
Text
UCAM-CL-TR-57
ISSN 1476-2986
Martin-Löf’s Intuitionistic Theory of Types is becoming popular for
formal reasoning about computer programs. To handle recursion
schemes other than primitive recursion, a theory of well-founded
relations is presented. Using primitive recursion over higher types,
induction and recursion are formally derived for a large class of
well-founded relations. Included are < on natural numbers, and
relations formed by inverse images, addition, multiplication, and
exponentiation of other relations. The constructions are given in
full detail to allow their use in theorem provers for Type Theory,
such as Nuprl. The theory is compared with work in the field of
ordinal recursion over higher types.
Categories of models for concurrency
Winskel, Glynn
University of Cambridge, Computer Laboratory
1984-10
en
Text
UCAM-CL-TR-58
ISSN 1476-2986
It is shown how a variety of models for concurrent processes can be
viewed as categories in which familiar constructions turn out to be
significant categorically. Constructions to represent various
parallel compositions are often based on a product construction, for
instance. In many cases different models can be related by a pair of
functors forming an adjunction between the two categories. Because
of the way in which such pairs of functors preserve categorical
constructions, the adjunction serves to translate between the
different models, so it is seen how semantics expressed in terms of
one model translate to semantics in terms of another.
On the composition and decomposition of
assertions
Winskel, Glynn
University of Cambridge, Computer Laboratory
1984-11
en
Text
UCAM-CL-TR-59
ISSN 1476-2986
Recently there has been a great deal of interest in the problem of
how to compose modal assertions, in order to deduce the truth of an
assertion for a composition of processes in a parallel programming
language, from the truth of certain assertions for its components.
This paper addresses that problem from a theoretical standpoint. The
programming language used is Robin Milner’s Synchronous Calculus of
Communicating Systems (called SCCS), while the language of
assertions is a fragment of dynamic logic which, despite its
simplicity, is expressive enough to characterise observational
equivalence. It is shown how, with respect to each operation ‘op’ in
SCCS, every assertion has a decomposition which reduces the problem
of proving the assertion holds of a compound process built up using
‘op’ to proving assertions about its components. These results
provide the foundations of a proof system for SCCS with assertions.
Memory and context mechanisms for automatic text
processing
Alshawi, Hiyan
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-60
ISSN 1476-2986
The thesis describes memory and context mechanisms for natural
language text processing. The mechanisms were implemented as part of
a computer system that successfully processed a number of short
descriptive English texts producing output that can be used to
create a relational database. The memory mechanism is concerned with
representing and retrieving various kinds of knowledge, while the
context mechanism is concerned with accumulating and applying
information specifying which fragments of knowledge are currently
more salient.
The mechanisms are used in the implemented system by an
interpretation component dealing with common language interpretation
problems that cannot be handled by simple sentence-level analysis.
These problems include resolving references, disambiguating word
senses, and discovering implicit relationships. The mechanisms are
also used by a task-specific component which carries out the
database capture application using database descriptions stored in
memory. The choice and handling of the particular application task,
interpretation operations, and types of context information, were
designed to check that the computational techniques developed for
memory and context provide appropriate apparatus for non-trivial
text processing involving a wide range of phenomena of language
interpretation in context.
The memory representation formalism is based on hierarchies for
classifying entities and the associations between them. It has the
advantage of simplicity and a well designed semantics. Retrieval
from memory is performed by marker processing on a network
structure. The context mechanism represents instances of various
types of context information as “context factors” which can be
combined to derive activation values for memory entities. Context
activation is used to choose the results of memory operations and to
restrict memory searches. Context factors are created and modified
as a result of text processing operations, leading to a gradual
alteration of the context representation. Both the memory and
context mechanisms utilize an indexing scheme that uses semantic
clustering criteria. This increases the efficiency of retrieval from
memory and allows efficient access to entities with high activations
derived from several factors while individual factors can be managed
independently.
User models and expert systems
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1984-12
en
Text
UCAM-CL-TR-61
ISSN 1476-2986
This paper analyses user models in expert systems in terms of the
many factors involved: user roles, user properties, model types,
model functions in relation to different aspects of system
performance, and sources, e.g. linguistic or non-linguistic, of
modelling information. The aim of the detailed discussion, with
extensive examples illustrating the complexity of modelling, is to
clarify the issues involved in modelling, as a necessary preliminary
to model building.
Constraint enforcement in a relational database management
system
Robson, Michael
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-62
ISSN 1476-2986
The dissertation describes the implementation of the data
structuring concept of domains, intra-tuple constraints and
referential constraints in a relational database management system
(DBMS). The need for constraints is discussed and it is shown how
they can be used to capture some of the semantics of the database’s
application. The implementation described was done within the
framework of the particular DBMS CODD, the main features of which
are presented.
Each class of constraint is described and it is shown how each of
them is specifed to the DBMS. The descriptions of the constraints
are stored in the database giving a centralised data model, which is
used in the enforcement of the constraints. This data model contains
descriptions not only of static structures but also of procedures to
be used to maintain constraints. A detailed account is given of how
each constraint is maintained.
The main focus of the dissertation is on referential constraints
since inter-relational structure is an area in which relational
systems are particularly weak. Referential constraints impose a
network structure on the database and it is shown how referential
constraints can be maintained by interpreting this network, using
the data-pipelining facilities provided by CODD. It is also shown
how referential constraints can be used to construct generalisation
hierarchies, themselves an important data modelling tool. Further,
some extensions to referential constraints, which allow them to
capture more semantics, are suggested. The usefulness of referential
constraints is illustrated by presenting a real database example
(that of the University Computing Service), on which the ideas
described in the dissertation have been tested.
Poly manual
Matthews, David C.J.
University of Cambridge, Computer Laboratory
1985-02
en
Text
UCAM-CL-TR-63
ISSN 1476-2986
Poly is a general purpose, High-level programming language. It has a
simple type system which is also very powerful. Higher order
procedures, polymorphic operations, parameterised abstract types and
modules are all supported by a single mechanism.
Poly is strongly typed. All objects have a specification which the
compiler can use to check that operations applied to them are
sensible. Type errors cannot cause run time faults. The language is
safe, meaning that any faults occuring at run time will result in
exceptions which can be caught. All veriables must be initialised
before use, so faults due to undefined variables cannot occur. Poly
allows higher order procedures to be declared and used; these take
another procedure as a parameter, or return a procedure as the
result. Since Poly is statically scoped, this may still refer to the
arguments and local variables of the procedure which returned it.
Poly allows polymorphic operations. Thus, it is possible to write
one program to perform an operation on data of any type, provided
only that the operation is available for the data type. Abstract
types may be created and manipulated. These can be specified in such
a way that only the functions to manipulate these objects are
available to the user. This has the advantage that the
implementation can easily be changed, provided that it has the same
external properties. Abstract types can be parameterised so that a
set of types can be defined in a single definition. Types in Poly
are similar to modules in other languages. For example, types can be
separately compiled. An abstract type which makes use of other types
can be written as though it were polymorphic; it will work if it is
given any type which has the required operations. Its operation may
be to return a new type which may be used directly or as a parameter
to other polymorphic abstract types.
A framework for inference in natural language front ends to
databases
Boguraev, Branimir K.
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1985-02
en
Text
UCAM-CL-TR-64
ISSN 1476-2986
Introduction to the programming language “Ponder”
Tillotson, Mark
University of Cambridge, Computer Laboratory
1985-05
en
Text
UCAM-CL-TR-65
ISSN 1476-2986
A formal hardware verification methodology and its
application to a network interface chip
Gordon, M.J.C.
Herbert, J.
University of Cambridge, Computer Laboratory
1985-05
en
Text
UCAM-CL-TR-66
ISSN 1476-2986
We describe how the functional correctness of a circuit design can
be verified by machine checked formal proof. The proof system used
is LCF_LSM [1], a version of Milner’s LCF [2] with a different
logical calculus called LSM. We give a tutorial introduction to LSM
in the paper.
Our main example is the ECL chip of the Cambridge Fast Ring (CFR)
[3]. Although the ECL chip is quite simple (about 360 gates) it is
nevertheless real. Minor errors were discovered as we performed the
formal proof, but when the corrected design was eventually
fabricated it was functionally correct first time. The main steps in
verification were: (1) Writing a high-level behavioural
specification in the LSM notation. (2) Translating the circuit
design from its Modula-2 representation in the Cambridge Design
Automation System [4] to LSM. (3) Using the LCF_LSM theorem proving
system to mechanically generate a proof that the behaviour
determined by the design is equivalent to the specified behaviour.
In order to accomplish the second of these steps, an interface
between the Cambridge Design Automation System and the LCF_LSM
system was constructed.
Natural deduction theorem proving via higher-order
resolution
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1985-05
en
Text
UCAM-CL-TR-67
ISSN 1476-2986
An experimental theorem prover is described. Like LCF it is embedded
in the metalanguage ML and supports backward proof using tactics and
tacticals. The prover allows a wide class of logics to be introduced
using Church’s representation of quantifiers in the typed
lambda-calculus. The inference rules are expressed as a set of
generalized Horn clauses containing higher-order variables.
Depth-first subgoaling along inference rules is essentially linear
resolution, but using higher-order unification instead of
first-order. This constitutes a higher-order Prolog interpreter.
The rules of Martin Löf’s Constructive Type Theory have been entered
into the Prover. Special tactics inspect a goal and decide which
type theory rules may be appropriate, avoiding excessive
backtracking. These tactics can automatically derive the types of
many Type Theory expressions. Simple functions can be derived
interactively.
HOL : A machine oriented formulation of higher order
logic
Gordon, Mike
University of Cambridge, Computer Laboratory
1985-07
en
Text
UCAM-CL-TR-68
ISSN 1476-2986
In this paper we describe a formal language intended as a basis for
hardware specification and verification. The language is not new;
the only originality in what follows lies in the presentation of the
details. Considerable effort has gone into making the formalism
suitable for manipulation by computer.
The logic described here underlies an automated proof generator
called HOL. The HOL logic is derived from Church’s Simple Type
Theory by: making the syntax more readable, allowing types to
contain variables, and building in the Axiom of Choice via Hilbert’s
ε-operator.
The exact syntax of the logic is defined relative to a theory, which
determines the types and constants that are available. Theories are
developed incrementally starting from the standard theories of
truth-values or booleans, and of individuals. This paper describes
the logic underlying the HOL system.
Proving termination of normalization functions for
conditional expressions
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1985-06
en
Text
UCAM-CL-TR-69
ISSN 1476-2986
Boyer and Moore have discussed a recursive function that puts
conditional expressions into normal form. It is difficult to prove
that this function terminates on all inputs. Three termination
proofs are compared: (1) using a measure function, (2) in domain
theory using LCF, (3) showing that its “recursion relation”, defined
by the pattern of recursive calls, is well-founded. The last two
proofs are essentially the same though conducted in markedly
different logical frameworks. An obviously total variant of the
normalize function is presented as the ‘computational meaning’ of
those two proofs.
A related function makes nested recursive calls. The three
termination proofs become more complex: termination and correctness
must be proved simultaneously. The recursion relation approach seems
flexible enough to handle subtle termination proofs where previously
domain theory seemed essential.
A remote procedure call system
Hamilton, Kenneth Graham
University of Cambridge, Computer Laboratory
1984-12
en
Text
UCAM-CL-TR-70
ISSN 1476-2986
The provision of a suitable means for communication between software
modules on different machines is a recognized problem in distributed
computing research. Recently the use of language-level Remote
Procedure Call (RPC) has been advocated as a solution to this
problem.
This thesis discusses the rationale, design, implementation and
supporting environment of a flexible RPC system for an extended
version of the CLU programming language. It is argued that earlier
RPC systems have adopted an undesirably rigid stance by attempting
to make remotre procedure calls look as similar as possible to local
procedure calls. It is suggested instead that the inevitable
differences in performance and failure properties between local and
remote calls should be regarded as being essentially different from
local calls. Following from this, it is proposed that RPC systems
should offer at least two complementary call mechanisms. One of
these should attempt to recover from network errors and should only
report unrecoverable failures. The other should never attempt
automatic recovery from network errors, thereby giving implementors
the convenience of a language-level mechanism without losing sight
of the underlying network.
Other specific areas that are discussed include binding issues,
protocols, transmission mechanisms for standard data types, and the
particular problems posed by abstract data types. A new transfer
mechanism for abstract types is proposed which would permit software
using new representations to communicate with software using earlier
representations. The provision of special operating system support
for the CLU RPC mechanism is also discussed.
Executing temporal logic programs
Moszkowski, Ben
University of Cambridge, Computer Laboratory
1985-08
en
Text
UCAM-CL-TR-71
ISSN 1476-2986
Logic programming and the specification of
circuits
Clocksin, W.F.
University of Cambridge, Computer Laboratory
1985-05
en
Text
UCAM-CL-TR-72
ISSN 1476-2986
Logic programming (see Kowalski, 1979) can be used for specification
and automatic reasoning about electrical circuits. Although
propositional logic has long been used for describing the truth
functions of combinational circuits, the more powerful Predicate
Calculus on which logic programming is based has seen relatively
little use in design automation. Previous researchers have
introduced a number of techniques similar to logic programming, but
many of the useful consequences of the logic programming methodology
have not been exploited. This paper first reviews and compares three
methods for representing circuits, which will be called here the
functional method, the extensional method, and the definitional
method. The latter method, which conveniently admits arbitrary
sequential circuits, is then treated in detail. Some useful
consequences of using this method for writing directly executable
specifications of circuits are described. These include the use of
quantified variables, verification of hypothetical states, and
sequential simulation.
Resource management in a distributed computing
system
Craft, Daniel Hammond
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-73
ISSN 1476-2986
The Cambridge Distributed System, based on the Cambridge Ring local
area network, includes a heterogeneous collection of machines known
as the processor bank. These machines may run network servers, or
may be loaded with services and allocated to users dynamically. The
machines and the variety of services they can support (eg. different
operating systems, compilers, formatters) are viewed as resources
available to other components of the distributed system.
By using a processor bank, two fundamental limitations of the
personal computer approach to distributed computing can be overcome:
responsiveness for computation-intensive tasks is not limited by the
single, personal machine because tasks may expand into processor
bank machines as necessary; and applications are not limited to the
operating system or languages available on the personal computer
because all of the systems or languages which run on processor bank
machines are at the users disposal, both for implementing new
applications and for importing applications from other systems.
Resource management is seen as one of the four areas which must be
addressed to realize these advantages.
The resource management system must match client requirements for
resources to those resources which are available on the network. To
do this it maintains two data bases: one contains information
describing existing resources, and the other contains information
indicating how to obtain resources from servers or have them
constructed from existing subresources by fabricators. The resource
management system accepts resource requirements from clients and
picks from the alternatives in these data bases the “best” match (as
defined by the resource management policy).
The resource management issues addressed include resource
description, location and allocation, construction, monitoring and
reclamation, authentication and protection, and policy. The design
and implementation of two resource management servers is discussed.
Hardware verification by formal proof
Gordon, Mike
University of Cambridge, Computer Laboratory
1985-08
en
Text
UCAM-CL-TR-74
ISSN 1476-2986
The use of mathematical proof to verify hardware designs is
explained and motivated. The hierarchical verification of a simple
n-bit CMOS counter is used as an example. Some speculations are made
about when and how formal proof will become used in industry.
Design and implementation of a simple typed language based
on the lambda-calculus
Fairbairn, Jon
University of Cambridge, Computer Laboratory
1985-05
en
Text
UCAM-CL-TR-75
ISSN 1476-2986
Despite the work of Landin and others as long ago as 1966, almost
all recent programming languages are large and difficult to
understand. This thesis is a re-examination of the possibility of
designing and implementing a small but practical language based on
very few primitive constructs.
The text records the syntax and informal semantics of a new language
called Ponder. The most notable features of the work are a powerful
type-system and an efficient implementation of normal order
reduction.
In contrast to Landin’s ISWIM, Ponder is statically typed, an
expedient that increases the simplicity of the language by removing
the requirement that operations must be defined for incorrect
arguments. The type system is a powerful extension of Milner’s
polymorphic type system for ML in that it allows local
quantification of types. This extension has the advantage that types
that would otherwise need to be primitive may be defined.
The criteria for the well-typedness of Ponder programmes are
presented in the form of a natural deduction system in terms of a
relation of generality between types. A new type checking algorithm
derived from these rules is proposed.
Ponder is built on the λ-calculus without the need for additional
computation rules. In spite of this abstract foundation an efficient
implementation based on Hughes’ super-combinator approach is
described. Some evidence of the speed of Ponder programmes is
included.
The same strictures have been applied to the design of the syntax of
Ponder, which, rather than having many pre-defined clauses, allows
the addition of new constructs by the use of a simple extension
mechanism.
Preserving abstraction in concurrent programming
Cooper, R.C.B.
Hamilton, K.G.
University of Cambridge, Computer Laboratory
1985-08
en
Text
UCAM-CL-TR-76
ISSN 1476-2986
Recent programming languages have attempted to provide support for
concurrency and for modular programming based on abstract
interfaces. Building on our experience of adding monitors to CLU, a
language orientated towards data abstraction, we explain how these
two goals conflict. In particular we discuss the clash between
conventional views on interface abstraction and the programming
style required for avoiding monitor deadlock. We argue that the best
compromise between these goals is a combination of a fine grain
locking mechanism together with a method for explicitly defining
concurrency properties for selected interfaces.
Why higher-order logic is a good formalisation for
specifying and verifying hardware
Gordon, Mike
University of Cambridge, Computer Laboratory
1985-09
en
Text
UCAM-CL-TR-77
ISSN 1476-2986
Higher order logic was originally developed as a foundation for
mathematics. In this paper we show how it can be used as: 1. a
hardware description language, and 2. a formalism for proving that
designs meet their specifications.
Examples are given which illustrate various specification and
verification techniques. These include a CMOS inverter, a CMOS full
adder, an n-bit ripple-carry adder, a sequential multiplier and an
edge-triggered D-type register.
A complete proof system for SCCS with model
assertions
Winskel, Glynn
University of Cambridge, Computer Laboratory
1985-09
en
Text
UCAM-CL-TR-78
ISSN 1476-2986
This paper presents a proof system for Robin Milner’s Synchronous
Calculus of Communicating Systems (SCCS) with modal assertions. The
language of assertions is a fragment of dynamic logic, sometimes
called Hennessy-Milner logic after they brought it to attention;
while rather weak from a practical point of view, its assertions are
expressive enough to characterise observation equivalence, central
to the work of Milner et al. on CCS and SCCS. The paper includes a
completeness result and a proof of equivalence between an
operational and denotational semantics for SCCS. Its emphasis is on
the theoretical issues involved in the construction of proof systems
for parallel programming langauges.
Petri nets, algebras and morphisms
Winskel, Glynn
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-79
ISSN 1476-2986
It is shown how a category of Petri nets can be viewed as a
subcategory of two sorted algebras over multisets. This casts Petri
nets in a familiar framework and provides a useful idea of morphism
on nets different from the conventional definition – the morphisms
here respect the behaviour of nets. The categorical constructions
with result provide a useful way to synthesise nets and reason about
nets in terms of their components; for example various forms of
parallel composition of Petri nets arise naturally from the product
in the category. This abstract setting makes plain a useful functor
from the category of Petri nets to a category of spaces of
invariants and provides insight into the generalisations of the
basic definition of Petri nets – for instance the coloured and
higher level nets of Kurt Jensen arise through a simple
modificationof the sorts of the algebras underlying nets. Further it
provides a smooth formal relation with other models of concurrency
such as Milner’s Calculus of Communicating Systems (CCS) and Hoare’s
Communicating Sequential Processes (CSP).
Interactive theorem proving with Cambridge LCF : A user’s
manual
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1985-11
en
Text
UCAM-CL-TR-80
ISSN 1476-2986
The implementation of functional languages using custom
hardware
Stoye, William Robert
University of Cambridge, Computer Laboratory
1985-12
en
Text
UCAM-CL-TR-81
ISSN 1476-2986
In recent years functional programmers have produced a great many
good ideas but few results. While the use of functional languages
has been enthusiastically advocated, few real application areas have
been tackled and so the functional programmer’s views and ideas are
met with suspicion.
The prime cause of this state of affairs is the lack of widely
available, solid implementations of functional languages. This in
turn stems from two major causes: (1) Our understanding of
implementation techniques was very poor only a few years ago, and so
any implementation that is “mature” is also likely to be unuseably
slow. (2) While functional languages are excellent for expressing
algorithms, there is still considerable debate in the functional
programming community over the way in which input and output
operations should be represented to the programmer. Without clear
guiding principles implementors have tended to produce ad-hoc,
inadequate solutions.
My research is concerned with strengthening the case for functional
programming. To this end I constructed a specialised processor,
called SKIM, which could evaluate functional programs quickly. This
allowed experimentation with various implementation methods, and
provided a high performance implementation with which to experiment
with writing large functional programs.
This thesis describes the resulting work and includes the following
new results: (1) Details of a practical turner-style combinator
reduction implementation featuring greatly improved storage use
compared with previous methods. (2) An implementation of Kennaway’s
director string idea that further enhances performance and increases
understanding of a variety of reduction strategies. (3)
Comprehensive suggestions concerning the representation of input,
output, and nondeterministic tasks using functional languages, and
the writing of operating systems. Details of the implementation of
these suggestions developed on SKIM. (4) A number of observations
concerning fuctional programming in general based on considerable
practical experience.
Natural deduction proof as higher-order
resolution
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1985-12
en
Text
UCAM-CL-TR-82
ISSN 1476-2986
An interactive theorem prover, Isabelle, is under development. In
LCF, each inference rule is represented by one function for forwards
proof and another (a tactic) for backwards proof. In Isabelle, each
inference rule is represented by a Horn clause. Resolution gives
both forwards and backwards proof, supporting a large class of
logics. Isabelle has been used to prove theorems in Martin-Löf’s
Constructive Type Theory.
Quantifiers pose several difficulties: substitution, bound
variables, Skolemization. Isabelle’s representation of logical
syntax is the typed lambda-calculus, requiring higher-order
unification. It may have potential for logic programming.
Depth-first search using inference rules constitutes a higher-order
Prolog.
Operating system design for large personal
workstations
Wilson, Ian David
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-83
ISSN 1476-2986
With the advent of personal computers in the mid 1970s, the design
of operating systems has had to change in order to take account of
the new machines. Traditional problems such as accounting and
protection are no longer relevant, but compactness, efficiency and
pertability have all become important issues as the number of these
small systems has grown.
Since that time, due to the reductions in the costs of computer
components and manufacture, personal workstations have become more
common with not only the number of machines having increased, but
also their CPU power and memory capacity. The work on software for
the new machines has not kept pace with the improvements in hardware
design, and this is particularly true in the area of operating
systems, where there is a tendency to treat the new machines as
small, inferior mainframes.
This thesis investigates the possibility of enhancing work done on
the original personal computer operating systems, so that better
utilisation of the new machines can be obtained. The work
concentrates on two main areas of improvement: the working
environment as perceived by the user, and the underlying primitives
and algorithms used by the operating system kernel.
The work is illustrated by two case studies, the user environment of
the TRIPOS operating system is described, along with a new command
line interpreter and command programming language, and a series of
techniques to make better use of the available hardware facilities
is discussed. The kernel of the TRIPOS operating system is examined
critically, particularly with respect to the way that machine
resources are used, and finally, a new set of kernel primitives and
algorithms is suggested, with reference to an experimental kernel
for the real time implementation of network protocol software.
BSPL: a language for describing the behaviour of synchronous
hardware
Richards, Martin
University of Cambridge, Computer Laboratory
1986-04
en
Text
UCAM-CL-TR-84
ISSN 1476-2986
Category theory and models for parallel
computation
Winskel, Glynn
University of Cambridge, Computer Laboratory
1986-04
en
Text
UCAM-CL-TR-85
ISSN 1476-2986
This report will illustrate two uses of category theory: Firstly the
use of category theory to define semantics in a particular model.
How semantic constructions can often be seen as categorical ones,
and, in particular, how parallel compositions are derived from a
categorical product and a nun-deterministic sum. These categorical
notions can provide a basis for reasoning about computations and
will be illustrated for the model of Petri nets.
Secondly, the use of category theory to relate different semantics
will be examined; specifically, how the relations between various
concrete models like Petri nets, event structures, trees and state
machines are expressed as adjunctions. This will be illustrated by
showing the coreflection between safe Petri nets and trees.
The Entity System: an object based filing system
Crawley, Stephen Christopher
University of Cambridge, Computer Laboratory
1986-04
en
Text
UCAM-CL-TR-86
ISSN 1476-2986
Developments in programming languages have provided increasingly
powerful facilities for algorithmic and data abstraction. Concepts
such as record declarations and formal type checking have been
developed by languages such as Pascal and Algol 68, while languages
such as Simula 67 and Smalltalk supported object based type systems.
Until recently, however, very little work has been done on extending
data typing concepts beyond a single program, where I/O is typically
performed by reading and writing data as an untyped stream of
characters.
By contrast, database systems have traditionally taken a data and
file based approach to storing complex data, and address the
problems of many programs using the same data, while handing
changing data descriptions and access requirements. Recently
attention has been focussed on extending data typing beyond the
bounds of a single program. The DTL language [Hughes 83] models a
program as a data transformer which converts one typed data stream
into another, while PS-Algol extends a representational type system
by allowing data in the heap to persist from one run of a program to
the next. None of these, however, really address the issues of
evolving programs and data requirements.
In order to build the desired functionality in programming
environments, the file system needs to provide considerably more
functionality, by joining together the components of a modular
program, and supporting both small and large components efficiently.
Finally a mechanism was needed for ensuring that files were treated
consistently. The term entity is used to describe an object held in
the file system, which are modelled as a collection of strongly
typed attributes with abstract interfaces. This thesis describes the
experience gained in constructing such a system and the requirements
of an effective persistent storage system.
Computer-aided type face design
Carter, Kathleen Anne
University of Cambridge, Computer Laboratory
1986-05
en
Text
UCAM-CL-TR-87
ISSN 1476-2986
This thesis tackles the problems encountered when trying to carry
out a creative and intuitive task, such as type face design, on a
computer. A brief history of printing and type design sets the scene
for a discussion of digital type. Existing methods for generating
and handling digital type are presented and their relative merits
are discussed. Consideration is also given to the nature of
designing, independent of the tools used. The importance of
intuition and experience in such a task is brought out. Any new
tools must allow the designer to exercise his skills of hand and
eye, and to judge the results visually. The different abstractions
that can be used to represent a typeface in a computer are discussed
with respect to the manner of working that they force upon the
designer.
In the light of this discussion some proposals are made for a new
system for computer-aided type face design. This system must be
highly interactive, providing rapid visual feedback in response to
the designer’s actions. Designing is a very unstructured task,
frequently with a number of activities being pursued at once. Hence
the system must also be able to support multiple activities, with
the user free to move between them at any time.
The characteristics of various types of interactive graphical
environment are then considered. This discussion leads on to
proposals for an environment suitable for supporting type face
design. The proposed anvironment is based on the provision of a
number of windows on the screen, each supporting a different
activity. A mouse, graphics tablet and keyboard are all continuously
available for interection with the system. The rest of the thesis
discusses the implementation of this graphical environment and the
type face design system that makes use of it. The final chapter
evaluates the success of both the underlying software and of the
type face design system itself.
A shallow processing approach to anaphor
resolution
Carter, David Maclean
University of Cambridge, Computer Laboratory
1986-05
en
Text
UCAM-CL-TR-88
ISSN 1476-2986
The thesis describes an investigation of the faesibility of
resolving anaphors in natural language texts by means of a “shallow
processing” approach which exploits knowledge of syntax, semantics
and local focussing as heavily as possible; it does not rely on the
presence of large amounts of world or domain knowledge, which are
notoriously hard to process accurately.
The ideas reported are implemented in a program called SPAR (Shallow
Processing Anaphor Resolver), which resolves anaphoric and other
linguistic ambiguities in simple English stories and generates
sentence-by-sentence paraphrases that show what interpretations have
been selected. Input to SPAR takes the form of semantic structures
for single sentences constructed by Boguraev’s English analyser.
These structures are integrated into a network-style text
representation as processing proceeds. To achieve anaphor
resolution, SPAR combines and develops several existing techniques,
most notably Sidner’s theory of local focussing and Wilks’
“preference semantics” theory of semantics and common sense
inference.
Consideration of the need to resolve several anaphors in the same
sentence results in Sidner’s framework being modified and extended
to allow focus-based processing to ineract more flexibly with
processing based on other types of knowledge. Wilks’ treatment of
common sense inference is extended to incorporate a wider range of
types of inference without jeopardizing its uniformity and
simplicity. Further his primitive-based formalism for word sense
meanings is developed in the interests of economy, accuracy and ease
of use.
Although SPAR is geared mainly towards resolving anaphors, the
design of the system allows many non-anaphoric (lexical and
structural) ambiguities that cannot be resloved during sentence
analysis to be resolved as a by-product of anaphor resolution.
Making form follow function : An exercise in functional
programming style
Fairbairn, Jon
University of Cambridge, Computer Laboratory
1986-06
en
Text
UCAM-CL-TR-89
ISSN 1476-2986
The combined use of user-defined infix operators and higher order
functions allows the programmer to invent new control structures
tailored to a particular problem area.
This paper is to suggest that such a combination has beneficial
effects on the ease of both writing and reading programmes, and
hence can increase programmer productivity. As an example, a parser
for a simple language is presented in this style.
It is hoped that the presentation will be palatable to people
unfamiliar with the concepts of functional programming.
The Cambridge Fast Ring networking system (CFR)
Hopper, Andy
Needham, Roger M.
University of Cambridge, Computer Laboratory
1986-06
en
Text
UCAM-CL-TR-90
ISSN 1476-2986
Local area networks have developed from slow systems operating at
below 1MBs to fast systems at 50MBs or more. We discuss the choices
facing a designer as faster speeds for networks are contemplated.
The 100MBs Cambridge Fast Ring is described. The ring protocol
allows one of a number of fixed size slots to be used once or
repeatedly. The network design allows sets of rings to be
constructed by pushing the bridge function to the lowest hardware
level. Low cost and ease of use is normally achieved by design of
special chips and we describe a two-chip VLSI implementation. This
VLSI hardware forms the basis of a kit-of-parts from which many
different network components can be constructed.
Hardware verification using higher-order logic
Camilleri, Albert
Gordon, Mike
Melham, Tom
University of Cambridge, Computer Laboratory
1986-09
en
Text
UCAM-CL-TR-91
ISSN 1476-2986
The Hardware Verification Group at the University of Cambridge is
investigating how various kinds of digital systems can be verified
by mechanised formal proof. This paper explains our approach to
representing behaviour and structure using higher order logic.
Several examples are described including a ripple carry adder and a
sequential device for computing the factorial function. The dangers
of inaccurate models are illustrated with a CMOS exclusive-or gate.
Implementation and programming techniques for functional
languages
Wray, Stuart Charles
University of Cambridge, Computer Laboratory
1986-06
en
Text
UCAM-CL-TR-92
ISSN 1476-2986
In this thesis I describe a new method of strictness analysis for
lazily evaluated functional languages, and a method of code
generation making use of the information provided by this analysis.
I also describe techniques for practical programming in lazily
evaluated functional languages, based on my experience of writing
substantial functional programs.
My new strictness analyser is both faster and more powerful than
that of Mycroft. It can be used on any program expressed as
super-combinator definitions and it uses the additional
classifications absent and dangerous as well as strict and lazy.
This analyser assumes that functional arguments to higher order
functions are completely lazy.
I describe an extension of my analyser which discovers more
strictness in the presence of higher order functions, and I compare
this with higher order analysers based on Mycroft’s work. I also
describe an extension of my analyser to lazy pairs and discuss
strictness analysers for lazy lists.
Strictness analysis brings useful performance improvements for
programs running on conventional machines. I have implemented my
analyser in a compiler for Ponder, a lazily evaluated functional
language with polymorphic typing. Results are given, including the
surprising result that higher order strictness analysis is no better
than first order strictness analysis for speeding up real programs
on conventional machines.
I have written substantial programs in Ponder and describe in some
detail the largest of these which is about 2500 lines long. This
program is an interactive spreadsheet using a mouse and bitmapped
display. I discuss programming techniques and practical problems
facing functional languages with illustrative examples from programs
I have written.
Automated design of an instruction set for BCPL
Bennett, J.P.
University of Cambridge, Computer Laboratory
1986-06
en
Text
UCAM-CL-TR-93
ISSN 1476-2986
A mechanized proof of correctness of a simple
counter
Cohn, Avra
Gordon, Mike
University of Cambridge, Computer Laboratory
1986-06
en
Text
UCAM-CL-TR-94
ISSN 1476-2986
Event structures : Lecture notes for the Advanced Course on
Petri Nets
Winskel, Glynn
University of Cambridge, Computer Laboratory
1986-07
en
Text
UCAM-CL-TR-95
ISSN 1476-2986
Event structures are a model of computational processes. They
represent a process as a set of event occurrences with relations to
express how events causally depend on others. This paper introduces
event structures, shows their relationship to Scott domains and
Petri nets, and surveys their role in denotational semantics, both
for modelling laguages like CCS and CSP and languages with higher
types.
Models and logic of MOS circuits : Lectures for the
Marktoberdorf Summerschool, August 1986
Winskel, Glynn
University of Cambridge, Computer Laboratory
1986-10
en
Text
UCAM-CL-TR-96
ISSN 1476-2986
Various models of hardware have been proposed though virtually all
of them do not model circuits adequately enough to support and
provide a formal basis for many of the informal arguments used by
designers of MOS circuits. Such arguments use rather crude discrete
notions of strength – designers cannot be too finicky about precise
resistances and capacitances when building a chip – as well as
subtle derived notions of information flow between points in the
circuit. One model, that of R.E. Bryant, tackles such issues in
reasonable generality and has been used as the basis of several
hardware simulators. However Bryant’s model is not compositional.
These lectures introduce Bryant’s ideas and present a compositional
model for the behaviour of MOS circuits when the input is steady,
show how this leads to a logic, and indicate the difficulties in
providing a full and accurate treatment for circuits with changing
inputs.
A study on abstract interpretation and “validating microcode
algebraically”
Mycroft, Alan
University of Cambridge, Computer Laboratory
1986-10
en
Text
UCAM-CL-TR-97
ISSN 1476-2986
This report attempts to perfrom two roles: the first part aims to
give a state-of-the-art introduction to abstract interpretation with
as little mathematics as possible. The question of the ‘best’
meta-language for abstract interpretation is, however, left open.
The second part gives a tutorial introduction to an application of
abstract interpretation based on the relational style of Mycroft and
Jones (1985). This report does not claim to have introduced any new
techniques, but rather aims to make the existing literature
understandable to a wider audience.
Power-domains, modalities and the Vietoris monad
Robinson, E.
University of Cambridge, Computer Laboratory
1986-10
en
Text
UCAM-CL-TR-98
ISSN 1476-2986
It is possible to divide the syntax-directed approaches to
programming language semantics into two classes, “denotational”, and
“proof-theoretic”. This paper argues for a different approach which
also has the effect of linking the two methods. Drawing on recent
work on locales as formal spaces we show that this provides a way in
which we can hope to use a proof-theoretical semantics to give us a
denotational one. This paper reviews aspects of the general theory,
before developing a modal construction on locales and discussing the
view of power-domains as free non-deterministic algebras. Finally,
the relationship between the present work and that of Winskel is
examined.
An overview of the Poly programming language
Matthews, David C.J.
University of Cambridge, Computer Laboratory
1986-08
en
Text
UCAM-CL-TR-99
ISSN 1476-2986
Poly is a general purpose programming language based on the idea of
treating types as first class values. It can support polymorphic
operations by passing types as parameters to procedures, and
abstract types and parameterised types by returning types as
results.
Although Poly is not intended specifically as a database programming
language it was convenient to implement it is a persistent storage
system. This allows the user to retain data structures from one
session to the next, and can support large programming systems such
as the Poly compiler and a Standard ML system.
Proving a computer correct in higher order logic
Joyce, Jeff
Birtwistle, Graham
Gordon, Mike
University of Cambridge, Computer Laboratory
1986-12
en
Text
UCAM-CL-TR-100
ISSN 1476-2986
Technical report no. 42, ‘Proving a computer correct using the
LSF_LSM hardware verification system’, describes the specification
and verification of a register-transfer level implementation of a
simple general purpose computer. The computer has a microcoded
control unit implementing eight user level instructions. We have
subsequently redone this example in higher order logic using the HOL
hardware verification system.
This report presents the specification and verification of Gordon’s
computer as an example of hardware specification and verification in
higher order logic. The report describes how the structure and
behaviour of digital circuits may be specified using the formalism
of higher order logic. The proof of correctness also shows how
digital behaviour at different granularities of time may be related
by means of a temporal abstraction.
This report should be read with Technical report no. 68, ‘HOL, a
machine oriented formulation of higher order logic’, which describes
the logic underlying the HOL hardware verification system.
Binary routing networks
Milway, David Russel
University of Cambridge, Computer Laboratory
1986-12
en
Text
UCAM-CL-TR-101
ISSN 1476-2986
Binary Routing Networks combine ideas from Wide Area Networks and
Interconnection Networks with the principles of Local Area Networks.
This results in a high performance network for use in the local and
wide area environment. Simulation of this form of network shows that
for certain structures the performance of the network can approach
or even exceed that obtained from a cross-bar switch. This
dissertation describes how network structures based on Binary
Routing Networks can be used in applications where a network capable
of high rates of throughput with low delay is required.
Binary Routing Networks use a switching fabric constructed from
simple routing nodes to route packets from a source to a
destination. Some network topologies allow many packets to pass
through the network simultanously, giving the network an aggregate
throughput mugh greater than the basic bit rate. Routing nodes do
not require knowledge of the topology and are thus simple to
construct. They use routing information in the packet to direct the
packet through the network. Packets pass through the nodes with
little delay except where contention for a link occurs when the
packet needs to be buffered.
A design for a non-buffered routing node is described where
contention is resolved by discarding one of the packets. Discarded
packets are retried later by the sending station. This form of
network removes the buffers from the routing nodes making them even
simpler to construct. Simulations of a network of 512 stations show
that for loads per station of up to 20% of the basic bit rate, a
non-buffered network can outperform a buffered network. This design
allows the construction of a fault tolerant network which can pass
packets through any number of different paths, avoiding broken links
or congensted areas in the network.
A prototype of a Binary Routing Network is discussed. This network
makes use of the non-buffered routing nodes and measurements of its
performance are compared with results obtained from the simulations.
A proposal for using this form of network in an Integrated Service
environment are also given.
Structures similar to Binary Routing Networks are fast becoming the
backbone of multiprocessor systems. Local Area Networks also need to
apply this technology to meet the requirements that they are being
asked to support.
A persistent storage system for Poly and ML
Matthews, David C.J.
University of Cambridge, Computer Laboratory
1987-01
en
Text
UCAM-CL-TR-102
ISSN 1476-2986
The conventional strategy for implementing interactive languages has
been based on the use of a “workspace” or “core-image” which is read
in at the start of a session and written out at the end. While this
is satisfactory for small systems it is inefficient for large
programs. This report describes how an idea originally invented to
simplify database programming, the persistent store, was adapted to
support program development in an interactive language.
Poly and ML are both semi-functional languages in the sense that
they allow functions as first class objects but they have variables
(references) and use call-by-value semantics. Implementing such
languages in a persistent store poses some problems but also allows
optimisations which would not be possible if their type systems did
not apply certain constraints.
The basic system is designed for single-users but the problems of
sharing data between users is discussed and an experimental system
for allowing this is described.
HOL : A proof generating system for higher-order
logic
Gordon, Mike
University of Cambridge, Computer Laboratory
1987-01
en
Text
UCAM-CL-TR-103
ISSN 1476-2986
HOL is a version of Robin Milner’s LCF theorem proving system for
higher-order logic. It is currently being used to investigate: how
various levels of hardware behaviour can be rigorously modelled; and
how the resulting behavioural representations can be the basis for
verification by mechanized formal proof.
This paper starts with a tutorial introduction to the meta-language
ML. The version of higher-order logic implemented in the HOL system
is then described. This is followed by an introduction to
goal-directed proof with tactics and tacticals. Finally, there is a
little example showing the system in action. This example
illustrates how HOL can be used for hardware verification.
A proof of correctness of the Viper microprocessor: the
first level
Cohn, Avra
University of Cambridge, Computer Laboratory
1987-01
en
Text
UCAM-CL-TR-104
ISSN 1476-2986
The Viper microprocessor designed at the Royal Signals and Radar
Establishment (RSRE) is one of the first commercially produced
computers to have been developed using modern formal methods. Viper
is specified in a sequence of decreasingly abstract levels. In this
paper a mechanical proof of the equivalence of the first two of
these levels is described. The proof was generated using a version
of Robin Milner’s LCF system.
A compositional model of MOS circuits
Winskel, Glynn
University of Cambridge, Computer Laboratory
1987-04
en
Text
UCAM-CL-TR-105
ISSN 1476-2986
This paper describes a compositional model for MOS circuits. Like
the model of Bryant (1984), it covers some of the effects of
capacitance and resistance used frequently in designs. Although this
has formed the basis of several hardware simulators, it suffers from
the inadequacy that it is not compositional, making it difficult to
reason in a structured way.
The present paper restricts its attention to the static behaviour of
circuits, representing this as the set of possible steady states the
circuit can settle into. A good understanding of such static
behaviour is necessary to treat sequential circuits. This paper
further takes the view that it is useful to have a language to
describe the construction of circuits, and to this end borrows ideas
from Hoare’s Communicating Sequential Processes, and Milner’s
Calculus of Communicating Systems.
Abstraction mechanisms for hardware verification
Melham, Thomas F.
University of Cambridge, Computer Laboratory
1987-05
en
Text
UCAM-CL-TR-106
ISSN 1476-2986
It is argued that techniques for proving the correctness of hardware
designs must use abstraction mechanisms for relating formal
descriptions at different levels of detail. Four such abstraction
mechanisms and their formalisation in higher order logic are
discussed.
DI-domains as a model of polymorphism
Coquand, Thierry
Gunter, Carl
Winskel, Glynn
University of Cambridge, Computer Laboratory
1987-05
en
Text
UCAM-CL-TR-107
ISSN 1476-2986
This paper investigates a model of the polymorphic lambda calculus
recently described by Girard (1985). This model differs from earlier
ones in that all the types are interpreted as domains rather than
closures or finitary projections on a universal domain. The
objective in this paper is to generalize Girard’s construction to a
larger category called dI-domains, and secondly to show how Girard’s
construction (and this generalization) can be done abstractly. It
demonstrates that the generalized construction can be used to do
denotational semantics in the ordinary way, but with the added
feature of type polymorphism.
Workstation design for distributed computing
Wilkes, Andrew John
University of Cambridge, Computer Laboratory
1987-06
en
Text
UCAM-CL-TR-108
ISSN 1476-2986
This thesis discusses some aspects of the design of computer systems
for local area networks (LANs), with particular emphasis on the way
such systems present themselves to their users. Too little attention
to this issue frequently results in computing environments that
cannot be extended gracefully to accommodate new hardware or
software and do not present consistent, uniform interfaces to either
their human users or their programmatic clients. Before computer
systems can become truly ubiquitous tools, these problems of
extensibility and accessibility must be solved. This dissertation
therefore seeks to examine one possible approach, emphasising
support for program development on LAN based systems.
Hardware verification of VLSI regular structures
Joyce, Jeffrey
University of Cambridge, Computer Laboratory
1987-07
en
Text
UCAM-CL-TR-109
ISSN 1476-2986
Many examples of hardware specification focus on hierarchical
specification as a means of controlling structural complexity in
design. Another method is the use of iteration. This paper, however,
presents a third method, namely the mapping of irregular
combinational functions to regular structures.
Regular structures often result in solutions which are economical in
terms of area and design time. The automatic generation of a regular
structure such as a ROM or PLA from a functional specification
usually accommodates minor changes to the functional specification.
The mapping of irregular combinational functions to a regular
structure separates function from circuit design. This paper shows
how this separation can be exploited to derive a behavioural
specification of a regular structure parameterized by the functional
specification.
Relating two models of hardware
Winskel, Glynn
University of Cambridge, Computer Laboratory
1987-07
en
Text
UCAM-CL-TR-110
ISSN 1476-2986
The idea of this note is to show how Winskel’s static-configuration
model of circuits is related formally to Gordon’s relational model.
Once so related, the simpler proofs in the relational model can, for
instance, be used to justify results in terms of the
static-configurations model. More importantly, we can exhibit
general conditions on circuits which ensure that assertions which
hold of a circuit according to the simpler model are correct with
respect to the more accurate model. The formal translation makes use
of a simple adjunction between (partial order) categories associated
with the two models, in a way reminiscient of abstract
interpretation. Preliminary results suggest similar lines of
approach may work for other kinds of abstraction such as temporal
abstraction used in e.g. Melham’s work to reason about hardware,
and, more generally, make possible a formal algebraic treatment of
the relationship between different models of hardware.
Realism about user modelling
Spärck Jones, K.
University of Cambridge, Computer Laboratory
1987-06
en
Text
UCAM-CL-TR-111
ISSN 1476-2986
This paper reformulates the framework for user modelling presented
in an earlier technical report, ‘User Models and Expert Systems’,
and considers the implications of the real limitations on the
knowledge likely to be available to a system for the value and
application of user models.
Reducing thrashing by adaptive backtracking
Wolfram, D.A.
University of Cambridge, Computer Laboratory
1987-08
en
Text
UCAM-CL-TR-112
ISSN 1476-2986
Adaptive backtracking dynamically reduces thrashing caused by blind
backtracking and recurring failures, by locating early backtrack
points and deleting choices which are not part of any solution.
Search problems with hereditary bounding properties are soluble by
this method. These problems include searches in theorem proving,
logic programming, reason maintenance, and planning. The location of
a backtrack point uses a particular minimal inconsistent subset,
which is called the cause set. A rejection set is computed from the
union of cause sets and rejection sets at a failure are used to
locate subsequent backtrack points. A choice is deleted when a
rejection set is a singleton. The worst case overhead is O(nf(n)) in
time if the bounding property can be tested in O(f(n)) time, and
O(n²) in space. An implementation confirms the expected exponential
speed-ups for problems whose solution involves much thrashing.
The representation of logics in higher-order
logic
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1987-08
en
Text
UCAM-CL-TR-113
ISSN 1476-2986
Intuitionistic higher-order logic — the fragment comtaining
implication, universal quantification, and equality — can serve as a
meta-logic for formalizing various logics. As an example, axioms
formalizing first-order logic are presented, and proved sound and
complete by induction on proof trees.
Proofs in higher-order logic represent derivations of rules as well
as proofs of theorems. A proof develops by deriving rules using
higher-order resolutions. The discharge of assumptions involves
derived meta-rules for ‘lifting’ a proposition.
Quantifiers require a similar lifting rule or else Hilbert’s
ε-operator. The alternatives are contrasted through several
examples. Hilbert’s ε underlies Isabelle’s original treatment of
quantifiers, but the lifting rule is logically simpler.
The meta-logic is used in the latest version of the theorem prover
Isabelle. It extends the logic used in earlier versions. Compared
with other meta-logics, higher-order logic has a weaker type system
but seems easier to implement.
An architecture for integrated services on the local area
network
Ades, Stephen
University of Cambridge, Computer Laboratory
1987-09
en
Text
UCAM-CL-TR-114
ISSN 1476-2986
This dissertation concerns the provision of integrated services in a
local area context, e.g. on business premises. The term integrated
services can be understood at several levels. At the lowest, one
network may be used to carry traffic of several media—voice, data,
images etc. Above that, the telephone exchange may be replaced by a
more versatile switching system, incorporating facilities such as
stored voice messages. Its facilities may be accessible to the user
through the interface of the workstation rather than a telephone. At
a higher level still, new services such as multi-media document
manipulation may be added to the capabilities of a workstation.
Most of the work to date has been at the lowest of these levels,
under the auspices of the Integrated Services Digital Network
(ISDN), which mainly concerns wide area communications systems. The
thesis presented here is that all of the above levels are important
in a local area context. In an office environment, sophisticated
data processing facilities in a workstation can usefully be combined
with highly available telecommunications facilities such as the
telephone, to offer the user new services which make the working day
more pleasant and productive. That these facilities should be
provided across one integrated network, rather than by several
parallel single medium networks is an important organisational
convenience to the system builder.
The work described in this dissertation is relevant principally in a
local area context—in the wide area economics and traffic balance
dictate that the emphasis will be on only the network level of
integration for some time now. The work can be split into three
parts:
i) the use of a packet network to carry mixed media. This has
entailed design of packet voice protocols which produce delays low
enough for the network to interwork with national telephone
networks. The system has also been designed for minimal cost per
telephone—packet-switched telephone systems have traditionally been
more expensive than circuit-switched types. The network used as a
foundation for this work has been the Cambridge Fast Ring.
ii) use of techniques well established in distributed computing
systems to build an ‘integrated services PABX (Private Automatic
Branch Exchange)’. Current PABX designs have a very short life
expectancy and an alarmingly high proportion of their costs is due
to software. The ideas presented here can help with both of these
problems, produce an extensible system and provide a basis for new
multi-media services.
iii) development of new user level Integrated Services. Work has
been done in three areas. The first is multi-media documents. A
voice editing interface is described along with the system structure
required to support it. Secondly a workstation display has been
built to support a variety of services based upon image manipulation
and transmission. Finally techniques have been demonstrated by which
a better interface to telephony functions can be provided to the
user, using methods of control typical of workstation interfaces.
Formal validation of an integrated circuit design
style
Dhingra, I.S.
University of Cambridge, Computer Laboratory
1987-08
en
Text
UCAM-CL-TR-115
ISSN 1476-2986
In dynamic circuit design many rules must be followed which govern
the correctness of the design. In this paper a dynamic CMOS design
style using a two phase non-overlapping clock with its intricate
design rules is presented together with formal means of showing that
a circuit follows these rules.
Domain theoretic models of polymorphism
Coquand, Thierry
Gunter, Carl
Winskel, Glynn
University of Cambridge, Computer Laboratory
1987-09
en
Text
UCAM-CL-TR-116
ISSN 1476-2986
The main point of this paper is to give an illustration of a
construction useful in producing and describing models of Girard and
Reynolds’ polymorphic λ-calculus. The key unifying ideas are that of
a Grothendieck fibration and the category of continuous sections
associated with it, constructions used in indexed category theory;
the universal types of the calculus are interpreted as the category
of continuous sections of the fibration. As a major example a new
model for the polymorphic λ-calculus is presented. In it a type is
interpreted as a Scott domain. The way of understanding universal
types of the polymorphic λ-calculus as categories of continuous
sections appears to be useful generally, and, as well as applying to
the new model introduced here, also applies, for instance, to the
retract models of McCracken and Scott, and a recent model of Girard.
It is hoped that by pin-pointing a key construction this paper will
help towards a deeper understanding of the models for the
polymorphic λ-calculus and the relations between them.
Distributed computing with RPC: the Cambridge
approach
Bacon, J.M.
Hamilton, K.G.
University of Cambridge, Computer Laboratory
1987-10
en
Text
UCAM-CL-TR-117
ISSN 1476-2986
The Cambridge Distributed Computing System (CDCS) is described and
its evolution outlined. The Mayflower project allowed CDCS
infrastructure, services and applications to be programmed in a high
level, object oriented, language, Concurrent CLU. The Concurrent CLU
RPC facility is described in detail. It is a non-transparent, type
checked, type safe system which employs dynamic binding and passes
objects of arbitrary graph structure. Recent extensions accomodate a
number of languages and transport protocols. A comparison with other
RPC schemes is given.
Material concerning a study of cases
Boguraev, B.K.
Spärck Jones, K.
University of Cambridge, Computer Laboratory
1987-05
en
Text
UCAM-CL-TR-118
ISSN 1476-2986
This note describes and illustrates a study of deep cases using a
large sample of sentences. We have used a language analyser which
builds meaning representations expressing semantic case roles;
specifically Boguraev’s (1979) analyser, which builds dependency
trees with word senses defined by semantic category primitive
formulae, and with case labels, i.e. semantic relation primitives.
This note highlights the importance of the source material for those
interested in case-based representations of sentence meaning, and
indicates the potential utility of the study results.
Pilgrim: a debugger for distributed systems
Cooper, Robert
University of Cambridge, Computer Laboratory
1987-07
en
Text
UCAM-CL-TR-119
ISSN 1476-2986
Pilgrim is a source level debugger for concurrent CLU programs which
execute in a distributed environment. It integrates conventional
debugging facilities with features for debugging remote procedure
calls and critical region based process interactions. Pilgrim is
unusual in that it functions on programs in the target environment
under conditions of actual use. This has caused a trade-off between
providing rich and detailed information to the programmer and
avoiding any unwanted alteration to the computation being debugged.
Another complication is debugging one client of a network server
while avoiding interference with the server’s other clients. A
successful methodology for this case requires assistance from the
server itself.
Block encryption
Wheeler, D.
University of Cambridge, Computer Laboratory
1987-11
en
Text
UCAM-CL-TR-120
ISSN 1476-2986
A fast and simple way of encrypting computer data is needed. The
UNIX crypt is a good way of doing this although the method is not
cryptographically sound for text. The method suggested here is
applied to larger blocks than the DES method which uses 64 bit
blocks, so that the speed of encyphering is reasonable. The
algorithm is designed for software rather than hardware. This
forgoes two advantages of the crypt algorithm, namely that each
character can be encoded and decoded independently of other
characters and that the identical process is used both for
encryption and decryption. However this method is better for coding
blocks directly.
A high-level petri net specification of the Cambridge Fast
Ring M-access service
Billington, Jonathan
University of Cambridge, Computer Laboratory
1987-12
en
Text
UCAM-CL-TR-121
ISSN 1476-2986
Numerical Petri Nets (a high level inhibitor net) are used to
characterise the Cambridge Fast Ring Hardware at a high level of
abstraction. The NPN model describes the service provided to users
of the hardware (stations, monitors, bridges and ring transmission
plant), known as the M-Access service definition in order to remove
ambiguities and as a basis for the development and verification of
the protocols using the M-Access service.
Temporal abstraction of digital designs
Herbert, John
University of Cambridge, Computer Laboratory
1988-02
en
Text
UCAM-CL-TR-122
ISSN 1476-2986
Formal techniques have been used to verify the function of
reasonably large digital devices ([Hunt85], [Cohn87]), and also to
describe and reason about digital signal behaviour at a detailed
timing level [Hanna85], [Herbert86]. Different models are used:
simple synchronous models of components are the basis for verifying
high-level functional specifications; more detailed models which
capture the behaviour of signals in real time are the basis of
proofs about timing. The procedure called temporal abstraction is a
technique for formally relating these two behavioural models.
The background to temporal abstraction is presented and the details
of its implementation in HOL. The HOL language ([Gordon85a]) is a
computerised version of higher-order logic which has an associated
proof assistant also called HOL. In HOL one may specify behaviour at
both the functional and timing levels. This work describes how the
relationship between these levels may also be described in HOL and
reasoned about using the HOL system.
The formal transformation of descriptions of behaviour at the timing
level to behaviour at the functional level involves generating and
verifying timing constraints. This process can be identified with
the conventional design activity of timing analysis. This work shows
that timing verification can be viewed, not as a separate phase of
design, but as part of a single verification process which
encompasses functional and timing verification. A single formal
language, HOL, is used to describe all aspects of the behaviour and
a single verification system provides all the proofs of correctness.
The use of uniform, formal techniques is shown to have a number of
advantages.
Case study of the Cambridge Fast Ring ECL chip using
HOL
Herbert, John
University of Cambridge, Computer Laboratory
1988-02
en
Text
UCAM-CL-TR-123
ISSN 1476-2986
This article describes the formal specification and verification of
an integrated circuit which is part of a local area network
interface. A single formal language is used to describe the
structure and behaviour at all levels in the design hierarchy, and
an associated proof assistant is used to generate all formal proofs.
The implementation of the circuit, described as a structure of gates
and flip-flops, is verified via a number of levels with respect to a
high-level formal specification of required behaviour. The
high-level formal specification is shown to be close to precise
natural language description of the circuit behaviour.
The specification language used, HOL [Gordon85a], has the advantage
of permitting partial specifications. It turns out that partial
specification has an important effect on the specification and
verification methodology and this is presented. We have also
evaluated aspects of conventional design, such as techniques for
locating errors and the use of simulation, within the case study of
formal methods. We assert that proof strategies must assist error
location and that simulation has a role alongside formal
verification.
Formal verification of basic memory devices
Herbert, John
University of Cambridge, Computer Laboratory
1988-02
en
Text
UCAM-CL-TR-124
ISSN 1476-2986
Formal methods have been used recently to verify high-level
functional specifications of digital systems. Such formal proofs
have used simple models of circuit components. In this article we
describe complementary work which uses a more detailed model of
components and demonstrates how hardware can be specified and
verified at this level.
In this model all circuits can be described as structures of gates,
each gate having an independent propagation delay. The behaviour of
digital signals in real time is captured closely. The function and
timing of asynchronous and synchronous memory elements implemented
using gates is derived. Formal proofs of correctness show that,
subject to certain constraints on gate delays and signal timing
parameters, these devices act as memory elements and exhibit certain
timing properties.
All the proofs have been mechanically generated using Gordon’s HOL
system.
An operational semantics for Occam
Camilleri, Juanito
University of Cambridge, Computer Laboratory
1988-02
en
Text
UCAM-CL-TR-125
ISSN 1476-2986
Occam is a programming language designed to support concurrent
applications, especially those implemented on networks of
communicating processors. The aim of this paper is to formulate the
meaning of the language constructs of Occam by semantic definitions
which are intended as a direct formalisation of the natural language
descriptions usually found in programming language manuals [Inmos
3]. This is done by defining a syntax directed transition system
where the transitions associated to a phrase are a function of the
transitions associated to its components. This method is by no means
novel. The concepts here were introduced in [Plotkin 8] and are
applied in [Plotkin 9] where an operational semantics for CSP [Hoare
2] was presented. The operational semantics for a subset of Ada is
defined in [Li 6], where tasking and exception handling are
modelled. For simplicity only a subset of Occam is defined. Timing,
priority, replicators and BYTE subscription are omitted. Other
features of Occam which deal with the associated components of an
Occam program with a set of physical resources (i.e. configurations)
are also omitted since they do not effect the semantic
interpretation of a program.
Reasoning about the function and timing of integrated
circuits with Prolog and temporal logic
Leeser, M.E.
University of Cambridge, Computer Laboratory
1988-02
en
Text
UCAM-CL-TR-126
ISSN 1476-2986
This article describes the application of formal methods to
transistor level descriptions of circuits. Formal hardware
verification uses techniques based on mathematical logic to formally
prove that a circuit correctly implements its behavioral
specification. In the approach described here, the structure of
circuits and their functional behavior are described with Interval
Temporal Logic. These specifications are expressed in Prolog, and
the logical manipulations of the proof process are achieved with a
Prolog system. To demonstrate the approach, the bahavior of several
example circuits is derived from the behavior of their components
down to the transistor level. These examples include a dynamic latch
which uses a 2-phase clocking scheme and exploits charge storage.
Timing as well as functional aspects of behavior are derived, and
constraints on the way a circuit interacts with its environment are
reasoned about formally.
A development environment for large natural language
grammars
Carroll, John
Boguraev, Bran
Grover, Claire
Briscoe, Ted
University of Cambridge, Computer Laboratory
1988-02
en
Text
UCAM-CL-TR-127
ISSN 1476-2986
The Grammar Development Environment (GDE) is a powerful software
tool designed to help a linguist or grammarian experiment with and
develop large natural language grammars. (It is also, however, being
used to help teach students on courses in computational
linguistics). This report describes the grammatical formalism
employed by the GDE, and contains detailed instructions on how to
use the system.
Source code for a Common Lisp version of the software is available
from the University of Edinburgh Artificial Intelligence
Applications Institute.
Debugging concurrent and distributed programs
Cooper, Robert Charles Beaumont
University of Cambridge, Computer Laboratory
1988-02
en
Text
UCAM-CL-TR-128
ISSN 1476-2986
This thesis aims to make one aspect of distributed programming
easier: debugging. The principles for designing and implementing an
interactive debugger for concurrent and distributed programs are
presented. These programs are written in a high-level language with
type-checked remote procedure calls. They execute on the nodes of a
local computer network and interact with the other programs and
services which exist on such a network.
The emphasis is on debugging programs in the environment in which
they will eventually operate, rather than some simulated environment
oriented specifically to the needs of debugging. Thus the debugging
facilities impose a low overhead on the program and may be activated
at any time.
Ideally the actions of the debugger should be transparent to the
execution of the program being debugged. The difficult problem of
avoiding any alteration to the relative ordering of inter-process
events is examined in detail. A method of breakpointing a
distributed computation is presented which achieves a high degree of
transparency in the face of arbitary process interactions through
shared memory.
The problems of debugging programs that interact with network
services, which are shared concurrently with other users of the
distributed environment, are examined. A range of debugging
techniques, some of which are directly supported by the debugger,
are discussed.
A set of facilities for debugging remote procedure calls is
presented, and the functions required of the operating system kernel
and runtime system to support debugging are also discussed. A
distributed debugger is itself an example of a distributed program
and so issues such as functional distribution and authentication are
addressed.
These ideas have been implemented in Pilgrim, a debugger for
Concurrent CLU programs running under the Mayflower supervisor
within the Cambridge Distributed Computing System.
A methodology for automated design of computer instruction
sets
Bennett, Jeremy Peter
University of Cambridge, Computer Laboratory
1988-03
en
Text
UCAM-CL-TR-129
ISSN 1476-2986
With semiconductor technology providing scope for increasingly
complex computer architectures, there is a need more than ever to
rationalise the methodology behind computer design. In the 1970’s,
byte stream architectures offered a rationalisation of computer
design well suited to microcoded hardware. In the 1980’s, RISC
technology has emerged to simplify computer design and permit full
advantage to be taken of very large scale integration. However, such
approaches achieve their aims by simplifying the problem to a level
where it is within the comprehension of a simple human being. Such
an effort is not sufficient. There is a need to provide a
methodology that takes the burden of design detail away from the
human designer, leaving him free to cope with the underlying
principles involved.
In this dissertation I present a methodology for the design of
computer instruction sets that is capable of automation in large
part, removing the drudgery of individual instruction selection. The
methodology does not remove the need for the designer’s skill, but
rather allows precise refinement of his ideas to obtain an optimal
instruction set.
In developing this methodology a number of pieces of software have
been designed and implemented. Compilers have been written to
generate trial instruction sets. An instruction set generator
program has been written and the instruction set it proposes
evaluated. Finally a prototype language for instruction set design
has been devised and implemented.
The foundation of a generic theorem prover
Paulson, Lawrence C
University of Cambridge, Computer Laboratory
1988-03
en
Text
UCAM-CL-TR-130
ISSN 1476-2986
Isabelle is an interactive theorem prover that supports a variety of
logics. It represents rules as propositions (not as functions) and
builds proofs by combining rules. These operations constitute a
meta-logic (or ‘logical framework’) in which the object-logics are
formalized. Isabelle is now based on higher-order logic – a precise
and well-understood foundation.
Examples illustrate use of this meta-logic to formalize logics and
proofs. Axioms for first-order logic are shown sound and complete.
Backwards proof is formalized by meta-reasoning about object-level
entailment.
Higher-order logic has several practical advantages over other
meta-logics. Many proof techniques are known, such as Huet’s
higher-order unification procedure.
Architecture problems in the construction of expert systems
for document retrieval
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1986-12
en
Text
UCAM-CL-TR-131
ISSN 1476-2986
The idea of an expert system front end offering the user effective
direct accessto a document retrieval system is an attractive one.
The paper considers two specific approaches to the construction of
such an expert interface, Belkin and Brooks and their colleagues’
treatment of the functions of such a front end based on the analysis
of human intermediaries, and Pollitt’s experimental implementation
of a query formulator for searching Cancerline. The distributed
expert system model proposed by Belkin and Brooks is a plausable
one, and Pollitt’s system can be regarded as a first step towards
it. But there are major problems about this type of architecture,
and the paper argues in particular that in seeking to develop more
powerful front ends of the kind envisaged there is one important
issue, the nature of the language used for communication between the
contributing experts, that requires for attention than it has
hitherto received.
Reasoning about the function and timing of integrated
circuits with Prolog and temporal logic
Leeser, Miriam Ellen
University of Cambridge, Computer Laboratory
1988-04
en
Text
UCAM-CL-TR-132
ISSN 1476-2986
The structure of circuits is specified with Prolog; their function
and timing behaviour is specified with interval temporal logic.
These structural and behavioural specifications are used to formally
verify the functionality of circut elements as well as their timing
characteristics. A circuit is verified by deriving its behaviour
from the behaviour of its components. The derived results can be
abstracted to functional descriptions with timing constraints. The
functional descriptions can then be used in proofs of more complex
hardware circuits.
Verification is done hierarchically, with transistors as primitive
elements. Transistors are modelled as switch-level devices with
delay. In order to model delay, the direction of signal flow through
the transistor must be assigned. This is done automatically by a set
of Prolog routines which also determine the inputs and outputs of
each circuit component.
Interval temporal logic descriptions are expressed in Prolog and
manipulated using PALM: Prolog Assistant for Logic Manipulation.
With PALM the user specifies rewrite rules and uses these rules to
manipulate logical terms. In the case of reasoning about circuits,
PALM is used to manipulate the temporal logic descriptions of the
components to derive a temporal logic description of the circuit.
These techniques are demonstrated by applying them to several
commonly used complementary metal oxide semiconductor (CMOS)
structures. Examples include a fully complementary dynamic latch and
a 1-bit adder. Both these circuits are implemented with transistors
and exploit 2-phase clocking and charge sharing. The 1-bit adder is
a sophisticated full adder implemented with a dynamic CMOS design
style. The derived timing and functional behaviour of the 1-bit
adder is abstracted to a purely functional behavior which can be
used to derive the behaviour of an arbitrary n-bit adder.
A preliminary users manual for Isabelle
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1988-05
en
Text
UCAM-CL-TR-133
ISSN 1476-2986
This is an early report on the theorem prover Isabelle and several
of its object-logics. It describes Isabelle’s operations, commands,
data structures, and organization. This information is fairly
low-level, but could benefit Isabelle users and implementors of
other systems.
Correctness properties of the Viper black model: the second
level
Cohn, Avra
University of Cambridge, Computer Laboratory
1988-05
en
Text
UCAM-CL-TR-134
ISSN 1476-2986
Viper [8,9,10,11,22] is a microprocessor designed by J. Cullyer, C.
Pygott and J. Kershaw at the Royal Signals and Radar Establishment
in Malvern (RSRE), and is now commericially available. Viper is
intended for use is safety-critical applications such as aviation
and nuclear power plant control. To this end, Viper has a
particularly simple design about which it is relatively easy to
reason using current techniques and models. The designers at RSRE,
who deserve much credit for the promotion of formal methods,
intended from the start that Viper be formally verified. This report
describes the partially completed correctness proof, in the HOL
system, of the Viper ‘block model’ with respect to Viper’s top level
functional specification. The (fully completed) correctness proof of
the Viper ‘major state’ model has already been reported in [5]. This
paper describes the analysis of the block model in some detail (in
sections 6 to 9), so is necessarily rather long. A less detailed
account is to appear in future [6]. Section 2 is a discussion of the
scope and limits of the word ‘verification’, and cautions against
careless use of the term. The paper includes a very brief
introduction to HOL (section 4), but does not attempt a description
or rationalization of Viper’s design. The possible uses of the paper
are as follows:
It includes enough detail to support an attempt to repeat the proof
in HOL, or possibly in other theorem-proving systems.
It serves as a guide for future analyses of Viper;
It completes the existing Viper documentation;
It covers some general issues in hardware verification;
It illustrates the probelms in managing large HOL proofs.
Using recursive types to reason about hardware in higher
order logic
Melham, Thomas F.
University of Cambridge, Computer Laboratory
1988-05
en
Text
UCAM-CL-TR-135
ISSN 1476-2986
The expressive power of higher order logic makes it possible to
define a wide variety of data types within the logic and to prove
theorems that state the properties of these types concisely and
abstractly. This paper describes how such defined data types can be
used to support formal reasoning in higher order logic about the
behaviour of hardware designs.
Formal specification and verification of asynchronous
processes in higher-order logic
Joyce, Jeffrey J.
University of Cambridge, Computer Laboratory
1988-06
en
Text
UCAM-CL-TR-136
ISSN 1476-2986
We model the interaction of a synchronous process with an
asynchronous memory process using a four-phase “handshaking”
protocol. This example demonstrates the use of higher-order logic to
reason about the behaviour of synchronous systems such as
microprocessors which communicate requests to asynchronous devices
and then wait for unpredictably long periods until these requests
are answered. We also describe how our model could be revised to
include some of the detailed timing requirements found in real
systems such as the M68000 microprocessor. One enhancement uses
non-determinism to model minimum setup times for asynchronous
inputs. Experience with this example suggests that higher-order
logic may also be a suitable formalism for reasoning about more
abstract forms of concurrency.
Mass terms and plurals: from linguistic theory to natural
language processing
Hasle, F.V.
University of Cambridge, Computer Laboratory
1988-06
en
Text
UCAM-CL-TR-137
ISSN 1476-2986
Two linguistic theories within the tradition of formal semantics are
investigated. One is concerned with mass terms, and the other with
plurals.
Special attention is paid to the possibility of implementing the
theories on a computer. With this goal in mind their basic ideas are
examined, and the linguistic implications are discussed. In the
process, various features of the theories are made formally precise.
This leads to two formal systems, one for representing the meanings
of sentences with mass terms, and another similar one for plurals.
The systems are specified by machine-executable translation
relations from fragments of natural language into logical
representations.
The underlying model-theoretic semantics of each theory is partially
axiomatised. From the axiomatisations all of the paradigmatic
inferences of each theory can be proved in a purely deductive
manner. This is demanstrated by a number of rigorous proofs of
natural language inferences.
Finally some methodological issues are raised. Both theories
recommend a particular approach within formal semantics for natural
language. I explore the methodological views underlying the
theories, and discuss whether the authors actually follow the
methods which they recommend.
Authentication: a practical study in belief and
action
Burrows, Michael
Abadi, Martín
Needham, Roger
University of Cambridge, Computer Laboratory
1988-06
en
Text
UCAM-CL-TR-138
ISSN 1476-2986
Questions of belief and action are essential in the analysis of
protocols for the authentication of principals in distributed
computing systems. In this paper we motivate, set out and exemplify
a logic specifically designed for this analysis; we show how
protocols differ subtly with respect to the required initial
assumptions of the participants and their final beliefs. Our
fomalism has enabled us to isolate and express these differences in
a way that was not previously possible, and it has drawn attention
to features of the protocols of which we were perviously unaware.
The reasoning about particular protocols has been mechanically
verified.
This paper starts with an informal account of the problem, goes on
to explain the formalism to be used, and gives examples of its
application to real protocols from the literature. The final
sections deal with a formal semantics of the logic and conclusions.
Petri net theory: a survey
Manson, Paul R.
University of Cambridge, Computer Laboratory
1988-06
en
Text
UCAM-CL-TR-139
ISSN 1476-2986
The intense interest in concurrent (or “parallel”) computation over
the past decade has given rise to a large number of languages for
concurrent programming, representing many conflicting views of
concurrency.
The discovery that concurrent programming is significantly more
difficult than sequential programming has prompted considerable
research into determining a tractable and flexible theory of
concurrency, with the aim of making concurrent processing more
accessible, and indeed the wide variety of concurrent languages
merely reflects the many different models of concurrency which have
also been developed.
This report, therefore introduces Petri nets, discussing their
behaviour, interpretation and relationship to other models of
concurrency. It defines and discusses several restrictions and
extensions of the Petri net model, showing how they relate to basic
Petri nets, while explaining why they have been of historical
importance. Finally it presents a survey of the analysis methods
applied to Petri nets in general and for some of the net models
introduced here.
Executing behavioural definitions in higher order
logic
Camilleri, Albert John
University of Cambridge, Computer Laboratory
1988-07
en
Text
UCAM-CL-TR-140
ISSN 1476-2986
Over the past few years, computer scientists have been using formal
verification techniques to show the correctness of digital systems.
The verification process, however, is complicated and expensive.
Even proofs of simple circuits can involve thousands of logical
steps. Often it can be extremely difficult to find correct device
specifications and it is desirable that one sets off to prove a
correct specification from the start, rather than repeatedly
backtrack from the verification process to modify the original
definitions after discovering they were incorrect or inadequate.
The main idea presented in the thesis is to amalgamate the
techniques of simulation and verification, rather than have the
latter replace the former. The result is that behavioural
definitions can be simulated until one is reasonably sure that the
specification is correct. Furthermore, proving the correctness with
respect to these simulated specifications avoids the inadequacies of
simulation where it may not be computationally feasible to
demonstrate correctness by exhaustive testing. Simulation here has a
different purpose: to get specifications correct as early as
possible in the verification process. Its purpose is not to
demonstrate the correctness of the implementation – this is done in
the verification stage when the very same specifications that were
simulated are proved correct.
The thesis discusses the implementation of an executable subset of
the HOL logic, the version of Higher Order Logic embedded in the HOL
theorem prover. It is shown that hardware can be effectively
described using both relations and functions; relations being
suitable for abstract specification and functions being suitable for
execution. The difference between relational and functional
specifications are discussed and illustrated by the verification of
an n-bit adder. Techniques for executing functional specifications
are presented and various optimisation strategies are shown which
make the execution of the logic efficient. It is further shown that
the process of generating optimised functional definitions from
relational definitions can be automated. Example simulations of
three hardware devices (a factorial machine, a small computer and a
communications chip) are presented.
Reliable management of voice in a distributed
system
Want, Roy
University of Cambridge, Computer Laboratory
1988-07
en
Text
UCAM-CL-TR-141
ISSN 1476-2986
The ubiquitous personal computer has found its way into most office
environments. As a result, widespread use of the Local Area Network
(LAN) for the purposes of sharing distributed computing resources
has become common. Another technology, the Private Automatic Branch
Exchange (PABX), has benefited from large research and development
by the telephone companies. As a consequence, it is cost effective
and has widely infiltrated the office world. Its primary purpose is
to switch digitised voice but, with the growing need for
communication between computers it is also being adapted to switch
data. However, PABXs are generally designed around a centralised
switch in which bandwidth is permanently divided between its
subscribers. Computing requirements need much larger bandwidths and
the ability to connect to several services at once, thus making the
conventional PABX unsuitable for this application.
Some LAN technologies are suitable for switching voice and data. The
additional requirement for voice is that point to point delay for
network packets should have a low upper-bound. The 10 Mb/s Cambridge
Ring is an example of this type of network, but is relatively low
bandwidth gives it limited application in this area. Networks with
larger bandwidths (up to 100 Mb/s) are now becoming available
comercially and could support a realistic population of clients
requiring voice and data communication.
Transporting voice and data in the same network has two main
advantages. Firstly, from a practical point of view, wiring is
minimised. Secondly, applications which integrate both media are
made possible, and hence digitised voice may be controlled by client
programs in new and interesting ways.
In addition to the new applications, the original telephony
facilities must also be available. They should, at least by default,
appear to work in an identical way to our tried and trusted
impression of a telephone. However, the control and management of a
network telephone is now in the domain of distributed computing. The
voice connections between telephones are virtual circuits. Control
and data information can be freely mixed with voice at a network
interface. The new problems that result are the management issues
related to the distributed control of real-time media.
This thesis describes the issues as a distributed computing problem
and proposes solutions, many of which have been demonstrated in a
real implementation. Particular attention has been paid to the
quality of service provided by the solutions. This amounts to the
design of helpful operator interfaces, flexible schemes for the
control of voice from personal workstations and, in particular, a
high reliability factor for the backbone telephony service. This
work demonstrates the advantages and the practicality of integrating
voice and data services within the Local Area Network.
A fast packet switch for the integrated services backbone
network
Newman, Peter
University of Cambridge, Computer Laboratory
1988-07
en
Text
UCAM-CL-TR-142
ISSN 1476-2986
With the projected growth in demand for bandwidth and
telecommunications services, will come the reguirement for a
multi-service backbone network of far greater efficiency, capacity
and flexibility than the ISDN is able to satisfy. This class of
network has been termed the Broadband ISDN, and the design of the
switching node of such a network is the subject of much current
research. This paper investigates one possible solution. The design
and performance, for multi-service traffic, is presented of a fast
packet switch based upon a non-buffered, multi-stage interconnection
network. It is shown that for an implementation in current CMOS
technology, operating at 50 MHz, switches with a total traffic
capacity of up to 150 Gbit/sec may be constructed. Furthermore, if
the reserved service traffic load is limited on each input port to a
maximum of 80% of switch port saturation, then a maximum delay
across the switch of the order of 100 μsecs may be guaranteed, for
99% of the reserved service traffic, regardless of the unreserved
service traffic load.
Experience with Isabelle : A generic theorem
prover
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1988-08
en
Text
UCAM-CL-TR-143
ISSN 1476-2986
The theorem prover Isabelle is described briefly and informally. Its
historical development is traced from Edinburgh LCF to the present
day. The main issues are unification, quantifiers, and the
representation of inference rules. The Edinburgh Logical Framework
is also described, for a comparison with Isabelle. An appendix
presents several Isabelle logics, including set theory and
Constructive Type Theory, with examples of theorems.
An operational semantics for occam
Camilleri, Juanito
University of Cambridge, Computer Laboratory
1988-08
en
Text
UCAM-CL-TR-144
ISSN 1476-2986
Occam is a programming language designed to support concurrent
applications, especially those implemented on networks of
communicating processors. The aim of this paper is to formulate the
meaning of the language constructs of Occam by semantic definitions
which are intended as a direct formalisation of the natural language
descriptions usually found in programming language manuals [Inmos
5]. This is done by defining a syntax directed transition system
where the transitions associated to a phrase are a function of the
transitions associated to its components. This method is by no means
novel. The concepts here were introduced in [Plotkin 10] and are
applied in [Plotkin 11] where an operational semantics for CSP
[Hoare 4] was presented. The operational semantics for a subset of
Ada is defined in [Li 6], where tasking and exception handling are
modelled. For simplicity only a subset of Occam is defined. Timing,
replicators and BYTE subscription are omitted. Other features of
Occam which deal with the association of components of an Occam
program with a set of physical resources (i.e. configurations) are
also omitted since they do not effect the semantic interpretation of
a program.
Mechanizing programming logics in higher order
logic
Gordon, Michael J.C.
University of Cambridge, Computer Laboratory
1988-09
en
Text
UCAM-CL-TR-145
ISSN 1476-2986
Formal reasoning about computer programs can be based directly on
the semantics of the programming language, or done in a special
purpose logic like Hoare logic. The advantage of the first approach
is that it guarantees that the formal reasoning applies to the
language being used (it is well known, for example, that Hoare’s
assignment axiom fails to hold for most programming languages). The
advantage of the second approach is that the proofs can be more
direct and natural.
In this paper, an attempt to get the advantages of both approaches
is described. The rules of Hoare logic are mechanically derived from
the semantics of a simple imperative programming language (using the
HOL system). These rules form the basis for a simple program
verifier in which verification conditions are generated by LCF-style
tactics whose validations use the derived Hoare rules. Because Hoare
logic is derived, rather than postulated, it is straightforward to
mix semantic and axiomatic reasoning. It is also straightforward to
combine the constructs of Hoare logic with other
application-specific notations. This is briefly illustrated for
various logical constructs, including termination statements,
VDM-style ‘relational’ correctness specifications, weakest
precondition statements and dynamic logic formulae.
The theory underlying the work presented here is well known. Our
contribution is to propose a way of mechanizing this theory in a way
that makes certain practical details work out smoothly.
Automating recursive type definitions in higher order
logic
Melham, Thomas F.
University of Cambridge, Computer Laboratory
1988-09
en
Text
UCAM-CL-TR-146
ISSN 1476-2986
The expressive power of higher order logic makes it possible to
define a wide variety of types within the logic and to prove
theorems that state the properties of these types concisely and
abstractly. This paper contains a tutorial introduction to the
logical basis for such type definitions. Examples are given of the
formal definitions in logic of several simple types. A method is
then described for systematically defining any instance of a certain
class of commonly-used recursive types. The automation of this
method in HOL, an interactive system for generating proofs in higher
order logic, is also discussed.
Formal specification and verification of microprocessor
systems
Joyce, Jeffrey
University of Cambridge, Computer Laboratory
1988-09
en
Text
UCAM-CL-TR-147
ISSN 1476-2986
This paper describes the use of formal methods to verify a very
simple microprocessor. The hierarchical structure of the
microprocessor implementation is formally specified in higher-order
logic. The behaviour of the microprocessor is then derived from a
switch level model of MOS (Metal Oxide Semiconductor) behaviour
using inference rules of higher-order logic with assistance from a
mechanical theorem proving system. The complexity of the formal
proof is controlled by a multi-level approach based on increasingly
abstract views of time and data. While traditional methods such as
multi-level simulation may reveal errors or inconsistencies, formal
verification can provide greater certainty about the correctness of
a design. The main difference with formal verification, and its
strength, is that behaviour at one level is formally dervied from
lower levels with a precise statement of the conditions under which
one level accurately models lower levels.
Extending coloured petri nets
Billington, Jonathan
University of Cambridge, Computer Laboratory
1988-09
en
Text
UCAM-CL-TR-148
ISSN 1476-2986
Jensen’s Coloured Petri Nets (CP-nets) are taken as the starting
point for the development of a specification technique for complex
concurrent systems. To increase its expressive power CP-nets are
extended by including capacity and inhibitor functions. A class of
extended CP-nets, known as P-nets, is defined that includes the
capacity function and the threshold inhibitor extension. The
inhibitor extension is defined in a totally symmetrical way to that
of the usual pre place map (or incidence function). Thus the
inhibitor and pre place maps may be equated by allowing a marking to
be purged by a single transition occurrence, useful when specifying
the abortion of various procedures. A chapter is devoted to
developing the theory and notation for the purging of a place’s
marking or part of its marking.
Two transformations from P-nets to CP-nets are presented and it is
proved that they preserve interleaving behaviour. These are based on
the notion of complementary places defined for PT-nets and involve
the definition and proof of a new extended complementary place
invariant for CP-nets
The graphical form of P-nets, known as a P-Graph, is presented
formally and draws upon the theories developed for algebraic
specification. Arc inscriptions are multiples of tuples of terms
generated by a many-sorted signature. Transition conditions are
Boolean expressions derived from the same signature. An
interpretation of the P-Graph is given in terms of a corresponding
P-net. The work is similar to that of Vautherin but includes the
inhibitor and capacity extension and a number of significant
differences. in the P-Graph concrete sets are associated with
places, rather than sorts and likewise there are concrete initial
marking and capacity functions. Vautherin associates equations with
transitions rather than the more general Boolean expressions.
P-Graphs are useful for specification at a concrete level. Classes
of the P-Graph, known as Many-sorted Algebraic Nets and Many-sorted
Predicate/Transition nets, are defined and illustrated by a number
of examples. An extended place capacity notation is developed to
allow for the convenient representation of resource bounds in the
graphical form.
Some communications-oriented examples are presented including queues
and the Demon Game of international standards fame.
The report concludes with a discussion of future work. In
particular, an abstract P-Graph is defined that is very similar to
Vautherin’s Petri net-like schema, but including the capacity and
inhibitor extensions and associating boolean expressions with
transitions. This will be useful for more abstract specifications
(eg classes of communications protocols) and for their analysis.
It is believed that this is the first coherent and formal
presentation of these extensions in the literature.
Improving security and performance for capability
systems
Karger, Paul Ashley
University of Cambridge, Computer Laboratory
1988-10
en
Text
UCAM-CL-TR-149
ISSN 1476-2986
This dissertation examines two major limitations of capability
systems: an inability to support security policies that enforce
confinement and a reputation for relatively poor performance when
compared with non-capability systems.
The dissertation examines why conventional capability systems cannot
enforce confinement and proposes a new secure capability
architecture, called SCAP, in which confinement can be enforced.
SCAP is based on the earlier Cambridge Capability System, CAP. The
dissertation shows how a non-discretionary security policy can be
implemented on the new architecture, and how the new architecture
can also be used to improve traceability of access and revocation of
access.
The dissertation also examines how capability systems are vulnerable
to discretionary Trojan horse attacks and proposes a defence based
on rules built into the command-language interpreter. System-wide
garbage collection, commonly used in most capability systems, is
examined in the light of the non-discretionary security policies and
found to be fundamentally insecure. The dissertation proposes
alternative approaches to storage management to provide at least
some of the benefits of system-wide garbage collection, but without
the accompanying security problems.
Performance of capability systems is improved by two major
techniques. First, the doctrine of programming generality is
addressed as one major cause of poor performance. Protection domains
should be allocated only for genuine security reasons, rather than
at every subroutine boundary. Compilers can better enforce
modularity and good programming style without adding the expense of
security enforcement to every subroutine call. Second, the ideas of
reduced instruction set computers (RISC) can be applied to
capability systems to simplify the operations required. The
dissertation identifies a minimum set of hardware functions needed
to obtain good performance for a capability system. This set is much
smaller than previous research had indicated necessary.
A prototype implementation of some of the capability features is
described. The prototype was implemented on a re-microprogrammed
VAX-11/730 computer. The dissertation examines the performance and
software compatibility implications of the new capability
architecture, both in the context of conventional computers, such as
the VAX, and in the context of RISC processors.
Simulation as an aid to verification using the HOL theorem
prover
Camilleri, Albert John
University of Cambridge, Computer Laboratory
1988-10
en
Text
UCAM-CL-TR-150
ISSN 1476-2986
The HOL theorem proving system, developed by Mike Gordon at the
University of Cambridge, is a mechanism of higher order logic,
primarily intended for conducting formal proofs of digital system
designs. In this paper we show that hardware specifications written
in HOL logic can be executed to enable simulation as a means of
supporting formal proof. Specifications of a small microprocessor
are described, showing how HOL logic sentences can be transformed
into executable code with minimum risk of introducing
inconsistencies. A clean and effective optimisation strategy is
recommended to make the executable specifications practical.
Formalising an integrated circuit design style in higher
order logic
Dhingra, Inderpreet-Singh
University of Cambridge, Computer Laboratory
1988-11
en
Text
UCAM-CL-TR-151
ISSN 1476-2986
If the activities of an integrated circuit designer are examined, we
find that rather than keeping track of all the details, he uses
simple rules of thumb which have been refined from experience. These
rules of thumb are guidelines for deciding which blocks to use and
how they are to be connected. This thesis gives a formal foundation,
in higher order logic, to the design rules of a dynamic CMOS
integrated circuit design style.
Correctness statements for the library of basic elements are
fomulated. These statements are based on a small number of
definitions which define the behaviour of transistors and capacitors
and the necessary axiomisation of the four valued algebra for
signals. The correctness statements of large and complex circuits
are then derived from the library of previously proved correctness
statements, using logical inference rules instead of rules of thumb.
For example, one gate from the library can drive another only if its
output constraints are satisfied by the input constraints of the
gate that it drives. In formalising the design rules, these
constraints are captured as predicates and are part of the
correctness statements of these gates. So when two gates are to be
connected, it is only necessary to check that the predicates match.
These ideas are fairly general and widely applicable for formalising
the rules of many systems.
A number of worked examples are presented based on these formal
techniques. Proofs are presented at various stages of development to
show how the correctness statement for a device evolves and how the
proof is constructed. In particular it is demonstrated how such
formal techniques can help improve and sharpen the final
specifications.
As a major case study to test all these techniques, a new design for
a gigital phase-locked loop is presented. This has been designed
down to the gate level using the above dynamic design style, and has
been described and simulated using ELLA. Some of the subcomponents
have been formally verified down to the detailed circuit level while
others have merely been specified without formal proofs of
correctness. An informal proof of correctness of this device is also
presented based on the formal specifications of the various
submodules.
Motion development for computer animation
Pullen, Andrew Mark
University of Cambridge, Computer Laboratory
1988-11
en
Text
UCAM-CL-TR-152
ISSN 1476-2986
This thesis examines the problems posed by the use of computers in
the production of animated sequences, and suggests possible
solutions to some of them. Over the years increasing emphasis has
been placed on the quest for visual realism, with the result that
other considerations—such as ease of motion specification—have been
overshadowed. Many current techniques put animation in the realm of
the scientist programmer rather than the animation artist. This
thesis in the main ignores image rendering issues but considers
instead the motion specification phase of generating an animated
sequence.
The thesis begins by examining the traditional hand animation
process and asks whether speed or quality benefits can be achieved
by automating parts of it. My own experiences in this area are
described based on the design and implementation of KAS, a
computer-assisted animation system, which was then used to produce
parts of a short animated film for Channel 4 television.
In the light of this experience, other computer animation techniques
are considered—beginning with a survey of animation systems
developed by other researchers over the years and a discussion of
their relative merits. This survey identifies the two techniques in
most common use today as being (i) an extension of the keyframing
technique used for traditional hand animation, and (ii) a scripting
approach, which essentially involves producing a textual description
of the desired motion in a specially developed animation language.
Both of these methods suffer from serious drawbacks—with keyframing
it is difficult to control complex motion, whereas scripting forces
artists into a style of working which does not exploit their
traditional intuition and experience and is more suited to computer
programmers than animators.
In an attempt to overcome these shortcomings, proposals are made for
a new style of motion development environment making heavy use of
interactive graphics and using computer simulation to guide the
motion of the objects being animated. Once suitable simulation rules
have been set up, the software becomes capable of dealing with the
majority of situations that an object will encounter. This means
that the animator need only intervene occasionally in order to steer
the animation to the desired conclusion. Two major experiments aimed
at determining the usefulness of this idea were conducted: one uses
fixed rules in a simple environment (the game of snooker); the other
considers a more general realm (cellular automata) and gives the
animator the freedom to change the simulation rules at any time.
The conclusion drawn from these experiments is that the proposed
method is capable of development to the stage where a powerful tool
can be provided for animators to use in a novel but intuitive
way—one which exploits their capability as artists and makes only
minor demands on them to understand the underlying technology.
Efficient data sharing
Burrows, Michael
University of Cambridge, Computer Laboratory
1988-12
en
Text
UCAM-CL-TR-153
ISSN 1476-2986
As distributed computing systems become widespread, the sharing of
data between people using a large number of computers becomes more
important. One of the most popular ways to facilitate this sharing
is to provide a common file system, accessible by all the machines
on the network. This approach is simple and reasonably effective,
but the performance of the system can degrade significantly if the
number of machines is increased. By using a hierarchical network,
and arranging that machines typically access files stored in the
same section of the network it is possible to build very large
systems. However, there is still a limit on the number of machines
that can share a single file server and a single network
effectively.
A good way to decrease network and server load is to cache file data
on client machines, so that data need not be fetched from the
centralized server each time it is accessed. This technique can
improve the performance of a distributed file system and is used in
a number of working systems. However, caching brings with it the
overhead of maintaining consistency, or cache coherence. That is,
each machine in the network must see the same data in its cache,
even though one machine may be modifying the data as others are
reading it. The problem is to maintain consistency without
dramatically increasing the number of messages that must be passed
between machines on the network.
Some existing file systems take a probabilistic approach to
consistency, some explicitly prevent the activities that can cause
inconsistency, while others provide consistency only at the some
cost in functionality or performance. In this dissertation, I
examine how distributed file systems are typically used, and the
degree to which caching might be expected to improve performance. I
then describe a new file system that attempts to cache significantly
more data than other systems, provides strong consistency
guarantees, yet requires few additional messages for cache
management.
This new file-system provides fine-grain sharing of a file
concurrently open on multiple machines on the network, at the
granularity of a single byte. It uses a simple system of
multiple-reader, single writer locks held in a centralized server to
ensure cache consistency. The problem of maintaining client state in
a centralized server are solved by using efficient data structures
and crash recovery techniques.
A natural language interface to an intelligent planning
system
Crabtree, I.B.
Crouch, R.S.
Moffat, D.C.
Pirie, N.J.
Pulman, S.G.
Ritchie, G.D.
Tate, B.A.
University of Cambridge, Computer Laboratory
1989-01
en
Text
UCAM-CL-TR-154
ISSN 1476-2986
An intelligent planning system is an example of a software aid
which, although developed by specialists, is intended to be used by
non-programmers for a wide variety of tasks. There is therefore a
need for a communication medium which allows the application
specialist, and the non-expert user to specify their needs without
knowing the details of the system.
This kind of system is one where the ‘mice and menus’ approach is
unlikely to be able to provide a very flexible interface since the
range and type of potential queries is not predictable in advance.
Clearly, therefore, some kind of language is a necessity here. The
aim of this project is to experiment with the use of English
language as the medium of communication. The kind of system we would
eventually be able to build would be one where the user could use
the planner to organise some external activity, trying out
alternative scenarios, and then interact with the system during the
execution of the resulting plans, making adjustments where
necessary.
Computational morphology of English
Pulman, S.G.
Russell, G.J.
Ritchie, G.D.
Black, A.W.
University of Cambridge, Computer Laboratory
1989-01
en
Text
UCAM-CL-TR-155
ISSN 1476-2986
This paper describes an implemented computer program which uses
various kinds of linguistic knowledge to analyse existing or novel
word forms in terms of their components. Three main types of
knowledge are required (for English): knowledge about spelling or
phonological changes consequent upon affixation (notice we are only
dealing with isolated word forms); knowledge about the syntactic or
semantic properties of affixation (i.e. inflexional and derivational
morphology), and knowledge about the properties of the stored base
forms of words (which in our case are always themselves words,
rather than more abstract entities). These three types of
information are stored as data files, represented in exactly the
form a linguist might employ. These data files are then compiled by
the system to produce a run-time program which will analyse
arbitrary word forms presented to it in a way consistent with the
original linguistic description.
Events and VP modifiers
Pulman, Steve
University of Cambridge, Computer Laboratory
1989-01
en
Text
UCAM-CL-TR-156
ISSN 1476-2986
This paper concerns the anaysis of adverbial and PP modifiers of VP
suggested by Davidson, where verbs are regarded as introducing
reference to events, and such modifiers are predicates of these
events. Several problems raised by it are described and a solution
is presented. The paper then goes on to suggest some extensions of
the theory in order to be able to cope with temporal and aspectual
modification of VPs.
Introducing a priority operator to CCS
Camilleri, Juanito
University of Cambridge, Computer Laboratory
1989-01
en
Text
UCAM-CL-TR-157
ISSN 1476-2986
In this paper we augment the syntax of CCS by introducing a priority
operator. We present a syntax directed operational semantics of the
language as a labelled transition system. A new equivalence relation
which is based on Milner’s strong observational equivalence [11] is
defined and proved to be a congruence. We also give some examples
which illustrate the use of the operator and emphasise the novelty
of the approach used to introduce the notion prior to process
algebras.
Tailoring output to the user: What does user modelling in
generation mean?
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1988-08
en
Text
UCAM-CL-TR-158
ISSN 1476-2986
This paper examines the implications for linguistic output
generation tailored to the interactive system user, of earlier
analyses of the components of user modelling and of the constraints
realism imposes on modelling. Using a range of detailed examples it
argues that tailoring based only on the actual dialogue and on the
decision model required for the system task is quite adequate, and
that more ambitious modelling is both dangerous and unnecessary.
Non-trivial power types can’t be subtypes of polymorphic
types
Pitts, Andrew M.
University of Cambridge, Computer Laboratory
1989-01
en
Text
UCAM-CL-TR-159
ISSN 1476-2986
This paper establishes a new, limitative relation between the
polymorphic lambda calculus and the kind of higher-order type theory
which is embodied in the logic of toposes. It is shown that any
embedding in a topos of the cartesian closed category of (closed)
types of a model of the polymorphic lambda calculus must place the
polymorphic types well away from the powertypes σ→Ω of the topos, in
the sense that σ→Ω is a subtype of a polymorphic type only in the
case that σ isempty (and hence σ→Ω is terminal). As corollaries we
obtain strengthenings of Reynold’s result on the non-existence of
set-theoretic models of polymorphism.
PFL+: A Kernal Scheme for Functions I/O
Gordon, Andrew
University of Cambridge, Computer Laboratory
1989-02
en
Text
UCAM-CL-TR-160
ISSN 1476-2986
In place of the common separation of functional I/O into
continuation and stream based schemes, an alternative division
between Data Driven and Strictness Driven mechanisms for I/O is
proposed. The data driven mechanism determines I/O actions by the
Weak Head Normal Form of programs, while strictness driven I/O is
based on suspensions – I/O actions are triggered when demand arises
for the value of a suspension during normal order reduction. The
data driven and strictness driven I/O mechanisms are exemplified by
the output list and input list, respectively, in Landin’s stream
based I/O scheme.
PFL+ is a functional I/O scheme, able to express arbitary I/O
actions and both data driven and strictness driven constructs in
terms of a small kernel of primitives. PFL+ could be added to any
functional language. It is based on Holmström’s PFL [5], a parallel
functional language with embedded communication and concurrency
operators from CCS. PFL+ adds non-strict communication, behaviours
with results and primitives to make suspensions.
Examples are given of how PFL+ can dervive from these primitives
both stream based I/O and the representation of the file system as a
function.
Papers on Poly/ML
Matthews, D.C.J.
University of Cambridge, Computer Laboratory
1989-02
en
Text
UCAM-CL-TR-161
ISSN 1476-2986
Various papers and research documents have been written while the
Poly/ML project was underway and now that the project is completed
it seemed appropriate to combine them into a single document. Some
of these papers have been published, while others were written
simply to help clarify thoughts on a particular subject.
Although Poly and ML have different syntax and type-systems their
execution models are remarkably similar. A new parser and
type-checker had to be written, but the code-generator and optimiser
could be shared between Poly and ML. The same run-time system was
used. Poly turned out to be an excellent language for the project,
and modules based around Poly abstract types were used extensively.
The lack of low level operations of a systems programming language
was not a problem as, in practice, compilers involve very few such
operations.
The papers in this report have been grouped into sections according
to their likely audience. The first section describes the Poly/ML
system and the extensions for windows and processes. Section two
contains various discussion papers about Poly and ML, although some
of the ideas covered were never implemented. The third section
contains two papers on the persistent storage system and its
implementation. The final section covers the implementation of Poly
and ML and the run-time system.
The Alvey natural language tools grammar (2nd
Release)
Grover, Claire
Briscoe, Ted
Carroll, John
Boguraev, Bran
University of Cambridge, Computer Laboratory
1989-04
en
Text
UCAM-CL-TR-162
ISSN 1476-2986
The ANLT grammar is a wide-coverage syntactic description of English
expressed in a computationally-tractable unification based
formalism. We describe the formalism and give a detailed account of
the analyses adopted for different English syntactic constructions
in the current version of the grammar. Appendices provide a complete
listing of the grammar, sample lexical entries, and a corpus of
parsable sentences. The grammar is fully compatible with the Grammar
Development Environment (Technical Report 127) which provides an
integrated software environment, supporting automated parsing,
generation, and modification of grammars expressed in the formalism
described here.
Inference in a natural language front end for
databases
Copestake, Ann
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1989-02
en
Text
UCAM-CL-TR-163
ISSN 1476-2986
This report describes the implementation and initial testing of
knowledge representation and inference capabilities within a modular
database front end designed for transportability.
A matrix key distribution system
Gong, Li
Wheeler, David J.
University of Cambridge, Computer Laboratory
1988-10
en
Text
UCAM-CL-TR-164
ISSN 1476-2986
A new key distribution scheme is presented. It is based on the
distinctive idea that lets each node have a set of keys of which it
shares a distinct subset with every other node. This has the
advantage that the numbers of keys that must be distributed and
maintained are reduced by a square root factor; moreover, two nodes
can start conversation with virtually no delay. Two versions of the
scheme are given. Their performance and security analysis shows it
is a practical solution to some key distribution problems.
Fast packet switching for integrated services
Newman, Peter
University of Cambridge, Computer Laboratory
1989-03
en
Text
UCAM-CL-TR-165
ISSN 1476-2986
As the communications industry continues to expand two current
trends are becoming apparent: the desire to support an increasing
diversity of communications services (voice, video, image, text,
etc.) and the consequent requirement for increased network capacity
to handle the expected growth in such multi-service traffic. This
dissertation describes the design, performance and implementation of
a high capacity switch which uses fast packet switching to offer the
integrated support of multi-service traffic. Applications for this
switch are considered within the public network, in the emerging
metropolitan area network and within local area networks.
The Cambridge Fast Packet Switch is based upon a non-buffered,
multi-path, switch fabric with packet buffers situated at the input
ports of the switch. This results in a very simple implementation
suitable for construction in current gate array technology. A
simulation study of the throughput at saturation of the switch is
first presented to select the most appropriate switch parameters.
Then follows an investigation of the swith performance for
multi-service traffic. It is shown, for example, that for an
implementation in current CMOS technology, operating at 50 Mhz,
switches with a total traffic capacity of up to 150 Gbits/sec may be
constructed. Furthermore, if the high priority traffic load is
limited on each input port to a maximum of 80% of switch port
saturation, then a maximum delay across the switch of the order of
100 µsecs may be guaranteed, for 99% of the high priority traffic,
regardless of the lower priority traffic load.
An investigation of the implementation of the switch by the
construction of the two fundamental components of the design in 3 µm
HCMOS gate arrays is presented and close agreement is demonstrated
between performance of the hardware implementation and the
simulation model. It is concluded that the most likely area of
application of this design is as a high capacity multi-service local
area network or in the interconnection of such networks.
Evolution of operating system structures
Bacon, Jean
University of Cambridge, Computer Laboratory
1989-03
en
Text
UCAM-CL-TR-166
ISSN 1476-2986
The development of structuring within operating systems is reviewed
and related to the simultaneous evolution of concurrent programming
languages. First traditional, multi-user systems are considered and
their evolution from monolithic closed systems to general domain
structured systems is traced. Hardware support for protected sharing
is emphasised for this type of system.
The technology directed trend towards single user workstations
requires a different emphasis in system design. The requirement for
protection in such systems is less strong than in multi-user systems
and, in a single language system, may to some extent be provided by
software at compile time rather than hardware at run time.
Distributed systems comprising single user workstations and
dedicated server machines are considered and the special
requirements for efficient implementation of servers are discussed.
The concepts of closed but structured and open system designs are
helpful. It is argued that the open approach is most suited to the
requirements of single user and distributed systems. Experiences of
attempting to implement systems over a closed operating system base
are presented.
Progress towards support for heterogeneity in distributed systems,
so that interacting components written in a range of languages may
interwork and may run on a variety of hardware, is presented.
The benefits of taking an object orientated view for system-level as
well as language-level objects and for specification, generation and
design of systems are discussed and work in this area is described.
An outline of formal approaches aimed at specification, verification
and automatic generation of software is given.
Finally, design issues are summarised and conclusions drawn.
A verified compiler for a verified microprocessor
Joyce, Jeffrey J.
University of Cambridge, Computer Laboratory
1989-03
en
Text
UCAM-CL-TR-167
ISSN 1476-2986
Distributed computing with a processor bank
Bacon, J.M.
Leslie, I.M.
Needham, R.M.
University of Cambridge, Computer Laboratory
1989-04
en
Text
UCAM-CL-TR-168
ISSN 1476-2986
The Cambridge Distributed Computing System (CDCS) was designed some
ten years ago and was in everyday use at the Computer Laboratory
until December 1988. An overview of the basic design of CDCS is
given, an outline of its evolution and a description of the
distributed systems research projects that were based on it.
Experience has shown that a design based on a processor bank leads
to a flexible and extensible distributed system.
Filing in a heterogeneous network
Seaborne, Andrew Franklin
University of Cambridge, Computer Laboratory
1989-04
en
Text
UCAM-CL-TR-169
ISSN 1476-2986
Heterogeneity is becoming a common feature in local area networks as
the variety of equipment that is marketed increases. Each such
system will have its own mix of hardware and software but it is
still desirable to be able to bring in new applications and machines
and integrate them with the existing equipment.
Careful design is required to be able to introduce new subsystems
into the network in a manner that reduces the manpower needed. If
binary compatibility for application programs is achieved then new
software can be introduced without the need for source code
alterations. If the design of the computing environment is correctly
constructed then the introduction of new hardware will not require
alterations or cause disruption to the rest of the system.
There is a reduction in the ability to share information through
files being accessible to many people and from many locations when
there are a number of filing systems present in the network. Ideally
a single filing system spanning all operating systems that exist in
the distributed computing environment would give maximum possible
sharing.
Any existing file service will provide a set of facilities for the
construction of a name space by its client or enforce a predefined
naming structure which is not compatible with any other. This thesis
describes a single network filing system that has been constructed
by separating file storage from file naming. By introducing a
directory service to manage the name space, and using file servers
only for file storage, the need for each client to be extended to
take account of every file service is avoided. A single network file
transfer protocol allows the directory service to authenticate each
request and allows for the introduction of new equipment with no
disruption to the existing system.
Ordered rewriting and confluence
Martin, Ursula
Nipkow, Tobias
University of Cambridge, Computer Laboratory
1989-05
en
Text
UCAM-CL-TR-170
ISSN 1476-2986
One of the major problems in term rewriting theory is what to do
with an equation which cannot be ordered into a rule. Many solutions
have been proposed, including the use of special unification
algorithms or of unfailing completion procedures.
If an equation cannot be ordered we can still use any instances of
it which can be ordered for rewriting. Thus for example x ∗ y = y ∗
x cannot be ordered, but if a, b are constants with b ∗ a > a ∗ b we
may rewrite b ∗ a → a ∗ b. This idea is used in unfailing
completion, and also appears in the Boyer-Moore system. In this
paper we define and investigate completeness with respect to this
notion of rewriting and show that many familiar systems are complete
rewriting systems in this sense. This allows us to decide equality
without the use of special unification algorithms. We prove
completeness by proving termination and local confluence. We
describe a confluence test based on recursive properties of the
ordering.
Some types with inclusion properties in ∀, →, μ
Fairbairn, Jon
University of Cambridge, Computer Laboratory
1989-06
en
Text
UCAM-CL-TR-171
ISSN 1476-2986
This paper concerns the ∀, →, μ type system used in the non-strict
functional programming language Ponder. While the type system is
akin to the types of Second Order Lambda-calculus, the absence of
type application makes it possible to construct types with useful
inclusion relationships between them.
To illustrate this, the paper contains definitions of a natural
numbers type with many definable subtypes, and of a record type with
inheritance.
A theoretical framework for computer models of cooperative
dialogue, acknowledging multi-agent conflict
Galliers, Julia Rose
University of Cambridge, Computer Laboratory
1989-07
en
Text
UCAM-CL-TR-172
ISSN 1476-2986
This thesis describes a theoretical framework for modelling
cooperative dialogue. The linguistic theory is a version of speech
act theory adopted from Cohen and Levesque, in which dialogue
utterances are generated and interpreted pragmatically in the
context of a theory of rational interaction. The latter is expressed
as explicitly and formally represented principles of rational
agenthood and cooperative interaction. The focus is the development
of strategic principles of multi-agent interaction as such a basis
for cooperative dialogue. In contrast to the majority of existing
work, these acknowledge the positive role of conflict to multi-agent
cooperation, and make no assumptions regarding the benevolence and
sincerity of agents. The result is a framework wherein agents can
resolve conflicts by negotiation. It is a preliminary stage to the
future building of computer models of cooperative dialogue for both
HCI and DAI, which will therefore be more widely and generally
applicable than those currently in existence.
The theory of conflict and cooperation is expressed in the different
patterns of mental states which characterise multi-agent conflict,
cooperation and indifference as three alternative postural
relations. Agents can recognise and potentially create these.
Dialogue actions are the strategic tools with which mental states
can be manipulated, whilst acknowledging that agents are autonomous
over their metal states; they have control over what they acquire
and reveal in dialogue. Strategic principles of belief and goal
adoption are described in terms of the relationships between
autonomous agents’ beliefs, goals, preference and interests, and the
relation of these to action. Veracity, mendacity, concealing and
revealing are defined as properties of acts. The role of all these
elements in reasoning about dialogue action and conflict resolution,
is tested in analysis of two example dialogues; a record of a real
trade union negotiation and an extract from “Othello” by
Shakespeare.
Programming in temporal logic
Hale, Roger William Stephen
University of Cambridge, Computer Laboratory
1989-07
en
Text
UCAM-CL-TR-173
ISSN 1476-2986
The idea of writing computer programs in logic is an attractive one,
for such programs may be designed, verified, and implemented using a
single formal language. This brings a number of practical benefits:
1. There is no room for ambiguity in the relationship between
specification and implementation, and no need to learn a different
language for each.
2. It is easy to test out specifications from the earliest stages of
development, which avoids attempting to implement or verify an
inapproptiate design.
3. Computerised tools can be applied directly to transform and
verify programs, using the established machinery of mathematical
logic.
4. Logic supports hierarchical design, so a large project can be
divided into smaller tasks which may be designed and verified
independently.
Similar benefits may be bestowed by any formal programming language,
but the idea only works if the language suits the intended
application. All too often the application is forced to fit the
language.
In this dissertation I describe an approach that suits the
development of parallel and real-time systems. The approach is based
on Tempura, a deterministic programming language developed by
Moszkowski from his work on hardware specification using Interval
Temporal Logic (ITL). I present the formal semantics of ITL in
higher-order logic, and show how programs can be transformed and
verified using the HOL theorem prover. Then I show how to represent
a number of familiar programming concepts in ITL. First, I show that
the language of while-programs can be embedded in temporal logic;
and that includes the destructive assignment statement with the
usual inertial assumption. An interesting corollary is that a simple
sequential program, written in Pascal, say, becomes a logic program
in Tempura. More advanced concepts include parallel processes and
message passing, as well as real-time phenomena such as timeouts,
interrupts and traps. Each idea is experimentally tested on a
suitable example, using an interpreter for Tempura. The examples
range from matrix multiplication and parallel sorting, to a
pipelined parser and a real-time lift-controller.
General theory relating to the implementation of concurrent
symbolic computation
Clarke, James Thomas Woodchurch
University of Cambridge, Computer Laboratory
1989-08
en
Text
UCAM-CL-TR-174
ISSN 1476-2986
The central result of this work is the discovery of a new class of
architectures, which I call D-RISC, sharing some characteristics of
both dataflow and Von Neumann RISC computers, for concurrent
computation. This rests on an original and simple theory which
relates the demands of concurrent computation on hardware resources
to the fundamental performance constraints of technology. I show
that dataflow and Von Neumann architectures have different
fundamental hardware constraints to performance, and that therefore
and D-RISC architecture, which balances these two constraints, is
likely to be optimum for concurrent computation.
The work forms four related sections:
A study of the nature of concurrent symbolic computation and the
demands which it makes from any implementation. Two new results
emerge from this. A model of computation which will be used
extensively in subsequent sections, and a way of incorporating
imperative updates in a functional language, similar but superior to
non-deterministic merge, which captures locally sequential updates
in a computation with minimum constraint on global concurrency.
The computational model is udes to contrast different policies for
localising data near a CPU. A new type of cache is proposed which
renames all of its cached addresses in order to reduce CPU
word-length.
CPU design is examined and a new class of architectures for
concurrent computation, called D-RISCs, are proposed.
The multiple-thread implementation problems encountered in the new
architectures are examined. A new analysis of the relationship
between scheduling and intermediate store use in a symbolic
concurrent computation is presented.
A formulation of the simple theory of types (for
Isabelle)
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1989-08
en
Text
UCAM-CL-TR-175
ISSN 1476-2986
Simple type theory is formulated for use with the generic theorem
prover Isabelle. This requires explicit type inference rules. There
are function, product, and subset types, which may be empty.
Descriptions (the eta-operator) introduce the Axiom of Choice.
Higher-order logic is obtained through reflection between formulae
and terms of type bool. Recursive types and functions can be
formally constructed.
Isabelle proof procedures are described. The logic appears suitable
for general mathematics as well as computational problems.
Implementing aggregates in parallel functional
languages
Clarke, T.J.W.
University of Cambridge, Computer Laboratory
1989-08
en
Text
UCAM-CL-TR-176
ISSN 1476-2986
Many constructions which are difficult to write efficiently in pure
functional languages have as underlying semantics an aggregate. An
aggregate is a collection of individual elements whose order does
not matter, it can thus be constructed functionally using a
commutative associative combining operator. Equivalent and more
efficient implementations for aggregates exist which are
operational. A new construction, the A-thread, an aggregate
specified operationally which introduces provably local data
indeterminacy, is defined. Operational specification of an
aggregate, in which each element is specified by a separate function
call, does not necessarily destroy referential transparency in a
functional language. Aggregates defined using joins on partial
orders allow early termination if an operational implementation is
used: Arvind’s ‘I-structures’ and Burton’s ‘improving values’ are
examples of this.
Experimenting with Isabelle in ZF Set Theory
Noel, P.A.J.
University of Cambridge, Computer Laboratory
1989-09
en
Text
UCAM-CL-TR-177
ISSN 1476-2986
The theorem prover Isabelle has been used to axiomatise ZF set
theory with natural deduction and to prove a number of theorems
concerning functions. In particular the axioms and inference rules
of four theories have been derived in the form of theorems of set
theory. The four theories are:
λ_βη, a form of typed lambda calculus with equality,
O_0, a form of simple type theory,
an intuitionistic first order theory with propositions interpreted
as the type of their proofs,
PPλ, the underlying theory of LCF.
Most of the theorems have deen derived using backward proofs, with a
small amount of automation.
Totally verified systems: linking verified software to
verified hardware
Joyce, Jeffrey J.
University of Cambridge, Computer Laboratory
1989-09
en
Text
UCAM-CL-TR-178
ISSN 1476-2986
We describe exploratory efforts to design and verify a compiler for
a formally verified microprocessor as one aspect of the eventual
goal of building totally verified systems. Together with a formal
proof of correctness for the microprocessor this yields a precise
and rigorously established link between the semantics of the source
language and the execution of compiled code by the fabricated
microchip. We describe in particular: (1) how the limitations of
real hardware influenced this proof; and (2) how the general
framework provided by higher order logic was used to formalize the
compiler correctness problem for a hierarchically structured
language.
Automating Squiggol
Martin, Ursula
Nipkow, Tobias
University of Cambridge, Computer Laboratory
1989-09
en
Text
UCAM-CL-TR-179
ISSN 1476-2986
The Squiggol style of program development is shown to be readily
automated using LP, an equational reasoning theorem prover.
Higher-order functions are handled by currying and the introduction
of an application operator. We present an automated version of
Bird’s development of the maximum segment sum algorithm, and a
similar treatment of a proof of the binomial theorem.
Formal verification of data type refinement: Theory and
practice
Nipkow, Tobias
University of Cambridge, Computer Laboratory
1989-09
en
Text
UCAM-CL-TR-180
ISSN 1476-2986
This paper develops two theories of data abstraction and refinement:
one for applicative types, as they are found in functional
programming languages, and one for state-based types found in
imperative languages. The former are modelled by algebraic
structures, the latter by automata. The automaton theoretic model
covers not just data types but distributed systems in general.
Within each theory two examples of data refinement are presented and
formally verified with the theorem prover Isabelle. The examples are
an abstract specification and two implementations of a memory
system, and a mutual exclusion algorithm.
Proof transformations for equational theories
Nipkow, Tobias
University of Cambridge, Computer Laboratory
1989-09
en
Text
UCAM-CL-TR-181
ISSN 1476-2986
This paper contrasts two kinds of proof systems for equational
theories: the standard ones obtained by combining the axioms with
the laws of equational logic, and alternative systems designed to
yield decision procedures for equational problems.
Two new matching alogorithms for (among other theories)
associativity, associativity + commutativity, and associativity +
commutativity + identity are presented, the emphasis is not so much
on individual theories but on the general method of proof
transformation as a tool for showing the equivalence of different
proof systems.
After studying proof translations defined by rewriting systems,
equivalence tests based on the notion of resolvant theories are used
to derive new matching and in some cases unification procedures for
a number of equational theories. Finally the combination of
resolvant systems is investigated.
The theory and implementation of a bidirectional question
answering system
Levine, John M.
Fedder, Lee
University of Cambridge, Computer Laboratory
1989-10
en
Text
UCAM-CL-TR-182
ISSN 1476-2986
This paper describes a question answering system which is a limited
instance of the general bidirectional architecture suggested by
Appelt (1987), The novel features of our approach include the use of
a linguistically well-motivated set of functional features; a
bidirectional grammar which encodes these features directly; a
question answering program which uses the thematic organisation of
the user’s input to construct a cooperative reply; and a tactical
generation component which can be used with Montague semantics.
The specification and verification of sliding window
protocols in higher order logic
Cardell-Oliver, Rachel
University of Cambridge, Computer Laboratory
1989-10
en
Text
UCAM-CL-TR-183
ISSN 1476-2986
This report describes the formal specification and verification of a
class of sliding window protocols using higher order logic. It is
proved that a model for implementations of the protocol locically
implies safety and liveness invariants, and that these invariants in
turm imply an abstract specification of the protocol. The
specification and implementation models used in the proof are based
on techniques developed for hardware verification in HOL at
Cambridge. This model and proof will eventually be used as the basis
for a more general sliding window protocol model.
Site interconnection and the exchange
architecture
Tennenhouse, David Lawrence
University of Cambridge, Computer Laboratory
1989-10
en
Text
UCAM-CL-TR-184
ISSN 1476-2986
The users of a site’s telecommunication facilities rely on a
colection of devices, transducers and computers, to provide the
primary communications interface. In the traditional approach to
site interconnection, some of these devices are directly attached to
specialised carrier networks. The remaining devices are attached to
local networks that are tailored to support communication amongst
compatible devices at remote sites. This arrangement does not reap
the full benefits of network and service integration: each local
network has its own common carrier interfaces; and there is no
provision for device independent processing, storage, and forwarding
elements.
This dissertation describes a layered approach to site
interconnection. Communication between peer sites is supported by
the lower layer carrier networks, and associations between upper
layer clients are supported by the local network layer. The site
interconnection layer, inserted between the lacal and carrier
layers, facilitates communication between peer local networks. This
layer is composed of independent subsystems that offer the site
interconnection service (SI-service) to their upper layer clients.
Each SI-subsystem is a funnel through which various device-dependent
symbol sets are encoded into a common digital format. The symbol
streams of concurrent upper layer associations are multiplexed
together for presentation at the shared carrier interfaces. Service
integration is achieved through the encoding of many different
styles of communication (vioce, video, facsimilie, tile transfer,
etc.) into a common symbol set.
The first part of this dissertation develops the connected argument
sharing this layered architecture. The second part describes the
experimental development and analysis of the exchange architecture,
which provides an SI-service that supports Asynchronous Transfer
Mode (ATM) communication. The ATM approach is characterized by the
use of small packets in conjunction with switch fabrics that provide
comprable performance to circuit switching, and permit much greater
variability in traffic patterns. The switch fabric of the pilot
implementation is based on the Cambridge Fast Ring: the CFR packet
structure is the basis of the ATM encoding; and the VLSI ring
technology has been used to construct the individual SI-subsystems.
In this application, the CFR provides ATM-based switching and
multiplexing facilities.
This work is distinguished by its emphasis on site independence and
universal access to telecommunication services. The principal
contributions to the thesis relate to site interconnection; ATM
encodings; out-of-band and non-invasive network management;
particular analysis methodologies; and the design of multi-service
networks.
Logics of domains
Zhang, Guo Qiang
University of Cambridge, Computer Laboratory
1989-12
en
Text
UCAM-CL-TR-185
ISSN 1476-2986
This dissertation studies the logical aspects of domains as used in
the denotational semantics of programming languages. Frameworks of
domain logics are introduced which serve as basic tools for the
systematic derivation of proof systems from the denotational
semantics of programming languages. The proof systems so derived are
guaranteed to agree with the denotational semantics in the sense
that the denotation of any program coincides with the set of
assertions true of it.
The study focuses on two frameworks for denotational semantics: the
SFP domains, and the less standard, but important, category of
dI-domains with stable functions.
An extended form of Scott’s information systems are introduced to
represent SFP objects. They provide better understanding of the
structure of finite elements and open sets of domains. These systems
generalise to a logic of SFP which uses inequational formulae to
axiomatise entailment and non-entailment of open-set assertions.
Soundness, completeness, and expressiveness results of the logic are
obtained, and possible applications are investigated. A μ-calculus
of Scott domains is introduced to extend the expressive power of the
assertion language.
Special kinds of open sets called stable neighbourhoods are
introduced and shown to determine stable functions in a similar
sense to that in which Scott-open sets determine continuous
functions. Properties and constructions of the stable neighbourhoods
on various categories of dI-domains are investigated. Logical
frameworks for Girard’s coherent spaces and Berry’s dI-domains are
given in which assertions are interpreted as stable neighbourhoods.
Various soundness, completeness, and expressiveness results are
provided.
Protocol design for high speed networks
McAuley, Derek Robert
University of Cambridge, Computer Laboratory
1990-01
en
Text
UCAM-CL-TR-186
ISSN 1476-2986
Improvements in fibre optic communication and in VLSI for network
switching components have led to the consideration of building
digital switched networks capable of providing point to point
communication in the gigabit per second range. Provision of
bandwidths of this magnitude allows the consideration of a whole new
range of telecommunications services, integrating video, voice,
image and text. These multi-service networks have a range of
requirements not met by traditional network architectures designed
for digital telephony or computer applications. This dissertation
describes the design, and an implementation, of the Multi-Service
Network architecture and protocol family, which is aimed at
supporting these services.
Asynchronous transfer mode networks provide the basic support
required for these integrated services, and the Multi-Service
Network architecture is designed primarily for these types of
networks. The aim of the Multi-Service protocol family is to provide
a complete architecture which allows use of the full facilities of
asynchronous transfer mode networks by multi-media applications. To
maintain comparable performance with the underlying media, certain
elements of the MSN protocol stack are designed with implementation
in hardware in mind. The interconnection of heterogeneous networks,
and networks belonging to different security and administrative
domains, is considered vital, so the MSN architecture takes an
internetworking approach.
Natural language interfaces to databases
Copestake, Ann
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1989-09
en
Text
UCAM-CL-TR-187
ISSN 1476-2986
This paper reviews the state of the art in natural language access
to databases. This has been a long-standing area of work in natural
language processing. But though some commercial systems are now
available, providing front ends has proved much harder than was
expected, and the necessary limitations on front ends have to be
recognised. The paper discusses the issues, both general to language
and task-specific, involved in front end design, and the way these
have been addressed, concentrating on the work of the last decade.
The focus is on the central process of translating a natural
language question into a database query, but other supporting
functions are also covered. The points are illustrated by the use of
a single example application. The paper concludes with an evaluation
of the current state, indicating that future progress will depend on
the one hand on general advances in natural language processing, and
on the other on expanding the capabilities of traditional databases.
Specification of computer architectures: a survey and
annotated bibliography
Leonard, Timothy E.
University of Cambridge, Computer Laboratory
1990-01
en
Text
UCAM-CL-TR-188
ISSN 1476-2986
I first define computer architecture and architecture specification,
explain how the conflict between clarity and ambiguity makes writing
specifications difficult, and introduce and consider the advantages
and problems of formal specifications. I then survey all the
literature on architecture specification, and introduce the
literature on technical writing and on formal specification in
general. I close with an annotated bibliography.
Isabelle tutorial and user’s manual
Paulson, Lawrence C.
Nipkow, Tobias
University of Cambridge, Computer Laboratory
1990-01
en
Text
UCAM-CL-TR-189
ISSN 1476-2986
This (obsolete!) manual describes how to use the theorem prover
Isabelle. For beginners, it explains how to perform simple
single-step proofs in the built-in logics. These include first-order
logic, a classical sequent calculus, ZF set theory, Constructie Type
Theory, and higher-order logic. Each of these logics is described.
The manual then explains how to develop advanced tactics and
tacticals and how to derive rules. Finally, it describes how to
define new logics within Isabelle.
Some notes on mass terms and plurals
Copestake, Ann
University of Cambridge, Computer Laboratory
1990-01
en
Text
UCAM-CL-TR-190
ISSN 1476-2986
This report describes a short investigation into some possible
treatments of mass nouns and plurals. It aims to provide a grammar
and axiomatisation with a reasonable coverage of these phenomena, so
that a range of sentences can be parsed, and inferences made
automatically.
The previous work on the subject, mainly due to Hasle (1988) is
reviewed, and the limitations of both the original theories and
Hasle’s implementation are demonstrated. Some more recent work,
especially that relevant to Link’s theory, is also discussed.
The present grammar and axiomatisation is described. Although it is
not the implementation of any particular theory, it draws on the
work of Link, Krifka and Roberts. Some of the problems with the
present approach are discussed, although possible solutions would
need to be considered in a wider context. The aim is to show what
types of phenomena can be treated by a relatively simple approach.
The implemented grammar covers everything that was treated by
Hasle’s implementation, and extends that coverage in a varietry of
ways, while providing a better integration of the treatment of mass
nouns and plurals than the earlier work. It was written in the CFG+
formalism, and some parts of the axiomatisation have been tested
using the HOL system.
An architecture for real-time multimedia communications
systems
Nicolaou, Cosmos
University of Cambridge, Computer Laboratory
1990-02
en
Text
UCAM-CL-TR-191
ISSN 1476-2986
An architecture for real-time multimedia communications systems is
presented. A multimedia communication systems includes both the
communication protocols used to transport the real-time data and
also the Distributed Computing system (DCS) within which any
applications using these protocols must execute. The architecture
presented attempts to integrate these protocols with the DCS in a
smooth fashion in order to ease the writing of multimedia
applications. Two issues are identified as being essential to the
success of this integration: namely the synchronisation of related
real-time data streams, and the management of heterogeneous
multimedia hardware. The synchronisation problem is tackled by
defining explicit synchronisation properties at the presentation
level and by providing control and synchronisation operations within
the DCS which operate in terms of these properties. The
heterogeneity problems are addressed by separating the data
transport semantics (protocols themselves) from the control
semantics (protocol interfaces). The control semantics are
implemented using a distributed, typed interface, scheme within the
DCS (i.e. above the presentation layer), whilst the protocols
themselves are implemented within the communication subsystem. The
interface between the DCS and communications subsystem is referred
to as the orchestration interface and can be considered to lie in
the presentation and session layers.
A conforming prototype implementation is currently under
construction.
Designing a theorem prover
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1990-05
en
Text
UCAM-CL-TR-192
ISSN 1476-2986
The methods and principles of theorem prover design are presented
through an extended example. Starting with a sequent calculus for
first-order logic, an automatic prover (called Folderol) is
developed. Folderol can prove quite a few complicated theorems,
although its search strategy is crude and limited. Folderol is coded
in Standard ML and consists largely of pure functions. Its complete
listing is included.
The report concludes with a survey of other research in theorem
proving: the Boyer/Moore theorem prover, Automath, LCF, and
Isabelle.
Belief revision and a theory of communication
Galliers, Julia Rose
University of Cambridge, Computer Laboratory
1990-05
en
Text
UCAM-CL-TR-193
ISSN 1476-2986
This report concerns choices about changing belief. It describes
research to establish and model a principled theoretical basis by
which rational agents autonomously choose whether, as well as how to
revise their beliefs. Aspects of the various problems in belief
revision are discussed, and solved in the context of an AI tool for
reason maintenance extended to cover situations of new evidence as
not assumed ‘truth’. Primarily this results from the inclusion of a
non numeric theory of strength of belief, which relates strength to
persistence in the context of challenge. Such autonomous belief
revision is presented as the basis of a theory of communication, as
a special case of reasoning about change in an uncertain world with
incomplete information, comprising others similarly constrained.
Proceedings of the First Belief Representation and Agent
Architectures Workshop
Galliers, Julia Rose
University of Cambridge, Computer Laboratory
1990-03
en
Text
UCAM-CL-TR-194
ISSN 1476-2986
The first Belief Representation and Agent Architectures workshop was
organised by Cambridge University Computer Laboratory, and held at
SRI International in Cambridge on the 22nd and 23rd March 1990. It
was designed as a closed meeting of 15 researchers, all currently
working in and familiar with this subfield of AI. The purpose of the
meeting was not so much to present completed work, as to exchange
ideas and explore issues with others equally as aware of the
relevant problems and background. Each presenter was given 90
minutes in which to lead a discussion on a topic related to their
research interests. Generally these were oriented around the
presenter’s current research projects, outlines of which had been
distributed prior to the meeting.
These proceedings comprise eight sections, each including the
discussion report followed by copies of the presenter’s overheads,
followed by the summaries of the presenter’s and rapporteur’s
current research projects. The sections are as follows: General
introduction, different styles of agent architectures, a minimalist
approach to agent architectures, models of belief revision, the
value of formal approaches, knowledge action chance and utility,
different value systems, and channels for dialogue.
Multi-level verification of microprocessor-based
systems
Joyce, Jeffrey J.
University of Cambridge, Computer Laboratory
1990-05
en
Text
UCAM-CL-TR-195
ISSN 1476-2986
The idea of using formal logic to reason about small fragments or
single layers of a software/hardware system is well-established in
computer science and computer engineering. Recently, formal logic
has been used to establish correctness properties for several
realistic systems including a commercially-available microprocessor
designed by the British Ministry of Defence for life-critical
applications. A challenging area of new research is to verify a
complete system by linking correctness results for multiple layers
of software and hardware into a chain of logical dependencies.
This dissertation focuses specifically on the use of formal proof
and mechanical proof-generation techniques to verify
microprocessor-based systems. We have designed and verified a
complete system consisting of a simple compiler for a hierarchically
structured programming language and a simple microprocessor which
executes code generated by this compiler. The main emphasis of our
discussion is on the formal verification of the microprocessor. The
formal verification of the compiler is described in a separate paper
included as an appendix to this dissertation.
Combining correctness results for the compiler with correctness
results for the microprocessor yields a precise and rigorously
established link between the formal semantics of the programming
language and the execution of compiled code by a model of the
hardware. The formal proof also links the hardware model to the
behavioural specification of an asynchronous memory interface based
on a four-phase handshaking protocol.
The main ideas of this research are (1) the use of generic
specification to filter out non-essential detail, (2) embedding
natural notations from special-purpose formalisations such as
temporal logic and denotational description, and (3) the use of
higher-order logic as a single unifying framework for reasoning
about complete systems.
Generic specification, in addition to supporting fundamental
principles of modularity, abstraction and reliable re-usability,
provides a mechanism for enforcing a sharp distinction between what
has and what has not been formally considered in a proof of
corectness. Furthermore, it is possible to create generic
specifications in a pure formalism with the expressive power of
higher-order logic without inventing new constructs.
Natural notations from special-purpose formalisms offer the
advantage of concise and meaningful specifications when applied to
particular areas of formal description. Semantic gaps between
different notations are avoided by embedding them in a single logic.
Special-purpose rules based on these notations can be derived as
theorems with the aim of implementing more efficient proof
strategies.
Finally it is argued that the primary purpose of using mechanical
proof generation techniques to reason about software and hardware is
to support the intelligent participation of a human verifier in the
rigorous analysis of a design at a level which supports clear
thinking.
The semantics of VHDL with Val and Hol: towards practical
verification tools
Van Tassell, John Peter
University of Cambridge, Computer Laboratory
1990-06
en
Text
UCAM-CL-TR-196
ISSN 1476-2986
The VHSIC Hardware Description Language (VHDL) is an emerging
standard for the design of Application Specific Integrated Circuits.
We examine the semantics of the language in the context of the VHDL
Annotation Language (VAL) and the Higher Order Logic (HOL) system
with the purpose of proposing methods by which VHDL designs may be
converted into these two forms for further validation and
verification. A translation program that utilizes these methods is
described, and several comprehensive VHDL design examples are shown.
The semantics and implementation of aggregates : or : how to
express concurrency without destroying determinism
Clarke, Thomas
University of Cambridge, Computer Laboratory
1990-07
en
Text
UCAM-CL-TR-197
ISSN 1476-2986
This paper investigates the relationship between declarative
semantics and concurrent computation. A fundamental programming
construction, the aggregate, is identified. Aggregates have a simple
declarative semantics, yet cannot be written in pure functional
languages. The addition of aggregates to a functional language
increases expressiveness without destroying determinism or
referential transparency. Specific aggregates can be used to
implememnt concurrent graph marking, time deterministic merge of
lazy lists, and write once locations.
Evaluation Logic
Pitts, Andrew M.
University of Cambridge, Computer Laboratory
1990-08
en
Text
UCAM-CL-TR-198
ISSN 1476-2986
A new typed, higher-order logic is described which appears
particularly well fitted to reasoning about forms of computation
whose operational behaviour can be specified using the Natural
Semantics style of structural operational semantics. The logic’s
underlying type system is Moggi’s computational metalanguage, which
enforces a distinction between computations and values via the
categorical structure of a strong monad. This is extended to a
(constructive) predicate logic with modal formulas about evaluation
of computations to values, called evaluation modalities. The
categorical structure corresponding to this kind of logic is
explained and a couple of examples of categorical models given.
As a first example of the naturalness and applicability of this new
logic to program semantics, we investigate the translation of a
(tiny) fragment of Standard ML into a theory over the logic, which
is proved computationally adequate for ML’s Natural Semantics.
Whilst it is tiny, the ML fragment does however contain both
higher-order functional and imperative features, about which the
logic allows us to reason without having to mention global states
explicitly.
The HOL verification of ELLA designs
Boulton, Richard
Gordon, Mike
Herbert, John
Van Tassel, John
University of Cambridge, Computer Laboratory
1990-08
en
Text
UCAM-CL-TR-199
ISSN 1476-2986
HOL is a public domain system for generating proofs in higher order
predicate calculus. It has been in experimental and commercial use
in several countries for a number of years.
ELLA is a hardware design language developed at the Royal Signals
and Radar Establishment (RSRE) and marketed by Computer General
Electronic Design. It supports simulation models at a variety of
different abstraction levels.
A preliminary methodology for reasoning about ELLA designs using HOL
is described. Our approach is to semantically embed a subset of the
ELLA language in higher order logic, and then to make this embedding
convenient to use with parsers and pretty-printers. There are a
number of semantic issues that may affect the ease of verification.
We discuss some of these briefly. We also give a simple example to
illustrate the methodology.
Type classes and overloading resolution via order-sorted
unification
Nipkow, Tobias
Snelting, Gregor
University of Cambridge, Computer Laboratory
1990-08
en
Text
UCAM-CL-TR-200
ISSN 1476-2986
We present a type inference algorithm for a haskell-like language
based on order-sorted unification. The language features
polymorphism, overloading, type classes and multiple inheritance.
Class and instance declarations give rise to an order-sorted algebra
of types. Type inference esentially reduces to the Hindley/Milner
algorithm where unification takes place in this order-sorted algebra
of types. The theory of order-sorted unification provides simple
sufficient conditions which ensure the existence of principal types.
The semantics of the language is given by a translation into
ordinary λ-calculus. We prove the correctness of our type inference
algorithm with respect to this semantics.
Formalizing abstraction mechanisms for hardware verification
in higher order logic
Melham, Thomas Frederick
University of Cambridge, Computer Laboratory
1990-08
en
Text
UCAM-CL-TR-201
ISSN 1476-2986
Recent advances in microelectronics have given designers of digital
hardware the potential to build devices of remarkable size and
complexity. Along with this however, it becomes increasingly
difficult to ensure that such systems are free from design errors,
where complete simulation of even moderately sized circuits is
impossible. One solution to these problems is that of hardware
verification, where the functional behaviour of the hardware is
described mathematically and formal proof is used to show that the
design meets rigorous specifications of the intended operation.
This dissertation therefore seeks to develop this, showing how
reasoning about the correctness of hardware using formal proof can
be achieved using fundamental abstraction mechanisms to relate
specifications of hardware at different levels. Therefore a
systematic method is described for defining any instance of a wide
class of concrete data types in higher order logic. This process has
been automated in the HOL theorem prover, and provides a firm
logical basis for representing data in formal specifications.
Further, these abstractions have been developed into a new technique
for modelling the behaviour of entire classes of hardware designs.
This is based on a formal representation in logic for the structure
of circuit designs using the recursive types defined by the above
method. Two detailed examples are presented showing how this work
can be applied in practice.
Finally, some techniques for temporal abstraction are explained, and
the means for asserting the correctness of a model containing
time-dependent behaviour is described. This work is then illustrated
using a case study; the formal verification on HOL of a simple ring
communication network.
[Abstract by Nicholas Cutler (librarian), as none was submitted with
the report.]
Three-dimensional integrated circuit layout
Harter, Andrew Charles
University of Cambridge, Computer Laboratory
1990-08
en
Text
UCAM-CL-TR-202
ISSN 1476-2986
Some recent developments in semiconductor process technology have
made possible the construction of three-dimensional integrated
circuits. Unlike other technological developments in two dimensional
integration, these circuits present a new and inherently richer
connection topology. This offers potential for improved layout in
terms of increased density and reduced interconnect length. These
circuits will be difficult and expensive to manufacture, at least in
the short term, and the scale of the improvement in layout is not
apparent. This dissertation presents a discussion of layout and
design for three-dimensional integrated circuits.
A number of materials and techniques can be used in the manufacture
of such circuits. This choice has a profound bearing on the topology
of circuit layout. A classification relating process technolgy to
layout topology is developed and illustrated with the design of a
number of circuits. A layout system is presented as the vehicle for
a series of experiments in three-dimensional layout. It is shown
that the system can be constrained to perform circuit layout in a
number of topologies in the classification.
Finally, some attempt to quantify the benefits of three-dimensional
layout is made. The layout model is calibrated by designing examples
of basic circuit elements. This is done using a set of design rules
corresponding to a proposed three-dimensional process technology.
Circuit layouts produced by the system are compared with
conventional two-dimensional layouts, and the variation in layout
quality as a function of the three-dimensionality of a layout is
explored.
Subtyping in Ponder (preliminary report)
de Paiva, Valeria C.V.
University of Cambridge, Computer Laboratory
1990-08
en
Text
UCAM-CL-TR-203
ISSN 1476-2986
This note starts the formal study of the type system of the
functional language Ponder. Some of the problems of proving
soundness and completeness are discussed and some preliminary
results, about fragments of the type system, shown.
It consists of 6 sections. In section 1 we review briefly Ponder’s
syntax and describe its typing system. In section 2 we consider a
very restricted fragment of the language for which we can prove
soundness of the type inference mechanism, but not completeness.
Section 3 describes possible models of this fragment and some
related work. Section 4 describes the type-inference algorithm for a
larger fragment of Ponder and in section 5 we come up against some
problematic examples. Section 6 is a summary of further work.
New foundations for fixpoint computations:
FIX-hyperdoctrines and the FIX-logic
Crole, Roy L.
Pitts, Andrew M.
University of Cambridge, Computer Laboratory
1990-08
en
Text
UCAM-CL-TR-204
ISSN 1476-2986
This paper introduces a new higher-order typed constructive
predicate logic for fixpoint computations, which exploits the
categorical semantics of computations introduced by Moggi and
contains a strong version of Martin Löf’s ‘iteration type’. The type
system enforces a separation of computations from values. The logic
contains a novel form of fixpoint induction and can express partial
and total correctness statements about evaluation of computations to
values. The constructive nature of the logic is witnessed by strong
metalogical properties which are proved using a category-theoretic
version of the ‘logical relations’ method.
Logic programming, functional programming and inductive
definitions
Paulson, Lawrence C.
Smith, Andrew W.
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-205
ISSN 1476-2986
This paper reports an attempt to combine logic and functional
programming. It also questions the traditional view that logic
programming is a form of first-order logic, arguing instead that the
essential nature of a logic program is an inductive definition. This
revised view of logic programming suggests the design of a combined
logic/functional language. A slow but working prototype is
described.
Formal verification of real-time protocols using higher
order logic
Cardell-Oliver, Rachel
University of Cambridge, Computer Laboratory
1990-08
en
Text
UCAM-CL-TR-206
ISSN 1476-2986
A protocol is a distributed program which controls communication
between machines in a computer network. Two or more programs are
executed on different computers which communicate only via the
medium connecting them.Protocol implementations are difficult to
understand and write correctly because the interaction between
programs and their non-deterministic, real-time environment is
complex. For this reason protocols are often specified using an
abstract model. However few abstract specification techniques model
the problems which occur in real implementations. In particular, the
correctness of many protocols depends on real-time issues such as
the correct setting of timers and fast responses to incoming
messages.
This paper presents techniques for modelling real-time protocols at
different levels of abstraction, from implementation behaviour to
abstract requirements specifications. The language used for these
models is higher order logic. The techniques are illustrated by the
specification and verification of the class of sliding window
protocols. The HOL system, a machine implementation of higher order
logic [2], as used to both specify and verify this example and a
full listing of the HOL theories for sliding window protocols is
given in Appendix B.
Video replay in computer animation
Hawkins, Stuart Philip
University of Cambridge, Computer Laboratory
1990-10
en
Text
UCAM-CL-TR-207
ISSN 1476-2986
This dissertation presents a design for an animation system that
supports video-rate replay of frame sequences within a frame buffer
based graphics architecture.
In recent years framebuffer architectures have become dominant,
largely displacing other forms of graphics display system. But a
framebuffer representation is not well suited to the support of
animation. In particular, two main problems are faced: (1) the
generation of each new frame within a single frame time (typically
40ms); and (2) the updating of the framebuffer with the new frame
representation, also within one frame time. Both of these problems
stem from the fact that the large amount of data required to
represent each frame has to be processed within a strictly limited
time. The difficulty with updating the frame buffer representation
has been largely addressed by the development of powerful new
display processor architectures, made possible by developments in
semiconductor technology. The generation of frames at replay rates,
however, represents a much greater challenge and there are numerous
situations for which real time animation is simply inpracticable. In
such cases an alternative approach is that of frame-by-frame
animation in which the frame sequence is pre-calculated off-line and
stored for later replay at the correct speed. This technique is
commonly referred to as real-time playback.
In this dissertation the requirements of real-time playback are
discussed and a number of distinct approaches to the design of such
systems identified. For each approach examples of previous real-time
playback systems are examined and their individual shortcomings
noted. In light of these observations the design of a new
hardware-based animation system is proposed and its implementation
described. In this system frames are stored digitally and image
compression is used to address the non-video-rate transfer rate and
storage capacity limitations of the frame storage device employed
(an unmodified 5 1/4 inch magnetic disc drive). Such an approach has
previously received little attention. Frame sequences are stored on
the disc in a compressed form and during replay are decompressed in
real-time using a hardware implementation of the coding algorithm. A
variety of image compression strategies are supported within a
generalised coding framework. This introduces operational
flexibility by allowing the system to be tailored according to the
needs of a particular application.
Categorical combinators for the calculus of
constructions
Ritter, Eike
University of Cambridge, Computer Laboratory
1990-10
en
Text
UCAM-CL-TR-208
ISSN 1476-2986
This report describes the derivation of a small and intuitive set of
categorical combinators for the Calculus of Constructions. The
choice of an appropriate categorical semantics is the crucial step.
A modification of Ehrhard’s higher-order closed summable fibrations,
yielding so called CC-categories, turns out to be the appropriate
underlying categorical structure. Standard techniques can then be
used to derive the combinators. The combinators can be turned
directly into the classifying category for the Calculus of
Constructions. This establishes a precise connection between the
calculus, the combinators and the CC-categories.
Efficient memory-based learning for robot control
Moore, Andrew William
University of Cambridge, Computer Laboratory
1990-11
en
Text
UCAM-CL-TR-209
ISSN 1476-2986
This dissertation is about the application of machine learning to
robot control. A system which has no initial model of the
robot/world dynamics should be able to construct such a model using
data received through its sensors—an approach which is formalized
here as the SAB (State-Action-Behaviour) control cycle. A method of
learning is presented in which all the experiences in the lifetime
of the robot are explicitly remembered. The experiences are stored
in a manner which permits fast recall of the closest previous
experience to any new situation, thus permitting very quick
predictions of the effects of proposed actions and, given a goal
behaviour, permitting fast generation of a candidate action. The
learning can take place in high-dimensional non-linear control
spaces with real-valued ranges of variables. Furthermore, the method
avoids a number of shortcomings of earlier learning methods in which
the controller can become trapped in inadequate performance which
does not improve. Also considered is how the system is made
resistant to noisy inputs and how it adapts to environmental
changes. A well founded mechanism for choosing actions is introduced
which solves the experiment/perform dilemma for this domain with
adequate computational efficiency, and with fast convergence to the
goal behaviour. The dissertation explains in detail how the SAB
control cycle can be integrated into both low and high complexity
tasks. The methods and algorithms are evaluated with numerous
experiments using both real and simulated robot domains. The final
experiment also illustrates how a compound learning task can be
structured into a hierarchy of simple learning tasks.
Higher-order unification, polymorphism, and
subsorts
Nipkow, Tobias
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-210
ISSN 1476-2986
This paper analyses the problems that arise in extending Huet’s
higher-order unification algorithm from the simply typed λ-calculus
to one with type variables. A simple, incomplete, but in practice
very useful extension to Huet’s algorithm is discussed. This
extension takes an abstract view of types. As a particular instance
we explore a type system with ML-style polymorphism enriched with a
notion of sorts. Sorts are partially ordered and classify types,
thus giving rise to an order-sorted algebra of types. Type classes
in the functional language Haskell can be understood as sorts in
this sense. Sufficient conditions on the sort structure to ensure
the existence of principal types are discussed. Finally we suggest a
new type system for the λ-calculus which may pave the way to a
complete unification algorithm for polymorphic terms.
The role of artificial intelligence in information
retrieval
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1990-11
en
Text
UCAM-CL-TR-211
ISSN 1476-2986
This paper reviews four potential roles for artificial intelligence
in information retrieval, evaluating AI from a realistic point of
view and within a wide information management context. The
conclusion is that AI has limited potential, not just because AI is
itself insufficiently developed, but because many information
management tasks are properly shallow information processing ones.
There is nevertheless an important place for specific applications
of AI or AI-derived technology when particular constraints can be
placed on the information management tasks involved.
A distributed and-or parallel Prolog network
Wrench, K.L.
University of Cambridge, Computer Laboratory
1990-12
en
Text
UCAM-CL-TR-212
ISSN 1476-2986
A model is proposed for the parallel execution of Prolog, exploiting
both dependent and- and full or-parallelism. The model is
implemented on a distributed network of loosely-coupled processors
and has no need of shared memory nor multiprocessor hardware.
Known as APPNet, the model makes use of oracles to partition the
search space dynamically, thereby enabling processing elements to be
allocated a unique portion of the computation. No communication
takes place between processing elements. In executing problems that
do not exhibit any and-parallelism, all solutions found represent
final answers to the query. When an and-parallel problem is
executed, the solutions generated are only partial solutions. The
sets of partial solution are then joined to produce consistent final
solutions. Back-unification is the process whereby partial solutions
are unified according to a template derived from the program.
Prolog source programs need not be modified by the user. Static
analysis is, however, carried out automatically on all programs by a
preprocessor before their execution in the APPNet to ensure that
clauses are not distributed before it is feasible to do so.
Side-effecting constructs are identified and the appropriate
restrictions are placed on the parallel execution strategy.
The Dialectica categories
de Paiva, Valeria Correa Vaz
University of Cambridge, Computer Laboratory
1991-01
en
Text
UCAM-CL-TR-213
ISSN 1476-2986
This work consists of two main parts. The first one, which gives it
its name, presents an internal categorical version of Gödel’s
“Dialectica interpretation” of higher-order arithmetic. The idea is
to analyse the Dialectica interpretation using a cetegory DC where
objects are relations on objects of a basic category C and maps are
pairs of maps of C satisfying a pullback condition. If C is finitely
complete, DC exists and has a very natural symmetric monoidal
structure. If C is locally cartesian closed then DC is symmetric
monoidal closed. If we assume C with stable and disjoint coproducts,
DC has cartesian products and weak-coproducts and satisfies a weak
form of distributivity. Using the structure above, DC is a
categorical model for intuitionistic linear logic.
Moreover if C has free monoids then DC has cofree comonoids and the
corresponding comonad “!” on DC, which has some special properties,
can be used to model the exponential “of course!” in Intuitionistic
Linear Logic. The category of “!”-coalgebras is isomorphic to the
category of comonoids in DC and, if we assume commutative monoids in
C, the “!”-Kleisli category, which is cartesian closed, corresponds
to the Diller-Nahm variant of the Dialectica interpretation.
The second part introduces the categories GC. The objects of GC are
the same objects of DC, but morphisms are easier to handle, since
they are maps in C in opposite directions. If C is finitely
complete, the category GC exists. If C is cartesian closed, we can
define a symmetric monoidal structure and if C is locally cartesian
closed as well, we can define inernal homs in GC that make it a
symmetric monoidal closed category. Supposing C with stable and
disjoint coproducts, we can define cartesian products and coproducts
in GC and, more interesting, we can define a dual operation to the
tensor product bifunctor, called “par”. The operation “par” is a
bifunctor and has a unit “⊥”, which is a dualising object. Using the
internal hom and ⊥ we define a contravariant functor “(−)⊥” which
behaves like negation and thus it is used to model linear negation.
We show that the category GC, with all the structure above, is a
categorical model for Linear Logic, but not exactly the classical
one.
In the last chapter a comonad and a monad are defined to model the
exponentials “!” and “?”. To define these endofunctors, we use
Beck’s distributive laws in an interesting way. Finally, we show
that the Kleisli category GC! is cartesian closed and that the
categories DC and GC are related by a Kleisli construction.
Integrating knowledge of purpose and knowledge of structure
for design evaluation
Bradshaw, J.A.
Young, R.M.
University of Cambridge, Computer Laboratory
1991-02
en
Text
UCAM-CL-TR-214
ISSN 1476-2986
This paper describes a knowledge representation strategy, for
mechanical devices, which combines Knowledge of Structure and
Knowledge of Purpose. Knowledge of Purpose specifies how devices are
expected to behave and Knowledge of Structure details how devices
are connected. Knowing ‘correct’ behaviour (Knowledge of Purpose) it
is possible to usefully comment on any generated behaviour,
predicted or actual. Generation of behaviour is a bottom up process
(from components to systems) whereas behaviour evaluation is top
down (from systems to components). Common purpose is used to group
devices into systems.
The core evaluation activity is the generation of an envisionment
graph (similar to that described by deKleer and Brown [deK84]). The
complete graph represents the full set of predicted behaviour states
for the represented device. These behaviour states are compared with
the Knowledge of Purpose behaviour descriptions; if conflicts are
found then these are described and the structure and purpose
descriptions of the device are scanned to establish the source of
the conflict. The ideas discussed in this paper are implemented in
the Doris system which is described.
A structured approach to the verification of low level
microcode
Curzon, Paul
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-215
ISSN 1476-2986
Errors in microprograms are especially serious since all higher
level programs on the machine depend on the microcode. Formal
verification presents one avenue which may be used to discover such
errors. Previous systems which have been used for formally verifying
microcode may be categorised by the form in which the microcode is
supplied. Some demand that it be written in a high level
microprogramming language. Conventional software verification
techniques are then applied. Other methods allow the microcode to be
supplied in the form of a memory image. It is treated as data to an
interpreter modelling the behaviour of the microarchitecture. The
proof is then performed by symbolic execution. A third solution is
for the code to be supplied in an assembly language and modelled at
that level. The assembler instructions are converted to commands in
a modelling language. The resulting program is verified using
traditional software verification techniques.
In this dissertation I present a new universal microprogram
verification system. It achieves many of the advantages of the other
kinds of systems by adopting a hybrid approach. The microcode is
supplied as a memory image, but it is transformed by the system to a
high level program which may be verified using standard software
verification techniques. The structure of the high level program is
obtained from user supplied documentation. I show that this allows
microcode to be split into small, independently validatable portions
even when it was not written in that way. I also demonstrate that
the techniques allow the complexity of detail due to the underlying
microarchitecture to be controlled at an early stage in the
validation process. I suggest that the system described would
combine well with other validation tools and provide help throughout
the firmware development cycle. Two case studies are given. The
first describes the verification of Gordon’s computer. This example
being fairly simple, provides a good illustration of the techniques
used by the system. The second case study is concerned with the High
Level Hardware Orion computer which is a commercially produced
machine with a fairly complex microarchitecture. This example shows
that the techniques scale well to production microarchitectures.
Exploiting OR-parallelism in Prolog using multiple
sequential machines
Klein, Carole Susan
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-216
ISSN 1476-2986
If the branches at each node of a tree are labelled, paths through
the tree can be represented by a sequence of labels called an
oracle. If an oracle leading to a node is followed, all of the
bindings and other state information associated with a node will be
recreated. Thus oracles are both a specification for a path through
the tree and a concide format for representing the environment at a
particular node.
This dissertation investigates the use of oracles for the parallel
execution of Prolog programs. The execution of a Prolog program can
be represented pictorially by an AND/OR tree. The branches of OR
nodes within this tree have no binding dependencies so their
evaluation can be performed on separate processors. If one of more
of these OR branches is explored in parallel, OR-parallelism is
exploited in the Prolog program.
A distributed system called the Delphi Machine has been designed and
implemented to exploit the OR-parallelism inherent in Prolog
programs. In the implementation described in this dissertation,
Delphi runs on a group of uniprocessors connected by Ethernet.
Various control strategies using oracles to control the parallel
search are investigated. The execution times for Prolog programs run
on the Delphi Machine are compared with those of a compiled and an
interpreted sequential Prolog system. The results show that a
distributed system using oracles to control the parallel search can
be an efficient way to exploit OR parallelism in nondeterministic
programs.
Because of overheads imposed by the Delphi algorithm, a program
executed on a single processor Delphi machine runs at approximately
one half the speed as the same program executed on the unmodified
prolog system. For a twenty processor configuration, the speed ups
obtained vary from approximately two to nine times depending on the
amount of OR-parallelism which can be exploited by Delphi. Problems
with large amounts of OR-parallelism show a nearly linear speedup.
Dynamic bandwidth management
Harita, Bhaskar Ramanathan
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-217
ISSN 1476-2986
Recent advances in semiconductor and optical technologies have
contributed greatly to the evolution of broadband integration of
multi-service traffic. The asynchronous transfer mode (ATM) has been
proposed as the target technique for broadband integrated services
digital networks (BISDNs) based on fast packet switching and optical
fibre transmission. A primary advantage of ATM is that variable bit
rate services can be supported efficiently, which meets the basic
needs of flexibility and service independence required of integrated
services networks. In order to fully exploit this flexibility and
enchance network efficiency by statistical multiplexing it is
important that there be effective methods of bandwidth management
and congestion control.
This dissertation describes the use of dynamic bandwidth management
to support an ATM overlay superimposed on a public, primary rate
ISDN. The overlay architecture provides for the flexible aggregation
of switched circuits into larger bandwidth channels. The channels
are formatted into a common packet encoding and packets from
different sources are statistically multiplexed onto them. In this
work, different control schemes that dynamically vary the bandwidth
of the channels in a transparent fashion, using out-of-band
signalling, are contrasted. The bandwidth is adjusted by adding or
deleting circuits in reaction to the traffic rates and the queue
sizes at the channels. Performance models of simple bandwidth
control schemes as queueing schemes are analysed by the use of
moment generating functions
Packet transfer on the overlay is virtual circuit based and
connection requests are accepted on the basis of their bandwidth
requirements. Dynamic bandwidth management is used to supplement
static bandwidth allocations in a congestion control framework
presented for the overlay. The cost effectiveness of dynamic
bandwidth control is examined for the tarrif structure implemented
by the underlying public ISDN.
The contributions of this dissertation are the development of
schemes for dynamic bandwidth management, their implementation on an
ATM testbed and the analysis of performance models for bandwidth
control validated by simulation and experiment.
Higher-order critical pairs
Nipkow, Tobias
University of Cambridge, Computer Laboratory
1991-04
en
Text
UCAM-CL-TR-218
ISSN 1476-2986
We consider rewrite systems over simply typed λ-terms with
restricted left-hand sides. This gives rise to a one-step reduction
relation whose transitive, reflexive and symmetric closure coincides
with equality. The main result of this paper is a decidable
confluence criterion which extends the well-known critical pairs to
a higher-order setting. Several applications to typed λ-calculi and
proof theory are shown.
Fairisle project working documents : Snapshot 1
Leslie, Ian M.
McAuley, Derek M.
Hayter, Mark
Black, Richard
Beller, Reto
Newman, Peter
Doar, Matthew
University of Cambridge, Computer Laboratory
1991-03
en
Text
UCAM-CL-TR-219
ISSN 1476-2986
This report contains the current versions of the documents
associated with the fairisle project. These include both papers and
draft documents. This collection of documents was made on March 21,
1991. Updated versions will be issued with later snapshot numbers
which will replace earlier versions. The present collection includes
the following documents:
Fairisle: Network architecture and components / Ian Leslie and Derek
McAuley.
Fairisle port controller: design and ideas / Mark Hayter and Richard
Black.
Fairisle VME interface (draft) / Reto Beeler.
A Slotted ring copy fabric for a multicast fast packet switch /
Peter Newman and Matthew Doar.
Universal Fairisle connector (proposed)
A distributed architecture for multimedia communication
systems
Nicolaou, Cosmos Andrea
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-220
ISSN 1476-2986
Technological advances in digital communications and in personal
computer workstations are beginning to allow the generation,
communication and presentation of multiple information media
simultaneously. In particular, the ability to support real-time
voice and video makes a new range of advanced and highly interactive
multimedia applications possible. These applications are not
restricted to the computer industry, but extend to other
technologically intensive industries which have some form of
multimedia communication requirement. Such industries include
medicine, conferencing, teaching, broadcasting, publishing and
printing. Each of these application areas has its own particular set
of requirements and makes corresponding demands on the computer
systems used.
Such a wide range of application areas leads to a correspondingly
large and diverse set of requirements of the systems used to
implement them. In addition, the real-time nature of voice, and
especially video, place heavy demands on the underlying systems.
Many of these requirements and demands are not met by existing
computer communication systems. This is due to the fact that the
architectural models used to design and implement these systems were
constructed before the technological advances making multimedia
communication possible took place. As a result existing multimedia
systems have tended to concentrate either on low level
implementation issues (e.g. communication networks and protocols)or
on a single restricted application area, without paying any regard
to their respective problems and requirements. The inevitable
consequence is that there is a mismatch between the functions
provided at the lower levels and those actually required by higher
level applications.
This dissertation presents an attempt to overcome these problems by
defining a new architecture for multimedia communication systems
which recognises and supports a wide range of application
requirements, in addition to satisfying the requirements made by the
information media themselves. A thorough survey of existing
multimedia systems was conducted in order to identify and understand
the requirements made by both applications and information media led
to the formulation of a set of design principles. In recognition of
the fact that any multimedia communication system is inherently
distributed in nature, the architecture is presented as an extension
of existing distributed systems.
The resulting architecture is called the Integrated Multimedia
Applications Communication architecture (IMAC) and a prototype
implementation of IMAC has been constructed and used to evaluate the
utility and feasibility of the architecture and to identify its
strength and weaknesses.
Transforming axioms for data types into sequential
programs
Milne, Robert
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-221
ISSN 1476-2986
A process is proposed for refining specifications of abstract data
types into efficient sequential implementations. The process needs
little manual intervention. It is split into three stages, not all
of which need always be carried out. The three stages entail
interpreting equalities as behavioural equivalences, converting
functions into procedures and replacing axioms by programs. The
stages can be performed as automatic transformations which are
certain to produce results that meet the specifications, provided
that simple conditions hold. These conditions describe the adequacy
of the specifications, the freedom from interference between the
procedures, and the mode of construction of the procedures.
Sufficient versions of these conditions can be checked
automatically. Varying the conditions could produce implementations
for different classes of specification. Though the transformations
could be automated, the intermediate results, in styles of
specification which cover both functions and procedures, have
interest in their own right and may be particularly appropriate to
object-oriented design.
Extensions to coloured petri nets and their application to
protocols
Billington, Jonathan
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-222
ISSN 1476-2986
This dissertation develops a net theoretic specification technique
for an area known as protocol engineering that covers the life-cycle
of protocols. After surveying the application of net theory to
protocol engineering, the fundamentals of the specification
technique are presented. The technique is based on Jensen’s Coloured
Petri Nets (CP-nets).
To increase their expressive power, CP-nets are extended by
including place capacities and an inhibitor function, leading to the
definition of a class of extended CP-nets, known as P-nets. To allow
the analysis techniques developed for CP-nets to be applied to
P-nets, a transformation from P-nets to CP-nets is formalised and it
is proved that it preserves interleaving behaviour. The
transformation is based on the notion of contemporary places (known
from Place/Transition-nets) and involves the definition and proof of
a new complementary place invariant for CP-nets. A class of P-nets
is defined where true concurrency is preserved under the
transformation.
A graphical form of P-nets, known as a P-graph, is formally defined,
drawing upon the notions developed for algebraic specification of
abstract data types. Arc inscriptions are multisets of terms
generated from a many-sorted signature. Transition conditions are
Boolean expressions derived from the same signature. An
interpretation of the P-Graph is given in terms of a corresponding
P-net. In the P-Graph, concrete sets are associated with places, and
likewise there are concrete initial marking and capacity multisets.
P-Graphs are useful for specification at a concrete level, and allow
classes of nets, such as CP-Graphs, many-sorted Algebraic nets and
many-sorted Predicate/Transition nets, to be defined as special
cases. They also provide the basis for a comparison with other
high-level nets such as Predicate/Transition nets and Algebraic
nets. An extended place capacity notation is developed to allow for
the convenient representation of resource bounds in the graphical
form.
Abstract P-Graphs are defined in a similar way to P-Graphs, but this
time sorts are associated with places, and markings and capacities
are defined at the syntactic level. This is useful for more abstract
specifications (such as classes of communication protocols) and for
their analysis.
Part of the motivation for the extensions to CP-nets has been to
develop convenient constructs for the purging of a place’s marking
(or part of the marking), by the occurrence of a single transition.
This is achieved by equating the inscriptions of the inhibitor and
normal arc. Some convenient notation is developed for the P-Graph
for purging parts of a place’s marking.
Some simple communications-oriented examples are presented including
queues and the Demon Game developed by the International
Organisation for Standardisation as a test case for formal
description techniques. A major case study of the M-Access service
of the Cambridge Fast Ring is specified with the P-Graph to
illustrate the utility of a number of the extensions developed for
P-nets.
Shallow processing and automatic summarising: a first
study
Gladwin, Philip
Pulman, Stephen
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1991-05
en
Text
UCAM-CL-TR-223
ISSN 1476-2986
This report describes a study of ten simple texts, investigating
various discourse phenomena to see how they might be exploited, in
shallow text processing, for summarising purposes. The processing
involved was a simulation of automatic analysis which is in
principle within reach of the state of the art. Each text was
treated by a version of Sidner’s focusing algorithm. The products of
this were fed into subsidiary stages of analysis to provide an
assessment of the activity of the various discourse entities within
each text. A concurrent process examined the occurrence of
orthographically identical noun phrase forms. Appendices give the
ten texts, a complete specification of the version of the focusing
algorithm in use, and the full experimental results. These suggest,
especially when the brevity of the test texts is taken into account,
that the type of information given by focusing has potential but
limited value for summarising.
Generalised probabilistic LR parsing of natural language
(corpora) with unification-based grammars
Briscoe, Ted
Carroll, John
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-224
ISSN 1476-2986
We describe work towards the construction of a very wide-coverage
probabilistic parsing system for natural language (NL), based on LR
parsing techniques. The system is intended to rank the large number
of syntactic analyses produced by NL grammars according to the
frequency of occurrence of the individual rules deployed in each
analysis. We discuss a fully automatic procedure for constructing an
LR parse table from a unification-based grammar formalism, and
consider the suitability of alternative LALR(1) parse table
construction methods for large grammars. The parse table is used as
the basis for two parsers; a user-driven interactive system which
provides a computationally tractable and labour-efficient method of
supervised learning of the statistical information required to drive
the probabilistic parser. The latter is constructed by associating
probabilities with the LR parse table directly. This technique is
superior to parsers based on probabilistic lexical tagging or
probabilistic context-free grammar because it allows for a more
context dependent probabilistic language model, as well as use of a
more linguistically adequate grammar formalism. We compare the
performance of an optimised variant of Tomita’s (1987) generalised
LR parsing algorithm to an (efficiently indexed and optimised) chart
parser. We report promising results of a pilot study training on 151
noun definitions from the Longman Dictionary of Contemporary English
(LDOCE) and retesting on these plus a further 54 definitions.
Finally we discuss limitations of the current system and possible
extensions to deal with lexical (syntactic and semantic) frequency
of occurrence.
Categorical multirelations, linear logic and petri nets
(draft)
de Paiva, Valeria
University of Cambridge, Computer Laboratory
1991-05
en
Text
UCAM-CL-TR-225
ISSN 1476-2986
This note presents a categorical treatment of multirelations, which
is, in a loose sense a generalisation of both our previous work on
the categories GC, and of Chu’s construction A_NC [Barr’79]. The
main motivation for writing this note was the utilisation of the
category GC by Brown and Gurr [BG90] to model Petri nets. We wanted
to extend their work to deal with multirelations, as Petri nets are
usually modelled using multirelations pre and post. That proved easy
enough and people interested mainly in concurrency theory should
refer to our joint work [BGdP’91], this note deals with the
mathematics underlying [BGdP’91]. The upshot of this work is that we
build a model of Intuitionistic Linear Logic (without modalities)
over any symmetric monoidal category C with a distinguished object
(N, ≤, ∘, e −∘) – a closed poset. Moreover, if the category C is
cartesian closed with free monoids, we build a model of
Intuitionistic Linear Logic with a non-trivial modality ‘!’ over it.
A new approach for improving system availability
Lam, Kwok-yan
University of Cambridge, Computer Laboratory
1991-06
en
Text
UCAM-CL-TR-226
ISSN 1476-2986
Priority in process calculi
Camilleri, Juanito Albert
University of Cambridge, Computer Laboratory
1991-06
en
Text
UCAM-CL-TR-227
ISSN 1476-2986
The desk area network
Hayter, Mark
McAuley, Derek
University of Cambridge, Computer Laboratory
1991-05
en
Text
UCAM-CL-TR-228
ISSN 1476-2986
A novel architecture for use within an end computing system is
described. This attempts to extend the concepts used in modern high
speed networks into computer system design. A multimedia workstation
is being built based on this concept to evaluate the approach.
Abstraction of image and pixel : The thistle display
system
Brown, David J.
University of Cambridge, Computer Laboratory
1991-08
en
Text
UCAM-CL-TR-229
ISSN 1476-2986
Proceedings of the Second Belief Representation and Agent
Architectures Workshop (BRAA ’91)
Galliers, J.
University of Cambridge, Computer Laboratory
1991-08
en
Text
UCAM-CL-TR-230
ISSN 1476-2986
Managing the order of transactions in widely-distributed
data systems
Yahalom, Raphael
University of Cambridge, Computer Laboratory
1991-08
en
Text
UCAM-CL-TR-231
ISSN 1476-2986
Mechanising set theory
Corella, Francisco
University of Cambridge, Computer Laboratory
1991-07
en
Text
UCAM-CL-TR-232
ISSN 1476-2986
Set theory is today the standard foundation of mathematics, but most
proof development sysems (PDS) are based on type theory rather than
set theory. This is due in part to the difficulty of reducing the
rich mathematical vocabulary to the economical vocabulary of the set
theory. It is known how to do this in principle, but traditional
explanations of mathematical notations in set theoretic terms do not
lead themselves easily to mechanical treatment.
We advocate the representation of mathematical notations in a formal
system consisting of the axioms of any version of ordinary set
theory, such as ZF, but within the framework of higher-order logic
with λ-conversion (H.O.L.) rather than first-order logic (F.O.L.).
In this system each notation can be represented by a constant, which
has a higher-order type when the notation binds variables. The
meaning of the notation is given by an axiom which defines the
representing constant, and the correspondence between the ordinary
syntax of the notation and its representation in the formal language
is specified by a rewrite rule. The collection of rewrite rules
comprises a rewriting system of a kind which is computationally well
behaved.
The formal system is justified by the fact than set theory within
H.O.L. is a conservative extension of set theory within F.O.L.
Besides facilitating the representation of notations, the formal
system is of interestbecause it permits the use of mathematical
methods which do not seem to be available in set theory within
F.O.L.
A PDS, called Watson, has been built to demonstrate this approach to
the mechanization of mathematics. Watson embodies a methodology for
interactive proof which provides both flexibility of use and a
relative guarantee of correctness. Results and proofs can be saved,
and can be perused and modified with an ordinary text editor. The
user can specify his own notations as rewrite rules and adapt the
mix of notations to suit the problem at hand; it is easy to switch
from one set of notations to another. As a case study, Watson has
been used to prove the correctness of a latch implemented as two
cross-coupled nor-gates, with an approximation of time as a
continuum.
A development environment for large natural language
grammars
Carroll, John
Briscoe, Ted
Grover, Claire
University of Cambridge, Computer Laboratory
1991-07
en
Text
UCAM-CL-TR-233
ISSN 1476-2986
Two tutorial papers: Information retrieval &
Thesaurus
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1991-08
en
Text
UCAM-CL-TR-234
ISSN 1476-2986
The first paper describes the characteristics of information
retrieval from documents or texts, the development and status of
automatic indexing and retrieval, and the actual and potential
relations between information retrieval and artificial intelligence.
The second paper discusses the properties, construction and actual
and potential uses of thesauri, as semantic classifications or
terminological knowledge bases, in information retrieval and natural
language processing.
Modelling and image generation
Wang, Heng
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-235
ISSN 1476-2986
Three dimensional (3D) volume representation, processing and
visualisation have gained growing attention during the last ten
years due to the rapid decrease in computer memory cost and the
enhancement of computation power. Recent developments in massively
parallel computer architectures and special purpose graphics
accelerators also facilitate the solution of 3D volume manuipulation
problems which usually have large memory and computation
requirements. Volumentric graphics is becoming practically possible
and finding many applications such as medical image processing,
computer aided design and scientific visualisation.
A volumetric object is usually represented in one of two forms: a
large 3D uniform grid of voxels (volume elements), and a relatively
compact non-uniform collection of volumes. Objects in the latter
form are obtained by adaptive, recursive decompositions. An octree
is a special case in which each non-terminal volume is subdivided
into eight sub-volumes. The problems of current implementation of
octrees concern the speed and complexity of memory management. This
dissertation looks into a novel approach of designing octree-related
volumetric graphics algorithms based on Content Addressable Memories
(CAMs). A CAM is an architecture consisting of elements which have
data storage capabilities and can be accessed simultaneously on the
basis of data contents instead of addresses. It is demonstrated that
the main features of CAMs, their parallel searching, pattern
matching and masked parallel updating capabilities, are suitable for
implementing octree related algorithms.
New CAM algorithms are presented for transforming octrees,
evaluating set operations (union, intersection, difference),
displaying volumetric objects, calculating volumes, constructing
octrees from other representations, and so on. These algorithms are
remarkably simple and conceptively intuitive. The simplicity plays
an important role in constructing robust solid 3D modelling systems.
In addition to their simplicity, many algorithms are more efficient
than their conventional counterparts.
A new method has been developed to speed up the image synthesis
algorithm of ray tracing using CAM octrees. It is aimed to reduce
the number of ray-object intersection tests without significantly
increasing the overheads of storage and computation which are
related to octree data structures and their traversals. The
simulation results confirm the expected improvements in speed and
memory management. Ray tracing can be accelerated by applying
parallelism. Preliminary analysis shows possibilities of
implementing the above CAM octree ray tracer on general parallel
machines such as MIMD (Multiple Instriction stream, Multiple Data
stream).
Using knowledge of purpose and knowledge of structure as a
basis for evaluating the behaviour of mechanical systems
Bradshaw, John Anthony
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-236
ISSN 1476-2986
Computing presuppositions in an incremantal language
processing system
Bridge, Derek G.
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-237
ISSN 1476-2986
This thesis describes the design and implementation of a natural
language analysis system for the computation of presuppositions. The
system is one in which syntactic, semantic and pragmatic processing
are interleaved with feedback to syntactic analysis from semantic
and pragmatic processing. The thesis begins by illustrating how the
system processes definite noun phrases. The mechanisms used for this
are then shown to be easily extensible to processing other parts of
speech such as indefinite noun phrases and verb phrases.
Definite noun phrases have been said to be presupposition triggers.
This means that traditionally they have been seen as licensing
certain inferences — presuppositions. In the system described
herein, presuppositions are treated as a special kind of inference:
preconditions. This treatment for definite noun phrases can be
extended to give a uniform account of all presupposition triggers
(e.g. factive verbs). It is a view that makes it clear that
presuppositions are not ‘optional extras’ that might or might not be
derived once a semantic representation of an utterance has been
produced. Rather, they play an essential role in driving the
utterance analysis process: the failure of a presupposition, i.e.
failure to satisfy a precondition, can direct the system to choose
an alternative reading of an utterance of an ambiguous sentence.
As it processes an utterance, the system builds and regularly
consults a representation of contextual knowledge referred to as a
discourse model. Importantly, the system checks whether
presuppositions are satisfied against the discourse model.
Presupposition failure, i.e. a presupposition not being satisfied by
the discourse model, is not necessarily the same as a presupposition
being false in, e.g., the ‘real’ world. Checking presuppositions for
satisfaction in a discourse model and not for truth in a possible
world offers new ideas on the behaviour of presuppositions in
utterances of negative and complex sentences.
In utterances of negative sentences, presuppositions must still be
satisfied by the discourse model. Presuppositions cannot be
cancelled as they can in other accounts. Rather, presupposition
“cancellation” data is explained in terms of utterances that make
metalinguistic statements about the model-theoretic interpretation
of the discourse model. It is shown that computing presuppositions
in an incremental system gives a simple account of most of the data
relating to the behaviour of presuppositions in utterancesof
compound sentences and longer stretches of text (the so-called
“projection problem”). Presuppositions must again be satisfied by
the discourse model, but they may be satisfied by virtue of changes
made to the discourse model by earlier parts of the utterance or
text.
Proceedings of the ACQUILEX Workshop on Default Inheritance
in the lexicon
Briscoe, Ted
Copestake, Ann
de Paiva, Valeria
University of Cambridge, Computer Laboratory
1991-10
en
Text
UCAM-CL-TR-238
ISSN 1476-2986
The ACQUILEX Esprit BRA (Basic Research Action) research project is
concerned with the acquisition and representation of lexical
information from machine readable dictionaries for use in Natural
Language Processing. The Cambridge group of the ACQUILEX project
organised a Workshop on Default Inheritance in April 1991, the main
purpose of which was to review approaches to default inheritance for
lexical organisation and representation. The emphasis from
ACQUILEX’s point of view was in implementing a practical system
capable of supporting substantial lexicons, based on existing
proposals to incorporate (default) inheritance into a
unification-based framework similar to DATR (Gazdar and Evans, 1989)
and HPSG (e.g. Carpenter, 1990).
The workshop consisted of two days of talks, where theoretical and
implementational issues on default inheritance were discussed, as
well as a last day of demonstrations of implemented systems. Papers
from several European colaborative projects on the topic of the
workshop were presented – see enclosed list of titles and
affiliations. The Cambridge ACQUILEX group presented and
demonstrated the ACQUILEX lexical knowledge base (LKB) system and
provided a tutorial on use of the software. The TFS system of the
project POLYGLOSS and the system ELU of the group at ISSCO were also
discussed and demonstrated.
Many thanks to all the participants for the lively discussions –
exactly what workshops are supposed to be for.
Planning multisentential English text using communicative
acts
Maybury, Mark Thomas
University of Cambridge, Computer Laboratory
1991-12
en
Text
UCAM-CL-TR-239
ISSN 1476-2986
The goal of this research is to develop explanation presentation
mechanisms for knowledge based systems which enable them to define
domain terminology and concepts, narrate events, elucidate plans,
processes, or propositions and argue to support a claim or advocate
action. This requires the development of devices which select,
structure, order and then linguistically realize explanation content
as coherent and cohesive English text.
With the goal of identifying generic explanation presentation
strategies, a wide range of naturally occurring texts were analyzed
with respect to their communicative structure, function, content and
intended effects on the reader. This motivated an integrated theory
of communicative acts which characterizes text at the level of
rhetorical acts (e.g. describe, define, narrate), illocutionary acts
(e.g. inform, request), and locutionary acts (ask, command). Taken
as a whole, the identified communicative acts characterize the
structure, content and intended effects of four types of text:
description, narration, exposition, argument. These text types have
distinct effects such as getting the reader to know about entities,
to know about events, to understand plans, processes, or
propositions, or to believe propositions or want to perform actions.
In addition to identifying the communicative function and effect of
text at multiple levels of abstraction, this dissertation details a
tripartite theory of focus of attention (discourse focus, temporal
focus and spatial focus) which constrains the planning and
linguistic realization of text.
To test the integrated theory of communicative acts and tripartite
theory of focus of attention, a text generation system TEXPLAN
(Textual EXplanation PLANner) was implemented that plans and
linguistically realizes multisentential and multiparagraph
explanations from knowledge based systems. The communicative acts
identified during text analysis were formalized over sixty
compositional and (in some cases) recursive plan operators in the
library of a hierarchical planner. Discourse, temporal and spatial
models were implemented to track and use attentional information to
guide the organization and realization of text. Because the plan
operators distinguish between the communicative function (e.g. argue
for a proposition) and the expected effect (e.g. the reader believes
the proposition) of communicative acts, the system is able to
construct a discourse model of the structure and function of its
textual responses as well as a user model of the expected effects of
its responses on the reader’s knowledge, beliefs, and desires. The
system uses both the discourse model and user model to guide
subsequent utterances. To test its generality, the system was
interfaced to a variety of domain applications including a
neuropsychological diagnosis system, a mission planning system, and
a knowledge based mission simulator. The system produces
descriptions, narratives, expositions and arguments from these
applications, thus exhibiting a broader ranger of rhetorical
coverage then previous text generation systems.
Symbolic compilation and execution of programs by proof: a
case study in HOL
Camilleri, Juanito
University of Cambridge, Computer Laboratory
1991-12
en
Text
UCAM-CL-TR-240
ISSN 1476-2986
This paper illustrates the symbolic compilation and execution of
programs by proof using the proof assistant HOL. We formalise the
operational semantics of an Occam-like programming language oc and
show how synchronous communication in oc compiles to an intermediate
programming language Safe, whose compilation yields instructions
intended to drive machines that communicate via shared memory. We
show how the symbolic formal manipulation of terms of a programming
language, subject to the definition of its semantics, can animate a
desired effect — be it compilation or execution. Needless to say,
such compilation and execution by proof is rather slow, but it is
fast enough to give vital feedback about the compilation algorithm
being used. Without such animation it is hard to anticipate whether
the compilation algorithm is reasonable before attempting to verify
it. This is particularly true when attempting to find a plausible
handshaking protocol that implements synchronous communication.
Learning in large state spaces with an application to biped
robot walking
Vogel, Thomas Ulrich
University of Cambridge, Computer Laboratory
1991-12
en
Text
UCAM-CL-TR-241
ISSN 1476-2986
Autonomous robots must be able to operate in complex, obstacle
cluttered environments. To do this the robots must be able to focus
on the important aspects of their environment, create basic
strategies to carry out their operations, generalise these
strategies and finally learn from successful experiences.
Based on simulated dynamic biped robot walking, this thesis
investigates these issues. An algorithm is given which analyses the
state space of the robot and orders the dimensions of the state
space by their importance relative to the task of the robot. Using
this analysis of its state space, the robot is able to generate a
set of macros (gaits) which enable it to operate in its immediate
environment. We then present a control algorithm which allows the
robot to control the execution of its gaits
Once the robot has learned to walk on an obstacle-free horizontal
surface, it uses its knowledge about gaits in order to derive
obstacle crossing gaits from existing gaits. A strategy based on the
qualitative equivalence between two behaviours is introduces in
order to derive new behavioural patterns from previous ones. This
enables the robot to reason about its actions at a higher level of
abstraction. This facilitates the transfer and adaptation of
existing knowledge to new situations. As a result, the robot is able
to derive stepping over an obstacle from stepping on a horizontal
surface.
Finally, the robot analyses its successful obstacle crossings in
order to generate a generic obstacle crossing strategy. The concept
of a virtual evaluation function is introduced in order to describe
how the robot has to change its search strategy in order to search
successfully for obstacle crossing behaviours. This is done by
comparing how the successful obstacle crossing of the robot differs
from its normal behaviour. By analysing and operationalising these
differences, the robot acquires the capability to overcome
previously unencountered obstacles. The robot’s obstacle crossing
capabilities are demonstrated by letting the robot walk across
randomly generated obstacle combinations
An object-oriented approach to virtual memory
management
Mapp, Glenford Ezra
University of Cambridge, Computer Laboratory
1992-01
en
Text
UCAM-CL-TR-242
ISSN 1476-2986
Advances in computer technology are being pooled together to form a
new computing environment which is characterised by powerful
workstations with vast amounts of memory connected to high speed
networks. This environment will provide a large number of diverse
services such as multimedia communications, expert systems and
object-oriented databases. In order to develop these complex
applications in an efficient manner, new interfaces are required
which are simple, fast and flexible and allow the programmer to use
an object-oriented approach throughout the design and implementation
of an application. Virtual memory techniques are increasingly being
used to build these new facilities.
In addition since CPU speeds continue to increase faster than disk
speeds, an I/O bottleneck may develop in which the CPU may be idle
for long periods waiting for paging requests to be satisfied. To
overcome this problem it is necessary to develop new paging
algorithms that better reflect how different objects are used. Thus
a facility to page objects on a per-object basis is required and a
testbed is also needed to obtain experimental data on the paging
activity of different objects.
Virtual memory techniques, previously only used in mainframe and
minicomputer architectures, are being employed in the memory
management units of modern microprocessors. With very large address
spaces becoming a standard feature of most systems, the use of
memory mapping is seen as an effective way of providing greater
flexibility as well as improved system efficiency.
This thesis presents an object-oriented interface for memory mapped
objects. Each object has a designated object type. Handles are
associated with different object types and the interface allows
users to define and manage new object types. Moving data between the
object and its backing store is done by user-level processes called
object managers. Object managers interact with the kernel via a
specified interface thus allowing users to build their own object
managers. A framework to compare different algorithms was also
developed and an experimental testbed was designed to gather and
analyse data on the paging activity of various programs. Using the
testbed, conventional paging algorithms were applied to different
types of objects and the results were compared. New paging
algorithms were designed and implemented for objects that are
accessed in a highly sequential manner.
Automating the librarian: a fundamental approach using
belief revision
Cawsey, Alison
Galliers, Julia
Reece, Steven
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1992-01
en
Text
UCAM-CL-TR-243
ISSN 1476-2986
This paper describes a current research project investigating belief
revision in intelligent systems by modelling the librarian in
interaction with a literature-seeking user. The work is designed to
both test a theory of agent behaviour based on belief revision
proposed by Galliers, and to evaluate a model of the librarian
developed by Belkin, Brooks and Daniels, through computational
implementation. Agent communication is seen as motivated by and
motivating belief changes, where belief revision is determined by
coherence, combining endorsement, connectivity and conservatism. The
librarian is viewed as a distributed expert system with many
individual specialised functions operating in particular belief
domains. The paper describes our first implementation of the belief
revision mechanism and of a very primative librarian, designed to
test the basic viability of our ideas and to allow us to explore
different forms of the distributed system architecture.
A mechanized theory of the π-calculus in HOL
Melham, T.F.
University of Cambridge, Computer Laboratory
1992-01
en
Text
UCAM-CL-TR-244
ISSN 1476-2986
The π-calculus is a process algebra developed at Edinburgh by
Milner, Parrow and Walker for modelling concurrent systems in which
the pattern of communication between processes may change over time.
This paper describes the results of preliminary work on a mechanized
formal theory of the π-calculus in higher order logic using the HOL
theorem prover.
System support for multi-service traffic
Dixon, Michael J.
University of Cambridge, Computer Laboratory
1992-01
en
Text
UCAM-CL-TR-245
ISSN 1476-2986
Digital network technology is now capable of supporting the
bandwidth requirements of diverse applications such as voice, video
and data (so called multi-service traffic). Some media, for example
voice, have specific transmission requirements regarding the maximum
packet delay and loss which they can tolerate. Problems arise when
attempting to multiplex such traffic over a single channel.
Traditional digital networks based on the Packet- (PTM) and
Synchronous- (STM) Transfer Modes prove unsuitable due to their
media access contention and inflexible bandwidth allocation
properties respectively. The Asynchronous Transfer Mode (STM) has
been proposed as a compromise between the PTM and STM techniques.
The current state of multimedia research suggests that a significant
amount of multi-service traffic will be handled by computer
operating systems. Unfortunately conventional operating systems are
largely unsuited to such a task. This dissertation is concerned with
the system organisation necessary in order to extend the benefits of
ATM networking through the endpoint operating system and up to the
application level. A locally developed micro-kernel, with ATM
network protocol support, has been used as a testbed for the ideas
presented. Practical results over prototype ATM networks, including
the 512 MHz Cambridge Backbone Network, are presented.
A relevance-based utterance processing system
Poznański, Victor
University of Cambridge, Computer Laboratory
1992-02
en
Text
UCAM-CL-TR-246
ISSN 1476-2986
This thesis presents a computational interpretation of Sperber and
Wilson’s relevance theory, based on the use of non-monotonic logic
supported by a reason maintenance system, and shows how the theory,
when given a specific form in this way, can provide a unique and
interesting account of discourse processing.
Relevance theory is a radical theory of natural language pragmatics
which attempts to explain the whole of human cognition using a
single maxim: the Principle of Optimal Relevance. The theory is seen
by its originators as a computationally more adequate alternative to
Gricean pragmatics. Much as it claims to offer the advantage of a
unified approach to utterance comprehension, Relevance Theory is
hard to evaluate because Sperber and Wilson only provide vague,
high-level descriptions of vital aspects of their theory. For
example, the fundamental idea behind the whole theory is that, in
trying to understand an utterance, we attempt to maximise
significant new information obtained from the utterance whilst
consuming as little cognitive effort as possible. However, Sperber
and Wilson do not make the nature of information and effort
sufficiently clear.
Relevance theory is attractive as a general theory of human language
communication and as a potential framework for computational
language processing systems. The thesis seeks to clarify and flesh
out the problem areas in order to develop a computational
implementation which is used to evaluate the theory.
The early chapters examine and criticise the important aspects of
the theory, emerging with a schema for an ideal relevance-based
system. Crystal, a computational implementation of an utterance
processing system based on this schema is then described. Crystal
performs certain types of utterance disambiguation and reference
resolution, and computes implicatures according to relevance theory.
An adequate reasoning apparatus is a key component of a relevance
based discourse processor, so a suitable knowledge representation
and inference engine are required. Various candidate formalisms are
considered, and a knowledge representation and inference engine
based on autoepistemic logic is found to be the most suitable. It is
then shown how this representation can be used to meet particular
discourse processing requirements, and how it provides a convenient
interface to a separate abduction system that supplies not
demonstrative inferences according to relevence theory. Crystal’s
powers are illustrated with examples, and the thesis shows how the
design not only implements the less precise areas of Sperber and
Wilson’s theory, but overcomes problems with the theory itself.
Crystal uses rather crude heuristics to model notions such as
salience and degrees of belief. The thesis thefore presents a
proposal and outline for a new kind of reason maintenance system
that supports non-monotonic logic whose formulae re labelled with
upper/lower probability ranges intended to represent strength of
belief. This system should facilitate measurements of change in
semantic information and shed some light on notions such as expected
utility and salience.
The thesis concludes that the design and implementation of crystal
provide evidence that relevance theory, as a generic theory of
language processing, is a viable alternative theory of pragmatics.
It therefore merits a greater level of investigation than has been
applied to it to date.
Programming metalogics with a fixpoint type
Crole, Roy Luis
University of Cambridge, Computer Laboratory
1992-02
en
Text
UCAM-CL-TR-247
ISSN 1476-2986
A programming metalogic is a formal system into which programming
languages can be translated and given meaning. The translation
should reflect both the structure of the language and make it easy
to prove properties of programs. This thesis develops certain
metalogics using techniques of category theory and treats recursion
in a new way.
The notion of a category with a fixpoint logic is defined.
Corresponding to this categorical structure there are type theoretic
equational rules which will be present in all of the metalogics
considered. These rules define the fixpoint type which will allow
the interpretation of recursive declarations. With these core
notions FIX categories are defined. These are the categorical
equivalent of an equational logic which can be viewed as a very
basic programming metalogic. Recursion is treated both syntactically
and categorically.
The expressive power of the equational logic is increased by
embedding it in an intuitionistic predicate calculus, giving rise to
the FIX logic. This contains propositions about the evaluation of
computations to values and an induction principle which is derived
from the definition of a fixpoint object as an initial algebra. The
categorical structure which accompanies the FIX logic is defined,
called a FIX hyperdoctrine, and certain existence and disjunction
properties of FIX are stated. A particular FIX hyperdoctrine is
constructed and used in the proof of the above properties.
PCF-style languages are translated into the FIX logic and
computational adequacy results are proved. Two languages are
studied: both are similar to PCF except one has call by value
recursive function declarations and the other higher order
conditionals.
A dependently typed equational logic containing a fixpoint type and
a universal type is given together with its related categorical
structure, namely a FIX category with attributes. A representation
theorem for Scott predomains is proved, which gives rise to a
concrete example of such a FIX category with attributes. Recursive
domain equations give rise to endofunctions on the universal type;
using the fixpoint type we may solve for fixpoints of such
endofunctions and thus obtain a solution the original domain as the
type coded by the fixpoint.
On efficiency in theorem provers which fully expand proofs
into primitive inferences
Boulton, Richard J.
University of Cambridge, Computer Laboratory
1992-02
en
Text
UCAM-CL-TR-248
ISSN 1476-2986
Theorem Provers which fully expand proofs into applications of
primitive inference rules can be made highly secure, but have been
criticized for being orders of magnitude slower than many other
theorem provers. We argue that much of this relative inefficiency is
due to the way proof procedures are typically written and not all is
inherent in the way the systems work. We support this claim by
considering a proof procedure for linear arithmetic. We show that
straightforward techniques can be used to significantly cut down the
computation required. An order of magnitude improvement in the
performance is shown by an implementation of these techniques.
A formalisation of the VHDL simulation cycle
Van Tassel, John P.
University of Cambridge, Computer Laboratory
1992-03
en
Text
UCAM-CL-TR-249
ISSN 1476-2986
The VHSIC Hardware Description Language (VHDL) has been gaining wide
acceptance as a unifying HDL. It is, however, still a language in
which the only way of validating a design is by careful simulation.
With the aim of better understanding VHDL’s particular simulation
process and eventually reasoning about it, we have developed a
formalisation of VHDL’s simulation cycle for a subset of the
language. It has also been possible to embed our semantics in the
Cambridge Higher-Order Logic (HOL) system and derive interesting
properties about specific VHDL programs.
TouringMachines: autonomous agents with attitudes
Ferguson, Innes A.
University of Cambridge, Computer Laboratory
1992-04
en
Text
UCAM-CL-TR-250
ISSN 1476-2986
It is becoming widely accepted that neither purely reactive nor
purely deliberative control techniques are capable of producing the
range of behaviours required of intelligent robotic agents in
dynamic, unpredictable, multi-agent worlds. We present a new
architecture for controlling autonomous, mobile agents – building on
previous work addressing reactive and deliberative control methods.
The proposed multi-layered control architecture allows a
resource-bounded, goal-directed agent to react promptlyto unexpected
changes in its environment; at the same time it allows the agent to
reason predictively about potential conflicts by contrasting and
projecting theories which hypothesise other agents’ goals and
intentions.
The line of research adopted is very much a pragmatic one. A single
common architecture has been implemented which, being extensively
parametrized allows an experimenter to study functionally- and
behaviourally-diverse agent configurations. A principal aim of this
research is to understand the role different functional capabilities
play in constraining an agent’s behaviour under varying
environmental conditions. To this end, we have constructed an
experimental testbed comprising a simulated multi-agent world in
which a variety of agent configurations and bahaviours have been
investigated. Some experience with the new control architecture is
described.
Multipoint digital video communications
Jiang, Xiaofeng
University of Cambridge, Computer Laboratory
1992-04
en
Text
UCAM-CL-TR-251
ISSN 1476-2986
Ever since the emergence of high-speed communication networks and
fast signal processing technology, digital video has been attracting
increased research interest. However, problems associated with its
use in a multipoint communication environment have not been
thouroughly investigated. In particular, these include the avoidance
of congestion on multicast paths when multiple wideband sources are
transmitting simultaneously, and the ability to interchange
different format signals properly and efficiently. This dissertation
addresses these issues with a two-level communications architecture.
The congestion issue at the network level is dealt with by several
stream multicast path finding algorithms which are either
centralised or distributed to suit various application environments.
Different ways of integrating communication link capacities are
investigated for supporting simultaneous transmission of broadband
signals with minimum effect on network traffic and maximum success
in path finding. Simulation results demonstrate performance
improvements over conventional multicast path finding algorithms.
The format issue at the presentation level is dealt with by an
intermediate format or general representation of digital video
streams. Signals under this scheme are organised in a form to
facilitate their interchange and scalable receiving in multipoint
communication applications. Issues including frame segmentation and
coding description are investigated. An experimental system
implementing a simple version of the scheme is presented along with
test results on picture quality degredation from conversion of
various types and related timing characteristics.
A co-induction principle for recursively defined
domains
Pitts, Andrew M.
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-252
ISSN 1476-2986
This paper establishes a new property of predomains recursively
defined using the cartesian product, disjoint union, partial
function space and convex powerdomain constructors. We prove that
the partial order on such a recuirsive predomain D is the greatest
fixed point of a certain monotone operator associated to D. This
provides a structurally defined family of proof principles for these
recursive predomains: to show that one element of D approximates
another, it suffices to find a binary relation containing the two
elements that is a post-fixed point for the associated monotone
operator. The statement of the proof principles is independent of
any of the various methods available for explicit construction of
recursive predomains. Following Milner and Tofte, the method of
proof is called co-induction. It closely resembles the way
bisimulations are used in concurrent process calculi.
Two specific instances of the co-induction principle already occur
in the work of Abramsky in the form of ‘internal full abstraction’
theorems for denotational semantics of SCCS and the lazy lambda
calculus. In the first case post-fixed binary relations are
precisely Abramsky’s partial bisimulations, whereas in the second
case they are his applicative bisimulations. The coinduction
principle also provides an apparently useful tool for reasoning
about the equality of elements of recursively defined datatypes in
(strict or lazy) higher order functional programming languages.
The (other) Cambridge ACQUILEX papers
Sanfilippo, Antonio
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-253
ISSN 1476-2986
A HOL semantics for a subset of ELLA
Boulton, Richard J.
University of Cambridge, Computer Laboratory
1992-04
en
Text
UCAM-CL-TR-254
ISSN 1476-2986
Formal verification is an important tool in the design of computer
systems, especially when the systems are safety or security
critical. However, the formal techniques currently available are not
well integrated into the set of tools more traditionally used by
designers. This work is aimed at improving the integration by
providing a formal semantics for a subset of the hardware
description language ELLA, and by supporting this semantics in the
HOL theorem proving system, which has been used extensively for
hardware verification.
A semantics for a subset of ELLA is described, and an outline of a
proof of the equivalence of parallel and recursive implementations
of an n-bit adder is given as an illustration of the semantics. The
proof has been performed in an extension of the HOL system. Some
proof tools written to support the verification are also described.
The formal verification of hard real-time systems
Cardell-Oliver, Rachel Mary
University of Cambridge, Computer Laboratory
1992
en
Text
UCAM-CL-TR-255
ISSN 1476-2986
MCPL programming manual
Richards, Martin
University of Cambridge, Computer Laboratory
1992-05
en
Text
UCAM-CL-TR-256
ISSN 1476-2986
MCPL is a systems programming language having much in common with
BCPL but augmented by the pattern matching ideas of both ML and
Prolog. Unlike ML, MCPL is typeless, runs using a contiguous runtime
stack and has no built in garbage collector, but it does make
extensive use of ML-like pattern matching. The low level aspects of
the language resemble those of BCPL and C. For efficiency, MCPL uses
its own function calling sequence, however a convenient mechanism
for mixing MCPL and C programs is provided.
Notable features of MCPL are its pattern matching facilities and the
simple way in which data structures are handled.
This document gives a complete definition of the language and
includes, at the end, several example programs to demonstrate its
capabilities.
Cut-free sequent and tableau systems for propositional
normal modal logics
Goré, Rajeev Prakhakar
University of Cambridge, Computer Laboratory
1992-05
en
Text
UCAM-CL-TR-257
ISSN 1476-2986
We present a unified treatment of tableau, sequent and axiomatic
formulations for many propositional normal modal logics, thus
unifying and extending the work of Hanson, Segerberg, Zeman, Mints,
Fitting, Rautenberg and Shvarts. The primary emphasis is on tableau
systems as the completeness proofs are easier in this setting. Each
tableau system has a natural sequent analogue defining a finitary
provability relation for each axiomatically formulated logic L.
Consequently, any tableau proof can be converted into a sequent
proof which can be read downwards to obtain an axiomatic proof. In
particular, we present cut-free sequent systems for the logics S4.3,
S4.3.1 and S4.14. These three logics have important temporal
interpretations and the sequent systems appear to be new.
All systems are sound and (weakly) complete with respect to their
known finite frame Kripke semantics. By concentrating almost
exclusively on finite tree frames we obtain finer characterisation
results, particularly for the logics with natural temporal
interpretations. In particular, all proofs of tableau completeness
are constructive and yield the finite model property and
decidability for each logic.
Most of these systems are cut-free giving a Gentzen cut-elimination
theorem for the logic in question. But even when the cut rule is
required, all uses of it remain analytic. Some systems do not
possess the subformula property. But in all such cases the class of
“superformulae” remains bounded, giving an analytic superformula
property. Thus all systems remain totally amenable to computer
implementation and immediately serve as nondeterministic decision
procedures for the logics they formulate. Furthermore, the
constructive completeness proofs yield deterministic decision
procedures for all the logics concerned.
In obtaining these systems we domonstrate that the subformula
property can be broken in a systematic and analytic way while still
retaining decidability. This should not be surprising since it is
known that modal logic is a form of second order logic and that the
subformula property does not hold for higher order logics.
Two papers on ATM networks
Greaves, David J.
McAuley, Derek
French, Leslie J.
University of Cambridge, Computer Laboratory
1992-05
en
Text
UCAM-CL-TR-258
ISSN 1476-2986
Private ATM networks / by David J. Greaves and Derek McAuley.
This paper advocates the use of local area networks which use 48
byte ATM cells. Hosts connected to the network are fitted with ATM
interfaces and run a new protocol stack up to the network level,
which avoids multiplexing and efficiently handles the out-of-band
signalling used by ATM.
The private network may be of WAN, MAN or LAN dimensions and contain
several different network technologies, provided each is able to
perform the basic function of carrying ATM cells from one point to
another. The private network may be connected to the B-ISDN at one
or more points.
Protocol and interface for ATM LANs / by David J. Greaves, Derek
McAuley and Leslie J. French.
This paper advocates the use of local area networks using the
Asynchronous Transfer Mode, where data is carried in the payloads of
48-byte cells. We describe the design and performance of a simple
ATM host interface for the DEC Turbochannel together with the MSNA
protocol architecture. We describe how MSNA creates a homogeneous
internet for ATM hosts and devices. We discuss the implementation of
an adaption layer for computer data which is able to take full
advantage of MSNA semantics, and which makes use of the end-to-end
ATM layer header bit which has recently been accepted.
Full abstraction in the Lazy Lambda Calculus
Abramsky, Samson
Ong, C.-H. Luke
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-259
ISSN 1476-2986
Local computation of alternating fixed-points
Anderson, Henrik Reif
University of Cambridge, Computer Laboratory
1992-06
en
Text
UCAM-CL-TR-260
ISSN 1476-2986
In this paper we consider the problem of alternating fixed-points of
monotone functions on finite boolean lattices. We describe a local
(demand-driven, lazy) algorithm for computing a boolean expression
with two alternating fixed-points, i.e. with a minimal and a maximal
fixed-point intertwined. Such expressions arise naturally in the
modal μ-calculus and are the main source of its expressive power –
and its difficult model checking problem. By a translation of the
model checking problem of the modal μ-calculus into a problem of
finding fixed-points on boolean lattices, we get a local model
checker for two alternating fixed-points which runs in time
O(|A|(|T|²)log(|A||T|)), where |A| is the size of the assertion and
|T| the size of the model, a labelled transition system. This
extends earlier results by the author and improves on earlier
published local algorithms. We also sketch how the algorithm can be
extended to arbitrary alternations.
Due to the generality of the algorithm it can be applied to other
(alternating or non-alternating) fixed-point problems.
Image resampling
Dodgson, Neil Anthony
University of Cambridge, Computer Laboratory
1992-08
en
Text
UCAM-CL-TR-261
ISSN 1476-2986
Image resampling is the process of geometrically transforming
digital images. This report considers several aspects of the
process.
We begin by decomposing the resampling process into three simpler
sub-processes: reconstruction of a continuous intensity surface from
a discrete image, transformation of that continuous surface, and
sampling of the transformed surface to produce a new discrete image.
We then consider the sampling process, and the subsidiary problem of
intensity quantisation. Both these are well understood, and we
present a summary of existing work, laying a foundation for the
central body of the report where the sub-process of reconstruction
is studied.
The work on reconstruction divides into four parts, two general and
two specific:
1. Piecewise local polynomials: the most studied group of
reconstructors. We examine these, and the criteria used in their
design. One new derivation is of two piecewise local quadratic
reconstructors.
2. Infinite extent reconstructors: we consider these and their local
approximations, the problem of finite image size, the resulting edge
effects, and the solutions to these problems. Amongst the
reconstructors discussed are the interpolating cubic B-spline and
the interpolating Bezier cubic. We derive the filter kernels for
both of these, and prove that they are the same. Given this kernel
we demonstrate how the interpolating cubic B-spline can be extended
from a one-dimensional to a two-dimensional reconstructor, providing
a considerable speed improvement over the existing method of
extension.
3. Fast Fourier transform reconstruction: it has long been known
that the fast Fourier transform (FFT) can be used to generate an
approximation to perfect scaling of a sample set. Donald Fraser (in
1987) took this result and generated a hybrid FFT reconstructor
which can be used for general transformations, not just scaling. We
modify Fraser’s method to tackle two major problems: its large time
and storage requirements, and the edge effects it causes in the
reconstructed intensity surface.
4. A priori knowledge reconstruction: first considering what can be
done if we know how the original image was sampled, and then
considering what can be done with one particular class of image
coupled with one particular type of sampling. In this latter case we
find that exact reconstruction of the image is possible. This is a
surprising result as this class of images cannot be exactly
reconstructed using classical sampling theory.
The final section of the report draws all of the strands together to
discuss transformations and the resampling process as a whole. Of
particular note here is work on how the quality of different
reconstruction and resampling methods can be assessed.
Term assignment for intuitionistic linear logic (preliminary
report)
Benton, Nick
Bierman, Gavin
de Paiva, Valeria
University of Cambridge, Computer Laboratory
1992-08
en
Text
UCAM-CL-TR-262
ISSN 1476-2986
In this paper we consider the problem of deriving a term assignment
system for Girard’s Intuitionistic Linear Logic for both the sequent
calculus and natural deduction proof systems. Our system differs
from previous calculi (e.g. that of Abramsky) and has two important
properties which they lack. These are the substitution property (the
set of valid deductions is closed under substitution) and subject
reduction (reduction on terms is well typed).
We define a simple (but more general than previous proposals)
categorical model for Intuitionistic Linear Logic and show how this
can be used to derive the term assignment system.
We also consider term reduction arising from cut-elimination in the
sequent calculus and normalisation in natural deduction. We explore
the relationship between these, as well as with the equations which
follow from our categorical model.
The Lazy Lambda Calculus: an investigation into the
foundations of functional programming
Ong, C.-H. Luke
University of Cambridge, Computer Laboratory
1992-08
en
Text
UCAM-CL-TR-263
ISSN 1476-2986
CCS with environmental guards
Camilleri, Juanito
University of Cambridge, Computer Laboratory
1992-08
en
Text
UCAM-CL-TR-264
ISSN 1476-2986
This paper investigates an extension of Milner’s CCS with agents
guarded by propositions on the environment. The agent g ≫ E,
pronounced E in an environment of which g holds, depends on the set
of actions the environment in ready to perform. This dependency is
realised by an operational semantics in which transitions carry
ready-sets (of the environment) as well as the normal action symbols
from CCS. A notion of strong bisimulation is defined on guarded
agents via this semantics. It is a congruence and satisfies new
equational laws (including a new expansion law) whicyh are shown to
be complete for finite guarded agents. The laws are conservative
over agents of traditional CCS. The guarding operator ≫ provides a
dynamic, local, and clean syntactic means of expressing the
behaviour of an agent depending on circumstance; it is more
expressive than the unless operator presented in [Cam91] and the
priority choice operator presented in [Cam90] and [CaW91], and
yields a much simpler expansion theorem.
Reasoning with inductively defined relations in the HOL
theorem prover
Camilleri, Juanito
Melham, Tom
University of Cambridge, Computer Laboratory
1992-08
en
Text
UCAM-CL-TR-265
ISSN 1476-2986
Automatic exploitation of OR-parallelism in
Prolog
Klein, Carole
University of Cambridge, Computer Laboratory
1992-09
en
Text
UCAM-CL-TR-266
ISSN 1476-2986
A path through a search space can be defined by a sequence of
integers called an oracle. The Delphi machine consists of a network
of individual workstations co-operating to execute a Prolog program.
Using oracles, these machines automatically partition the search
space between them, thereby exploiting OR-parallelism. This report
provides a brief description of the tree-searching algorithms
(control strategies) implemented in the Delphi machine.
Untyped strictness analysis
Ernoult, Christine
Mycroft, Alan
University of Cambridge, Computer Laboratory
1992-10
en
Text
UCAM-CL-TR-267
ISSN 1476-2986
We re-express Hudak and Young’s higher-order strictness analysis for
the untyped λ-calculus in a conceptually simpler and more
semantically-based manner. We show our analysis to be a sound
abstraction of Hudak and Young’s whichis also complete in a sense we
make precise.
Network file server design for continuous media
Jardetzky, Paul W.
University of Cambridge, Computer Laboratory
1992-10
en
Text
UCAM-CL-TR-268
ISSN 1476-2986
This dissertation concentrates on issues related to the provision of
a network based storage facility for digital audio and video data.
The goal is to demonstrate that a distributed file service in
support of these media may be built without special purpose
hardware. The main objective is to identify those parameters that
affect file system performance and provide the criteria for making
desirable design decisions.
Optimising compilation
Mycroft, Alan
Norman, Arthur
University of Cambridge, Computer Laboratory
1992-10
en
Text
UCAM-CL-TR-269
ISSN 1476-2986
This report consists of pre-prints of two tutorial lectures on
optimising compilation to be presented at the Czechoslovak ‘SOFSEM
92’ conference. The first discusses optimising compilers based on
dataflow analysis for classical imperative languages like ‘C’. The
second turns attention to optimisation of lazy functional languages
by ‘strictness analysis’.
Part 1: Classical imperative languages
This tutorial considers the design of modern machine-independent
optimising compilers for classical (C-like) languages. We draw from
two sources (1) the literature and lectures by the authors at
Cambridge and (2) the ‘Norcroft’ compiler suite jointly constructed
by the authors.
Part 2: Lazy functional languages
This lecture considers the optimisation of functional programming
languages (particularly ‘lazy’ languages) based on ‘strictness
analysis’. Such optimisations alter evaluation order to allow more
efficient translation into von Neumann architecture or to increase
the parallelism in a sequentially specified system (such as that
implied by lazy-semantics).
Designing a universal name service
Ma, Chaoying
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-270
ISSN 1476-2986
Generally speaking, naming in computing systems deals with the
creation of object identifiers at all levels of system architecture
and the mapping among them. Two of the main purposes of having names
in computer systems are (a) to identify objects; (b) to accomplish
sharing. Without naming no computer system design can be done.
The rapid development in the technology of personal workstations and
computer communication networks has placed a great number of demands
on designing large computer naming systems. In this dissertation,
issues of naming in large distributed computing systems are
addressed. Technical aspects as well as system architecture are
examined. A design of a Universal Name Service (UNS) is proposed and
its prototype implementation is described. Three major issues on
designing a global naming system are studied. Firstly, it is
observed that none of the existing name services provides enough
flexibility in restructuring name spaces, more research has to be
done. Secondly it is observed that although using stale naming data
(hints) at the application level is acceptable in most cases as long
as it is detectable and recoverable, stronger naming data integrity
should be maintained to provide a better guarantee of finding
objects, especially when a high degree of availability is required.
Finally, configuring the name service is usually done in an ad hoc
manner, leading to unexpected interruptions or a great deal of human
intervention when the system is reconfigured. It is necessary to
make a systematic study of automatic configuration and
reconfiguration of name services.
This research is based on a distributed computing model, in which a
number of computers work cooperatively to provide the service. The
contributions include: (a) the construction of a Globally Unique
Directory Identifier (GUDI) name space. Flexible name space
restructuring is supported by allowing directories to be added to or
removed from the GUDI name space. (b) The definition of a two class
name service infrastructure which exploits the semantics of naming.
It makes the UNS replication control more robust, reliable as well
as highly available. (c) The identification of two aspects in the
name service configuration: one is concerned with the replication
configuration, and the other is concerned with the server
configuration. It is notable that previous work only studied these
two aspects individually but not in combination. A distinguishing
feature of the UNS is that both issues are considered at the design
stage and novel methods are used to allow dynamic service
configuration to be done automatically and safely.
Set theory as a computational logic: I. from foundations to
functions
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1992-11
en
Text
UCAM-CL-TR-271
ISSN 1476-2986
A logic for specification and verification is derived from the
axioms of Zermelo-Fraenkel set theory. The proofs are performed
using the proof assistant Isabelle. Isabelle is generic, supporting
several different logics. Isabelle has the flexibility to adapt to
variants of set theory. Its higher-order syntax supports the
definition of new binding operators. Unknowns in subgoals can be
instantiated incrementally. The paper describes the derivation of
rules for descriptions, relations and functions, and discusses
interactive proofs of Cantor’s Theorem, the Composition of
Homomorphisms challenge, and Ramsey’s Theorem. A generic proof
assistant can stand up against provers dedicated to particular
logics.
Interactive program derivation
Coen, Martin David
University of Cambridge, Computer Laboratory
1992-11
en
Text
UCAM-CL-TR-272
ISSN 1476-2986
As computer programs are increasingly used in safety critical
applications, program correctness is becoming more important; as the
size and complexity of programs increases, the traditional approach
of testing is becoming inadequate. Proving the correctness of
programs written in imperative languages is awkward; functional
programming languages, however, offer more hope. Their logical
structure is cleaner, and it is practical to reason about
terminating functional programs in an internal logic.
This dissertation describes the development of a logical theory
called TPT for reasoning about the correctness of terminating
functional programs, its implementation using the theorem prover
Isabelle, and its use in proving formal correctness. The theory
draws both from Martin-Löf’s work in type theory and Manna and
Waldinger’s work in program synthesis. It is based on classical
first-order logic, and it contains terms that represent classes of
behaviourally equivalent programs, types that denote sets of
terminating programs and well-founded orderings. Well-founded
induction is used to reason about general recursion in a natural way
and to separate conditions for termination from those for
correctness.
The theory is implemented using the generic theorem prover Isabelle,
which allows correctness proofs to be checked by machine and
partially automated using tactics. In particular, tactics for type
checking use the structure of programs to direct proofs. Type
checking allows both the verification and derivation of programs,
reducing specifications of correctness to sets of correctness
conditions. These conditions can be proved in typed first-order
logic, using well-known techniques of reasoning by induction and
rewriting, and then lifted up to TPT. Examples of program
termination are asserted and proved, using simple types. Behavioural
specifications are expressed using dependent types, and the
correctness of programs asserted and then proved. As a non-trivial
example, a unification algorithm is specified and proved correct by
machine.
The work in this dissertation clearly shows how a classical theory
can be used to reason about program correctness, how general
recursion can be reasoned about, and how programs can direct proofs
of correctness.
TouringMachines: an architecture for dynamic, rational,
mobile agents
Ferguson, Innes A.
University of Cambridge, Computer Laboratory
1992-11
en
Text
UCAM-CL-TR-273
ISSN 1476-2986
It is becoming widely accepted that neither purely reactive nor
purely deliberative control techniques are capable of producing the
range of behaviours required of intelligent computational or robotic
agents in dynamic, unpredictable, multi-agent worlds. We present a
new architecture for controlling autonomous, mobile agents –
building on previous work addressing reactive and deliberative
control methods. The proposed multi-layered control architecture
allows a resource-bounded, goal-directed agent to react promptly to
unexpected changes in its environment; at the same time it enables
the agent to reason predictively about potential conflicts by
constructing and projecting causal models or theories which
hypothesise other agents’ goals and intentions.
The line of research adopted is very much a pragmatic one. A single,
common architecture has been implemented which, being extensively
parametrized, allows an experimenter to study functionally- and
behaviourally-diverse agent configurations. A principal aim of this
research is to understand the role different functional capabilities
play in constraining an agent’s behaviour under varying
environmental conditions. To this end, we have constructed an
experimental testbed comprising a simulated multi-agent world in
which a variety of agent configurations and behaviours have been
investigated. Experience with the new control architecture is
described.
Of what use is a verified compiler specification?
Curzon, Paul
University of Cambridge, Computer Laboratory
1992-11
en
Text
UCAM-CL-TR-274
ISSN 1476-2986
Program verification is normally performed on source code. However,
it is the object code which is executed and so which ultimately must
be correct. The compiler used to produce the object code must not
introduce bugs. The majority of the compiler correctness literature
is concerned with the verification of compiler specifications rather
than executable implementations. We discuss different ways that
verified specifications can be used to obtain implementations with
varying degrees of security. In particular we describe how a
specification can be executed by proof. We discuss how this method
can be used in conjunction with an insecure production compiler so
as to retain security without slowing the development cycle of
application programs. A verified implementation of a compiler in a
high-level language is not sufficient to obtain correct object code.
The compiler must itself be compiled into a low level language
before it can be executed. At first sight it appears we need an
already verified compiler to obtain a secure low-level
implementation of a compiler. We describe how a low-level
implementation of a compiler can be securely obtained from a
verified compiler implementation.
Exploratory learning in the game of GO
Pell, Barney
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-275
ISSN 1476-2986
This paper considers the importance of exploration to game-playing
programs which learn by playing against opponents. The central
question is whether a learning program should play the move which
offers the best chance of winning the present game, or if it should
play the move which has the best chance of providing useful
information for future games. An approach to addressing this
question is developed using probability theory, and then implemented
in two different learning methods. Initial experiments in the game
of Go suggest that a program which takes exploration into account
can learn better against a knowledgeable opponent than a program
which does not.
METAGAME: a new challenge for games and learning
Pell, Barney
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-276
ISSN 1476-2986
In most current approaches to Computer Game-Playing, including those
employing some form of machine learning, the game analysis mainly is
performed by humans. Thus, we are sidestepping largely the
interesting (and difficult) questions. Human analysis also makes it
difficult to evaluate the generality and applicability of different
approaches.
To address these problems, we introduce a new challenge: Metagame.
The idea is to write programs which take as input the rules of a set
of new games within a pre-specified class, generated by a program
which is publicly available. The programs compete against each other
in many matches on each new game, and they can then be evaluated
based on their overall performance and improvement through
experience.
This paper discusses the goals, research areas, and general concerns
for the idea of Metagame.
METAGAME in symmetric chess-like games
Pell, Barney
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-277
ISSN 1476-2986
I have implemented a game generator that generates games from a wide
but still restricted class. This class is general enough to include
most aspects of many standard games, including Chess, Shogi, Chinese
Chess, Checkers, Draughts, and many variants of Fairy Chess. The
generator, implemented in Prolog is transparent and publicly
available, and generates games using probability distributions for
parameters such as piece complexity, types of movement, board size,
and locality.
The generator is illustrated by means of a new game it produced,
which is then subjected to a simple strategic analysis. This form of
analysis suggests that programs to play Metagame well will either
learn or apply very general game-playing principles. But because the
class is still restricted, it may be possible to develop a naive but
fast program which can outplay more sophisticated opponents.
Performance in a tournament between programs is the deciding
criterion.
A formalization of the process algebra CCS in high order
logic
Nesi, Monica
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-278
ISSN 1476-2986
This paper describes a mechanization in higher order logic of the
theory for a subset of Milner’s CCS. The aim is to build a sound and
effective tool to support verification and reasoning about process
algebra specifications. To achieve this goal, the formal theory for
pure CCS (no value passing) is defined in the interactive theorem
prover HOL, and a set of proof tools, based on the algebraic
presentation of CCS, is provided.
The transition assertions specification method
Carreño, Victor A.
University of Cambridge, Computer Laboratory
1992-12
en
Text
UCAM-CL-TR-279
ISSN 1476-2986
A modelling and specification method for real-time, reactive systems
is described. Modelling is performed by constructing time dependent
relations of the system parameters. A textual formal notation using
higher order logic and a graphical notation are presented. The
formal notation allows the use of rigorous mathematical methods on
the specification, one of the primary sources of design errors. A
cruise control case example is included in the paper and the HOL
mechanised theorem prover is used to show that the specification
comply with some top level requirements.
Introduction to Isabelle
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1993-01
en
Text
UCAM-CL-TR-280
ISSN 1476-2986
Isabelle is a generic theorem prover, supporting formal proof in a
variety of logics. Through a variety of examples, this paper
explains the basic theory demonstrates the most important commands.
It serves as the introduction to other Isabelle documentation.
Pegasus project description
Mullender, Sape J.
Leslie, Ian M.
McAuley, Derek
University of Cambridge, Computer Laboratory
1992-09
en
Text
UCAM-CL-TR-281
ISSN 1476-2986
Pegasus – Operating system support for distributed
multimedia systems
Leslie, Ian M.
McAuley, Derek
Mullender, Sape J.
University of Cambridge, Computer Laboratory
1992-12
en
Text
UCAM-CL-TR-282
ISSN 1476-2986
The Isabelle reference manual
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1993-02
en
Text
UCAM-CL-TR-283
ISSN 1476-2986
This manual is a comprehensive description of Isabelle, including
all commands, functions and packages. It is intended for reference
rather than for reading through, and is certainly not a tutorial.
The manual assumes familiarity with the basic concepts explained in
Introduction to Isabelle. Functions are organized by their purpose,
by their operands (subgoals, tactics, theorems), and by their
usefulness. In each section, basic functions appear first, then
advanced functions, and finally esoteric functions.
The Alvey Natural Language Tools grammar (4th
Release)
Grover, Claire
Carroll, John
Briscoe, Ted
University of Cambridge, Computer Laboratory
1993-01
en
Text
UCAM-CL-TR-284
ISSN 1476-2986
Functional programming and input/output
Gordon, Andrew Donald
University of Cambridge, Computer Laboratory
1993-02
en
Text
UCAM-CL-TR-285
ISSN 1476-2986
Isabelle’s object-logics
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1993-02
en
Text
UCAM-CL-TR-286
ISSN 1476-2986
Several logics come with Isabelle. Many of them are sufficiently
developed to serve as comfortable reasoning environments. They are
also good starting points for defining new logics. Each logic is
distributed with sample proofs, some of which are presented in the
paper. The logics described include first-order logic,
Zermelo-Fraenkel set theory, higher-order logic, constructive type
theory, and the classical sequent calculus LK. A final chapter
explains the fine points of defining logics in Isabelle.
A mechanised definition of Silage in HOL
Gordon, Andrew D.
University of Cambridge, Computer Laboratory
1993-02
en
Text
UCAM-CL-TR-287
ISSN 1476-2986
If formal methods of hardware verification are to have any impact on
the practices of working engineers, connections must be made between
the languages used in practice to design circuits, and those used
for research into hardware verification. Silage is a simple dataflow
language marketed for specifying digital signal processing circuits.
Higher Order Logic (HOL) is extensively used for research into
hardware verification. This paper presents a formal definition of a
substantial subset of Silage, by mapping Silage declarations into
HOL predicates. The definition has been mechanised in the HOL
theorem prover to support the transformational design of Silage
circuits as theorem proving in HOL.
Cut-free sequent and tableau systems for propositional
Diodorean modal logics
Gore, Rajeev
University of Cambridge, Computer Laboratory
1993-02
en
Text
UCAM-CL-TR-288
ISSN 1476-2986
We present sound, (weakly) complete and cut-free tableau systems for
the propositional normal modal logics S4.3, S4.3.1 and S4.14. When
the modality □ is given a temporal interpretation, these logics
respectively model time as a linear dense sequence of points; as a
linear discrete sequence of points; and as a branching tree where
each branch is a linear discrete sequence of points.
Although cut-free, the last two systems do not possess the
subformula property. But for any given finite set of formulae X the
“superformulae” involved are always bounded by a finite set of
formulae X*L depending only on X and the logic L. Thus each system
gives a nondeterministic decision procedure for the logic in
question. The completeness proofs yield deterministic decision
procedures for each logic because each proof is constructive.
Each tableau system has a cut-free sequent analogue proving that
Gentzen’s cut-elimination theorem holds for these logics. The
techniques are due to Hintikka and Rautenberg.
The semantics of noun phrase anaphora
Elworthy, David Alan Howard
University of Cambridge, Computer Laboratory
1993-02
en
Text
UCAM-CL-TR-289
ISSN 1476-2986
Anaphora is a linguistic phenomenon in which one expression, called
an anaphor, gains some or all of its meaning from another, its
antecedent. In this thesis, I study the semantics of one particular
sort of anaphor, where both antecedent and anaphor are noun phrases.
Most research in the past has dealt with singular anaphora; I also
address plurals.
The two major theories of anaphora are Kamp’s Discourse
Representation Theory (DRT) and dynamic logics. While they have
yielded many valuable insights into the phenomenon, I think it is
time to subject them to some critical scrutiny. There are two main
criticisms. Firstly, the interpretation assigned to the lingustic
data is not always consistent with language users’ intuitions about
it. Secondly the current theories employ semantic formalisms which
rely on either specific representational devices or on
unconventional logics. I develop a new theory, TAI (Theory of
Anaphoric Information), which attempts to rectify both problems.
This thesis starts with a critical re-examination of the linguistic
data, and in particular of the so-called “donkey sentences”, which
exhibit complex interactions between quantification and anaphora.
The following chapter examines DRT and dynamic logics in some
detail, considering their successes and failings from both empirical
and methodological perspectives.
TAI itself is presented in chapter 4. The theory starts from a
conceptual model, which specifies the information needed to
interpret anaphors correctly. A logic, L(GQA), is then developed,
which derives both truth conditions and constraints on the anaphoric
information from formulae derived from natural language sentences.
The logic is static and does not rely on structured representations
of the sort found in DRT. The translation procedure from linguistic
input to L(GQA) formulae captures a significant part of the
emprirical weight of the theory, and provides sufficient flexibility
to make the required range of readings available.
The last chapter evaluates TAI from a variety of standpoints. The
conceptual model is used as a baseline for comparing DRT, dynamic
logics and TAI. The relation between semantic logics of TAI and
pragmatic aspects of interpreting anaphors is considered.
Computational aspects of TAI are also examined: how it relates to
Webber’s theory of anaphora, and how the logic could be implemented
efficiently. Finally, some directions in which research based on TAI
could proceed are identified.
Discourse modelling for automatic summarising
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1993-02
en
Text
UCAM-CL-TR-290
ISSN 1476-2986
Automatic abstracting is a challenging task for natural language
processing. It depends not only on sentence interpretation and the
local context representation this requires, but also on the
recognition and use of large-scale discourse structure. This paper
describes research investigating the nature of different approaches
to discourse representation and their value for summarising. This
work is focussed on comparative analysis, illustrated in the paper
through the provision of different forms of representation, and
different strategies for summary formation, for a short example
text.
Evaluating natural language processing systems
Galliers, J.R.
Spärck Jones, K.
University of Cambridge, Computer Laboratory
1993-02
en
Text
UCAM-CL-TR-291
ISSN 1476-2986
This report presents a detailed analysis and review of NLP
evaluation, in principle and in practice. Part 1 examines evaluation
concepts and establishes a framework for NLP system evaluation. This
makes use of experience in the related area of information retrieval
and the analysis also refers to evaluation in speech processing.
Part 2 surveys significant evaluation work done so far, for instance
in machine translation, and discusses the particular problems of
generic system evaluation. The conclusion is that evaluation
strategies and techniques for NLP need much more development, in
particular to take proper account of the influence of system tasks
and settings. Part 3 develops a general approach to NLP evaluation,
aimed at methodologically-sound strategies for test and evaluation
motivated by comprehensive performance factor identification. The
analysis throughout the report is supported by extensive
illustrative examples.
Synchronisation services for digital continuous
media
Sreenan, Cormac John
University of Cambridge, Computer Laboratory
1993-03
en
Text
UCAM-CL-TR-292
ISSN 1476-2986
The development of broadband ATM networking makes it attractive to
use computer communication networks for the transport of digital
audio and motion video. Coupled with advances in workstation
technology, this creates the opportunity to integrate these
continuous information media within a distributed computing system.
Continuous media have an inherent temporal dimension, resulting in a
set of synchronisation requirements which have real-time
constraints. This dissertation identifies the role and position of
synchronisation, in terms of the support which is necessary in an
integrated distributed system. This work is supported by a set of
experiments which were performed in an ATM inter-network using
multi-media workstations, each equipped with an Olivetti Pandora
Box.
Objects and transactions for modelling distributed
applications: concurrency control and commitment
Bacon, Jean
Moody, Ken
University of Cambridge, Computer Laboratory
1993-04
en
Text
UCAM-CL-TR-293
ISSN 1476-2986
The concepts of object and transaction form an ideal basis for
reasoning about the behaviour of distributed applications. An object
model allows the semantics of an application to be used to specify
the required concurrency behaviour of each object. A transaction
model covers multi-component computations where the components are
distributed and therefore subject to concurrent execution and
partial failure.
This tutorial establishes an object model for a distributed system
in which transactions are used. It focusses on the alternative
methods of concurrency control that might be employed and shows how
each method might be appropriate for certain application
characteristics and system behaviour. The background for this
discussion is eatablished in [Bacon 1993].
OPERA : Storage, programming and display of multimedia
objects
Moody, Ken
Bacon, Jean
Adly, Noha
Afshar, Mohamad
Bates, John
Feng, Huang
Hayton, Richard
Lo, Sai Lai
Schwiderski, Scarlet
Sultana, Robert
Wu, Zhixue
University of Cambridge, Computer Laboratory
1993-04
en
Text
UCAM-CL-TR-294
ISSN 1476-2986
This project aims to support the interactive display of synchronised
multiple media types in workstation windows. This style of
application needs high speed ATM networks and suitable protocols and
operating systems; an infrastructure that exists at the University
of Cambridge Computer Laboratory. Above this infrastructure we have
designed and are building storage services (MSSA), a platform to
support the creation and display of multimedia presentations (IMP)
and a persistent programming language (PC++), for reliable and
convenient programming of multimedia applications. This paper gives
an overview of the work of the OPERA project in these three areas.
OPERA : Storage and presentation support for multimedia
applications in a distributed, ATM network environment
Bacon, Jean
Bates, John
Lo, Sai Lai
Moody, Ken
University of Cambridge, Computer Laboratory
1993-04
en
Text
UCAM-CL-TR-295
ISSN 1476-2986
We are building a display platform for multimedia applications above
a multi-service storage architecture (MSSA). This style of
application needs high speed ATM networks and suitable protocols and
operating systems; an infrastructure that exists at the University
of Cambridge Computer Laboratory.
An open storage architecture gives flexibility and extensibility.
Conventional files, audio, video and structured objects are
supported within a common architectural framework and composite
objects, such as a display representation, may have components of
any of these storage types. The two-level hierarchy of servers
provides storage media and a byte-segment abstraction at the low
level and a variety of abstractions at the high level. Quality of
service guarantees, which are essential for continuous media file
types, are supported by sessions and tickets. These are arranged via
the high level servers and used directly with the low level servers.
A platform for the creation and interactive display of multimedia
presentations (IMP) is being developed. A script language allows a
multimedia presentation to be specified in terms of objects, the
relationships between them and the (composite) events that drive it.
Presentation data is stored on the structured data service of MSSA
and component objects are stored on appropriate servers, and
accepted and retrieved at guaranteed rates. The presentation
requirements of an application are managed by applying a script to
the data representing the presentation to create a display.
A persistent programming language for multimedia databases
in the OPERA project
Wu, Z.
Moody, K.
Bacon, J.
University of Cambridge, Computer Laboratory
1993-04
en
Text
UCAM-CL-TR-296
ISSN 1476-2986
The advent of high bandwidth local area ATM networks has transformed
the potential of distributed computing systems. At the Computer
Laboratory we are moving towards a world in which multimedia
displays are managed by editing, browsing and composing tools [Bates
93]. The recently completed Pandora project [Hopper 90] has given us
some experience of multimedia applications, and an idea of their
scope.
We have developed a persistent programming language PC++ [Wu 93], an
extension of C++, to help programmers developing multimedia
applications to make use of the MSSA. In this paper we present the
design of PC++ and show how its special features meet the
requiremets to effectively manage data in a distributed, real-time,
context.
Categorical abstract machines for higher-order typed lambda
calculi
Ritter, Eike
University of Cambridge, Computer Laboratory
1993-04
en
Text
UCAM-CL-TR-297
ISSN 1476-2986
We define in this thesis categorical abstract machines for the
calculus of constructions, a special higher-order lambda-calculus.
We start with the derivation of categorical combinators, i.e. an
equational theory based on a categorical structure for the calculus.
It turns out that only a generalization of Ehrhard’s D-categories
can be used for this purpose; all other categorical structures
modelling the calculus yield only conditional equations or no
equations at all. Next we orient the equations to obtain reduction
rules. When we want to show that this reduction corresponds to
reduction in the calculus, we run into difficulties in proving
strong normalization. We can only show that any reduction that leads
first to a combinator corresponding to a weak head-normal form is
finite. These results are the key to formulate an eager and a lazy
strategy for the reduction of a combinator to its normal form.
We then construct abstract machines for the eager and lazy strategy.
Their correctness proof consists of an induction over the definition
of the reduction strategies. These machines specialize to the CAM
and Krivine’s machine in the first order case respectively. The
original construction of the CAM is based on cartesian closed
categories (CCCs). They model both environments and terms by
morphisms regardless of their conceptual difference, whereas the
D-categories separate these two notions. Hence the correspondence
between the D-categories and the abstract machines described in this
thesis is closer than that between the CAM and the CCCs. We also
obtain an abstract machine for type checking of these combinators,
which uses the above reduction machines. Preliminary tests suggest
that the abstract machines are quite efficient compared to other
implementations.
Multicast in the asynchronous transfer mode
environment
Doar, John Matthew Simon
University of Cambridge, Computer Laboratory
1993-04
en
Text
UCAM-CL-TR-298
ISSN 1476-2986
In future multimedia communication networks, the ability to
multicast information will be useful for many new and existing
services. This dissertation considers the design of multicast
switches for Asynchronous Transfer Mode (ATM) networks and proposes
one design based upon a slotted ring. Analysis and simulation
studies of this design are presented and details of its
implementation for an experimental ATM network (Project Fairisle)
are described, together with the modifications to the existing
multi-service protocol architecture necessary to provide multicast
connections. Finally, a short study of the problem of multicast
routing is presented, together with some simulations of the
long-term effect upon the routing efficiency of modifying the number
of destinations within a multicast group.
Pragmatic reasoning in bridge
Gamback, Bjorn
Rayner, Manny
Pell, Barney
University of Cambridge, Computer Laboratory
1993-04
en
Text
UCAM-CL-TR-299
ISSN 1476-2986
In this paper we argue that bidding in the game of Contract Bridge
can profitably be regarded as a micro-world suitable for
experimenting with pragmatics. We sketch an analysis in which a
“bidding system” is treated as the semantics of an artificial
language, and show how this “language”, despite its apparent
simplicity, is capable of supporting a wide variety of common speech
acts parallel to those in natural languages; we also argue that the
reason for the relatively unsuccessful nature of previous attempts
to write strong Bridge playing programs has been their failure to
address the need to reason explicitly about knowledge, pragmatics,
probabilities and plans. We give an overview of Pragma, a system
currently under development, which embodies these ideas in concrete
form, using a combination of rule-based inference, stochastic
simulation, and “neural-net” learning. Examples are given
illustrating the functionality of the system in its current form.
Formal verification of VIPER’s ALU
Wong, Wai
University of Cambridge, Computer Laboratory
1993-04
en
Text
UCAM-CL-TR-300
ISSN 1476-2986
This research describes the formal verification of an arithmetic
logic unit of the VIPER microprocessor. VIPER is one of the first
processors designed using formal methods. A formal model in HOL has
been created which models the ALU at two levels: on the higher
level, the ALU is specified as a function taking two 32-bit operands
and returning a result; on the lower level the ALU is implemented by
a number of 4-bit slices which should take the same operands and
return the same results. The ALU is capable of performing thirteen
different operations. A formal proof of functional equivalence of
these two levels has been completed successfully. The complete HOL
text of the ALU formal model and details of the proof procedures are
included in this report. It has demonstrated that the HOL system is
powerful and efficient enough to perform formal verification of
realistic hardware design.
The dual-level validation concurrency control
method
Wu, Zhixue
Moody, Ken
Bacon, Jean
University of Cambridge, Computer Laboratory
1993-06
en
Text
UCAM-CL-TR-301
ISSN 1476-2986
Atomic data types permit maximum concurrency among transations by
exploiting the semantics of object operations. Concurrency control
is needed to ensure both object level atomicity and transaction
level atomicity. It must be possible to regard each operation on an
object as elementary. Recovery methods for transactions which are
based on atomic objects must take into account that partial results
of a transaction might be seen by other transactions.
This paper presents, formalises and verifies a protocol called the
dual-level validation method which can be used to provide atomicity
for atomic data types. It is optimistic and has a number of
advantages over previous methods. It permits maximum concurrency at
the low level by allowing non-conflicting operations to be scheduled
concurrently. It allows applications to cope with very large objects
by supporting multi-granularity shadowing. Transaction recovery is
simple to implement. The method performs well, particularly when
different transactions are unlikely to access the same (sub)objects
concurrently. Finally, it is well suited to a distributed
environment since validation and commit are not implemented
atomically.
Logic programming for general game-playing
Pell, Barney
University of Cambridge, Computer Laboratory
1993-06
en
Text
UCAM-CL-TR-302
ISSN 1476-2986
Meta-Game Playing is a new approach to games in Artificial
Intelligence, where we construct programs to play new games in a
well-defined class, which are output by an automatic game generator.
As the specific games to be played are not known in advance, a
degree of human bias is eliminated, and playing programs are
required to perform any game-specific optimisations without human
assistance.
The attempt to construct a general game-playing program is made
difficult by the opposing goals of generality and efficiency. This
paper shows how application of standard techniques in
logic-programming (abstract interpretation and partial evaluation)
makes it possible to achieve both of these goals. Using these
techniques, we can represent the semantics of a large class of games
in a general and declarative way, but then have the program
transform this representation into a more efficient version once it
is presented with the rules of a new game. This process can be
viewed as moving some of the responsibility for game analysis (that
concerned with efficiency) from the researcher to the program
itself.
Drawing trees — a case study in functional
programming
Kennedy, Andrew
University of Cambridge, Computer Laboratory
1993-06
en
Text
UCAM-CL-TR-303
ISSN 1476-2986
This report describes the application of functional programming
techniques to a problem previously studied by imperative
programmers, that of drawing general trees automatically. We first
consider the nature of the problem and the ideas behind its
solution, independent of programming language implementation. The
functional language implementation is described in a bottom up style
starting with very general functions over trees and then narrowing
in on the particular tree layout algorithm. Its correctness is
considered informally. Finally we discuss the implementation’s
computational complexity and possible improvements.
Co-induction and co-recursion in higher-order
logic
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1993-07
en
Text
UCAM-CL-TR-304
ISSN 1476-2986
A theory of recursive and corecursive definitions has been developed
in higher-order logic (HOL) and mechanised using Isabelle. Least
fixedpoints express inductive data types such as strict lists;
greatest fixedpoints express co-inductive data types, such as lazy
lists. Well-founded recursion expresses recursive functions over
inductive data types; co-recursion expresses functions that yield
elements of co-inductive data types. The theory rests on a
traditional formalization of infinite trees. The theory is intended
for use in specification and verification. It supports reasoning
about a wide range of computable functions, but it does not
formalize their operational semantics and can express noncomputable
functions also. The theory is demonstrated using lists and lazy
lists as examples. The emphasis is on using co-recursion to define
lazy list functions, and on using co-induction to reason about them.
Strong normalisation for the linear term calculus
Benton, P.N.
University of Cambridge, Computer Laboratory
1993-07
en
Text
UCAM-CL-TR-305
ISSN 1476-2986
We provide a strong normalisation result for the linear term
calculus which was introduced in (Benton et al. 1992). Rather than
prove the result from first principles, we give a translation of
linear terms into terms in the second order polymorphic lambda
calculus (λ2) which allows the result to be proved by appealing to
the well known strong normalisation property of λ2. An interesting
feature of the translation is that it makes use of the λ2 coding of
a coinductive datatype as the translation of the !-types
(exponentials) of the linear calculus.
Recording HOL proofs
Wong, Wai
University of Cambridge, Computer Laboratory
1993-07
en
Text
UCAM-CL-TR-306
ISSN 1476-2986
This paper describes a text file format for recording HOL proofs. It
is intended to become an interface between HOL and proof checkers.
Modification to HOL-88 has been carried out to incorporate a proof
recorder to generate a proof file in this format. The usage of this
new feature is explained by a simple example. A more substantial
proof has been recorded, and benchmark data is presented here.
Natural language processing for information
retrieval
Lewis, David D.
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1993-07
en
Text
UCAM-CL-TR-307
ISSN 1476-2986
The paper summarizes the essential properties of document retrieval
and reviews both conventional practice and research findings, the
latter suggesting that simple statistical techniques can be
effective. It then considers the new opportunities and challenges
presented by the ability to search full text directly (rather than
e.g. titles and abstracts), and suggests appropriate approaches to
doing this, with a focus on the role of natural language processing.
The paper also comments on possible connections with data and
knowledge retrieval, and concludes by emphasizing the importance of
rigorous performance testing.
A case study of co-induction in Isabelle HOL
Frost, Jacob
University of Cambridge, Computer Laboratory
1993-08
en
Text
UCAM-CL-TR-308
ISSN 1476-2986
The consistency of the dynamic and static semantics for a small
functional programming language was informally proved by R. Milner
and M. Tofte. The notions of co-inductive definitions and the
associated principle of co-induction played a pivotal role in the
proof. With emphasis on co-induction, the work presented here deals
with the formalisation of this result in the higher-order logic of
the generic theorem prover Isabelle.
Strictness analysis of lazy functional programs
Benton, Peter Nicholas
University of Cambridge, Computer Laboratory
1993-08
en
Text
UCAM-CL-TR-309
ISSN 1476-2986
Strictness analysis is a compile-time analysis for lazy functional
languages. The information gained by a strictness analyser can be
used to improve code generation for both sequential and parallel
implementations of such languages.
After reviewing the syntax and semantics of a simply typed lambda
calculus with constants, we describe previous work on strictness
analysis. We then give a new formulation of higher order strictness
analysis, called strictness logic. This is inspired by previous work
on static analysis by non-standard type inference, and by work on
logic of domains. We investigate some proof theoretic and semantic
properties of our logic, and relate it to the conventional approach
using abstract interpretation. We also consider extending the logic
with disjunction.
We then describe how to extend the simply typed lambda calculus with
lazy algebraic datatyped. A new construction of lattices of
strictness properties of such datatypes is described. This arises
from the characterisation of the solutions to the recursive domain
equations associated with these types as initial algebras.
Next we consider first order (ML-style) ploymorphism and show how
Wadler’s ‘theorems for free’ parametricity results may be obtained
from a simple extension of the semantics of monomorphic language. We
then prove a polymorphic invariance result relating the derivable
strictness properties of different substitution instances of
polymorphic terms.
HARP: a hierarchical asynchronous replication protocol for
massively replicated systems
Adly, Noha
University of Cambridge, Computer Laboratory
1993-08
en
Text
UCAM-CL-TR-310
ISSN 1476-2986
This paper presents a new asynchronous replication protocol that is
especially suitable for wide area and mobile systems, and allows
reads and writes to occur at any replica. Updates reach other
replicas using a propagation scheme based on nodes organized into a
logical hierarchy. The hierarchical structure enables the scheme to
scale well for thousands of replicas, while ensuring reliable
delivery. A new service interface is proposed that provides
different levels of asynchrony, allowing strong consistency and weak
consistency to be integrated into the same framework. Further, due
to the hierarchical pattern of propagation, the scheme provides the
ability to locate replicas that are more up-to-date than others,
depending on the needs of various applications. Also, it allows a
selection from a number of reconciliation techniques based on
delivery order mechanisms. Restructuring operations are provided to
build and reconfigure the hierarchy dynamically without disturbing
normal operations. The scheme tolerates transmission failures and
network partitions.
A verified Vista implementation
Curzon, Paul
University of Cambridge, Computer Laboratory
1993-09
en
Text
UCAM-CL-TR-311
ISSN 1476-2986
We describe the formal verification of a simple compiler using the
HOL theorem proving system. The language and microprocessor
considered are a subset of the structured assembly language Vista,
and the Viper microprocessor, respectively. We describe how our work
is directly applicable to a family of languages and compilers and
discuss how the correctness theorem and verified compiler fit into a
wider context of ensuring that object code is correct. We first show
how the compiler correctness result can be formally combined with a
proof system for application programs. We then show how our verified
compiler, despite not being written in a traditional programming
language, can be used to produce compiled code. We also discuss how
a dependable implementation might be obtained.
Set theory for verification: II : Induction and
recursion
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1993-09
en
Text
UCAM-CL-TR-312
ISSN 1476-2986
A theory of recursive definitions has been mechanized in Isabelle’s
Zermelo-Fraenkel (ZF) set theory. The objective is to support the
formalization of particular recursive definitions for use in
verification, semantics proofs and other computational reasoning.
Inductively defined sets are expressed as least fixedpoints,
applying the Knaster-Tarski Theorem over a suitable set. Recursive
functions are defined by well-founded recursion and its derivatives,
such as transfinite recursion. Recursive data structures are
expressed by applying the Knaster-Tarski Theorem to a set that is
closed under Cartesian product and disjoint sum.
Worked examples include the transitive closure of a relation, lists,
variable-branching trees and mutually recursive trees and forests.
The Schröder-Bernstein Theorem and the soundness of propositional
logic are proved in Isabelle sessions.
Proof by pointing
Bertot, Yves
Kahn, Gilles
Théry, Laurent
University of Cambridge, Computer Laboratory
1993-10
en
Text
UCAM-CL-TR-313
ISSN 1476-2986
A number of very powerful and elegant computer programs to assist in
making formal proofs have been developed. While these systems
incorporate ever more sophisticated tactics, proofs that can be
carried out without any user directions are the exception. In this
paper we present a principle called proof by pointing that allows
the user to guide the proof process using the mouse in the
user-interface. This idea is widely applicable and has been
implemented by the authors in user-interfaces for several proof
development systems.
Practical unification-based parsing of natural
language
Carroll, John Andrew
University of Cambridge, Computer Laboratory
en
Text
UCAM-CL-TR-314
ISSN 1476-2986
The thesis describes novel techniques and algorithms for the
practical parsing of realistic Natural Language (NL) texts with a
wide-coverage unification-based grammar of English. The thesis
tackles two of the major problems in this area: firstly, the fact
that parsing realistic inputs with such grammars can be
computationally very expensive, and secondly, the observation that
many analyses are often assigned to an input, only one of which
usually forms the basis of the correct interpretation.
The thesis starts by presenting a new unification algorithm,
justifies why it is well-suited to practical NL parsing, and
describes a bottom-up active chart parser which employs this
unification algorithm together with several other novel processing
and optimisation techniques. Empirical results demonstrate that an
implementation of this parser has significantly better practical
performance than a comparable, state-of-the-art unification-based
parser. Next, techniques for computing an LR table for a large
unification grammar are described, a context free non-deterministic
LR parsing algorithm is presented which has better time complexity
than any previously reported using the same approach, and a
unification-based version is derived. In experiments, the
performance of an implementation of the latter is shown to exceed
both the chart parser and also that of another efficient LR-like
algorithm recently proposed.
Building on these methods, a system for parsing text taken from a
given corpus is described which uses probabilistic techniques to
identify the most plausible syntactic analyses for an input from the
often large number licensed by the grammar. New techniques
implemented include an incremental approach to semi-supervised
training, a context-sensitive method of scoring sub-analyses, the
accurate manipulation of probabilities during parsing, and the
identification of the highest ranked analyses without exhaustive
search. The system attains a similar success rate to approaches
based on context-free grammar, but produces analyses which are more
suitable for semantic processing.
The thesis includes detailed analyses of the worst-case space and
time complexities of all the main algorithms described, and
discusses the practical impact of the theoretical complexity
results.
Strategy generation and evaluation for meta-game
playing
Pell, Barney Darryl
University of Cambridge, Computer Laboratory
1993-11
en
Text
UCAM-CL-TR-315
ISSN 1476-2986
Meta-Game Playing (METAGAME) is a new paradigm for research in
game-playing in which we design programs to take in the rules of
unknown games and play those games without human assistance. Strong
performance in this new paradigm is evidence that the program,
instead of its human designer, has performed the analysis of each
specific game.
SCL-METAGAME is a concrete METAGAME research problem based around
the class of symmetric chess-like games. The class includes the
games of chess, checkers, noughts and crosses, Chinese-chess, and
Shogi. An implemented game generator produces new games in this
class, some of which are objects of interest in their own right.
METAGAMER is a program that plays SCL-METAGAME. The program takes as
input the rules of a specific game and analyses those rules to
construct for that game an efficient representation and an
evaluation function, both for use with a generic search engine. The
strategic analysis performed by the program relates a set of general
knowledge sources to the details of the particular game. Among other
properties, this analysis determines the relative value of the
different pieces in a given game. Although METAGAMER does not learn
from experience, the values resulting from its analysis are
qualitatively similar to values used by experts on known games, and
are sufficient to produce competitive performance the first time the
program actually plays each game it is given. This appears to be the
first program to have derived useful piece values directly from
analysis of the rules of different games.
Experiments show that the knowledge implemented in METAGAMER is
useful on games unknown to its programmer in advance of the
competition and make it seem likely that future programs which
incorporate learning and more sophisticated active-analysis
techniques will have a demonstrable competitive advantage on this
new problem. When playing the known games of chess and checkers
against humans and specialised programs, METAGAMER has derived from
more general principles some strategies which are familiar to
players of those games and which are hard-wired in many
game-specific programs.
The Compleat LKB
Copestake, Ann
University of Cambridge, Computer Laboratory
1993-08
en
Text
UCAM-CL-TR-316
ISSN 1476-2986
This report is a full description of the lexical knowledge base
system (LKB) and the representation language (LRL) developed on the
Esprit ACQUILEX project. The LKB system is designed to allow the
representation of multilingual lexical information in a way which
integrates lexical semantics with syntax and formal semantics. The
LRL is a typed feature structure language which makes it possible to
represent the lexicon as a highly structured object and to capture
relationships between individual word senses by (default)
inheritance and by lexical rules. The extension to multilingual
representation allows a concise and natural description of
translation mismatches. Most of this report consists of a detailed
formal description of the LRL — this is augmented with appendices
containing the user manual, an implementation outline and a
discussion of some of the algorithms used, and a bibliography of
papers which describe the LKB and its use within ACQUILEX. (Some of
this material has been published previously, but is included here to
make this report a convenient reference source.)
Femto-VHDL: the semantics of a subset of VHDL and its
embedding in the HOL proof assistant
Van Tassel, John Peter
University of Cambridge, Computer Laboratory
1993-11
en
Text
UCAM-CL-TR-317
ISSN 1476-2986
The design of digital devices now resembles traditional computer
programming. Components are specified in a specialised form of
programming language known as a Hardware Description Language.
Programs written in such languages are then executed to simulate the
behaviour of the hardware they describe. These simulations cannot be
exhaustive in most situations, so result in high, yet incomplete,
confidence that the proper behaviour has been achieved.
The formal analysis of programming languages provides ways of
mathematically proving properties of programs. These properties
apply to behaviours resulting from all possible inputs rather then
just a subset of them. The prerequisite for such an analysis is a
formal understanding of the semantics of the language.
The Very High Speed Hardware Description Language (VHDL) is
currently used to specify and simulate a wide range of digital
devices. The language has no formal mathematical semantics as part
of its definition, hence programs written in it have not been
amenable to formal analysis.
The work presented here defines a structural operational semantics
for a subset of VHDL. The semantics is then embedded in a mechanical
proof assistant. This mechanisation allows one not only to reason
about individual programs but also to express equivalences between
programs. Examples which highlight the methodology used in this
reasoning are provided as a series of case studies.
A method of program refinement
Grundy, Jim
University of Cambridge, Computer Laboratory
1993-11
en
Text
UCAM-CL-TR-318
ISSN 1476-2986
A method of specifying the desired behaviour of a computer program,
and of refining such specifications into imperative programs is
proposed. The refinement method has been designed with the intention
of being amenable to tool support, and of being applicable to
real-world refinement problems.
Part of the refinement method proposed involves the use of a style
of transformational reasoning called ‘window inference’. Window
inference is particularly powerful because it allows the information
inherent in the context of a subexpression to be used in its
transformation. If the notion of transformational reasoning is
generalised to include transformations that preserve relationships
weaker than equality, then program refinement can be regarded as a
special case of transformational reasoning. A generalisation of
window inference is described that allows non-equivalence preserving
transformations. Window inference was originally proposed
independently from, and as an alternative to, traditional styles of
reasoning. A correspondence between the generalised version of
window inference and natural deduction is described. This
correspondence forms the basis of a window inference tool that has
been built on top of the HOL theorem proving system.
This dissertation adopts a uniform treatment of specifications and
programs as predicates. A survey of the existing approaches to the
treatment of programs as predicates is presented. A new approach is
then developed based on using predicates of a three-valued logic.
This new approach can distinguish more easily between specifications
of terminating and nonterminating behaviour than can the existing
approaches.
A method of program refinement is then described by combining the
unified treatment of specifications and programs as three-valued
predicates with the window inference style of transformational
reasoning. The result is a simple method of refinement that is well
suited to the provision of tool support.
The method of refinement includes a technique for developing
recursive programs. The proof of such developments is usually
complicated because little can be assumed about the form and
termination properties of a partially developed program. These
difficulties are side-stepped by using a simplified meaning for
recursion that compels the development of terminating programs. Once
the development of a program is complete, the simplified meaning for
recursion is refined into the true meaning.
The dissertation concludes with a case study which presents the
specification and development of a simple line-editor. The case
study demonstrates the applicability of the refinement method to
real-world problems. The line editor is a nontrivial example that
contains features characteristic of large developments, including
complex data structures and the use of data abstraction. Examination
of the case study shows that window inference offers a convenient
way of structuring large developments.
A workstation architecture to support multimedia
Hayter, Mark David
University of Cambridge, Computer Laboratory
1993-11
en
Text
UCAM-CL-TR-319
ISSN 1476-2986
The advent of high speed networks in the wide and local area enables
multimedia traffic to be easily carried between workstation class
machines. The dissertation considers an architecture for a
workstation to support such traffic effectively. In addition to
presenting the information to a human user the architecture allows
processing to be done on continuous media streams.
The proposed workstation architecture, known as the Desk Area
Network (DAN), extends ideas from Asynchronous Transfer Mode (ATM)
networks into the end-system. All processors and devices are
connected to an ATM interconnect. The architecture is shown to be
capable of supporting both multimedia data streams and more
traditional CPU cache line traffic. The advocated extension of the
CPU cache which allows caching of multimedia data streams is shown
to provide a natural programming abstraction and a mechanism for
synchronising the processor with the stream.
A prototype DAN workstation has been built. Experiments have been
done to demonstrate the features of the architecture. In particular
the use of the DAN as a processor-to-memory interconnect is closely
studied to show the practicality of using ATM for cache line traffic
in a real machine. Simple demonstrations of the stream cache ideas
are used to show its utility in future applications.
A fixedpoint approach to implementing (co)inductive
definitions (updated version)
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1995-07
en
Text
UCAM-CL-TR-320
ISSN 1476-2986
Several theorem provers provide commands for formalizing recursive
datatypes or inductively defined sets. This paper presents a new
approach, based on fixedpoint definitions. It is unusually general:
it admits all monotone inductive definitions. It is conceptually
simple, which has allowed the easy implementation of mutual
recursion and other conveniences. It also handles coinductive
definitions: simply replace the least fixedpoint by a greatest
fixedpoint. This represents the first automated support for
coinductive definitions.
The method has been implemented in Isabelle’s formalization of ZF
set theory. It should be applicable to any logic in which the
Knaster-Tarski Theorem can be proved. The paper briefly describes a
method of formalizing non-well-founded data structures in standard
ZF set theory.
Examples include lists of n elements, the accessible part of a
relation and the set of primitive recursive functions. One example
of a coinductive definition is bisimulations for lazy lists.
Recursive datatypes are examined in detail, as well as one example
of a “codatatype”: lazy lists. The appendices are simple user’s
manuals for this Isabelle/ZF package.
Relational properties of domains
Pitts, Andrew M.
University of Cambridge, Computer Laboratory
1993-12
en
Text
UCAM-CL-TR-321
ISSN 1476-2986
New tools are presented for reasoning about properties of
recursively defined domains. We work within a general,
category-theoretic framework for various notions of ‘relation’ on
domains and for actions of domain constructors on relations. Freyd’s
analysis of recursive types in terms of a property of mixed
initiality/finality is transferred to a corresponding property of
invariant relations. The existence of invariant relations is proved
under completeness assumptions about the notion of relation. We show
how this leads to simpler proofs of the computational adequacy of
denotational semantics for functional programming languages with
user-declared datatypes. We show how the initiality/finality
property of invariant relations can be specialized to yield an
induction principle for admissible subsets of recursively defined
domains, generalizing the principle of structural induction for
inductively defined sets. We also show how the initiality/finality
property gives rise to the co-induction principle studied by the
author (in UCAM-CL-TR-252), by which equalities between elements of
recursively defined domains may be proved via an appropriate notion
of ‘bisimulation’.
Supporting distributed realtime computing
Li, Guangxing
University of Cambridge, Computer Laboratory
1993-12
en
Text
UCAM-CL-TR-322
ISSN 1476-2986
Computers have been used for realtime systems for almost 50 years.
However, it is only recently that computer research institutions are
becoming interested in realtime computing, realizing the
significance of realtime systems and their increasing practical
importance. Realtime systems engineering still faces many
challenges: current systems concepts and functions are unfavourable
for the development of a general and consistent framework for
realtime systems engineering. The realtime problem domain has also
been further complicated by the rapid spread of distributed
computing.
This dissertation is concerned with the design and construction of a
distributed system environment for supporting realtime applications.
The contributions range from high-level programming abstractions
down to an operating system kernel interface through the detailed
engineering tradeoffs required to create, implement, and integrate
the mechanisms within the environment. The contributions consist of
a realtime programming model, a timed RPC protocol, a temporal
synchronisation facility and empirical validations.
The realtime programming model provides a framework to facilitate
the enforcement of the stringent timing constraints found in
distributed realtime applications. The model incorporates tasks and
communication channels as its basic programming components. It
synthesises aspects of resource requirements, resource allocation
and resource scheduling into an object based programming paradigm.
The development of the timed RPC protocol allows a programmer to
express and enforce reasonable timing requirements (representing
different tradeoffs between consistency and strictness) with object
invocations.
The definition and infrastructure support of the timed automata to
provide a temporal synchronisation facility. This facility
contributes to the understanding of temporal synchronisations in a
distributed world.
A prototype implementation of the system environment has been
constructed and used to evaluate the feasibility of the
architectural concepts of the system.
Representing higher-order logic proofs in HOL
von Wright, J.
University of Cambridge, Computer Laboratory
1994-01
en
Text
UCAM-CL-TR-323
ISSN 1476-2986
When using a theorem prover based on classical logic, such as HOL
[2], we are generally interested in the facts that are proved (the
theorems) than in the way in which they were proved (the proofs).
However we may be interested in checking the correctness of the
proofs. Since machine-generated proofs are generaly very long we
need a computer program, a proof checker, to do this. However, we
would also want the correctness of the proof checker to be verified
formally. One way of doing this is by specifying it in a mechanised
logic (such as that of the HOL system) and then doing a correctness
proof in that logic. While this may seem circular, it is acceptable
provided we have a theory of proofs embedded in the logic.
This paper describes an attempt to formalise the notion of HOL
proofs within HOL. The aim is to be able to verify (inside HOL) that
what is claimed to be a proof really is a proof.
Verifying modular programs in HOL
von Wright, J.
University of Cambridge, Computer Laboratory
1994-01
en
Text
UCAM-CL-TR-324
ISSN 1476-2986
This paper describes a methodology for verifying imperative programs
that are modular, i.e., built using separately defined functions and
procedures.
The verification methodology is based on a simple programming
notation with a weak precondition semantics. This notation has been
semantically embedded in the HOL theorem prover [3] and a number of
laws have been derived from the semantics.
These semantic laws are used to prove the correctness of functional
procedures, by showing that a call to the procedure in question is
equivalent to a call to the corresponding function as it is defined
in the logic. This makes it possible to specify a program in an
essentially functional style, but the functions are then implemented
as imperative procedures (like user-defined functions in FORTRAN or
Pascal).
We also show how to define non-functional procedures and calls to
such procedures. Procedures may be recursive. Altogether, this gives
us a basis for mechanical verification of modular imperative
programs.
The temporal properties of English conditionals and
modals
Crouch, Richard
University of Cambridge, Computer Laboratory
1994-01
en
Text
UCAM-CL-TR-325
ISSN 1476-2986
This thesis deals with the patterns of temporal reference exhibited
by conditional and modal sentences in English, and specifically with
the way that past and present tenses can undergo deictic shift in
these contexts. This shifting behaviour has consequences both for
the semantics of tense and for the semantics of conditionals and
modality.
Asymmetries in the behaviour of the past and present tenses under
deictic shift are explained by positing a primary and secondary
deictic centre for tenses. The two deictic centres, the assertion
time and the verification time, are given independent motivation
through an information based view of tense. This holds that the
tense system not only serves to describe the way that the world
changes over time, but also the way that information about the world
changes. Information change takes place in two stages. First, it is
asserted that some fact holds. And then, either at the same time or
later, it is verified that is assertion is correct.
Typically, assertion and verification occur simultaneously, and most
sentences convey verified information. Modals and conditionals allow
delayed assertion and verification. “If A, then B” means roughly:
suppose you were now to assert A; if and when A is verified, you
will be in a position to assert B, and in due course this assertion
will also be verified. Since A and B will both be tensed clauses,
the shifting of the primary and secondary deictic centres leads to
shifted interpretations of the two clauses.
The thesis presents a range of temporal properties of indicative and
subjunctive conditionals that have not previously been discussed,
and shows how they can be explained. A logic is presented for
indicative conditionals, based around an extension of intuitionistic
logic to allow for both verified and unverified assertions. This
logic naturally gives rise to three forms of epistemic modality,
corresponding to “must”, “may” and “will”.
A modular and extensible network storage
architecture
Lo, Sai-Lai
University of Cambridge, Computer Laboratory
1994-01
en
Text
UCAM-CL-TR-326
ISSN 1476-2986
Most contemporary distributed file systems are not designed to be
extensible. This work asserts that the lack of extensibility is a
problem because:
– New data types, such as continuous-medium data and structured
data, are significantly different from conventional unstructured
data, such as text and binary, that contemporary distributed file
systems are built to support.
– Value-adding clients can provide functional enhancements, such as
convenient and reliable persistent programming and automatic and
transparent file indexing, but cannot be integrated smoothly with
contemporary distributed file systems.
– New media technologies, such as the optical jukebox and RAID disk,
can extend the scale and performance of a storage service but
contemporary distributed file systems do not have a clear framework
to incorporate these new technologies and to provide the necessary
user level transparency.
Motivated by these observations, the new network storage
architecture (MSSA) presented in this dissertation, is designed to
be extensible. Design modularity is taken as the key to achieve
service extensibility. This dissertation examines a number of issues
related to the design of the architecture. New ideas, such as a
flexible access control mechanism based on temporary capabilities, a
low level storage substrate that uses non-volatile memory to provide
atomic update semantics at high performance, a concept of sessions
to differentiate performance requirements of different data types,
are introduced. Prototype implementations of the key components are
evaluated.
A new application for explanation-based generalisation
within automated deduction
Baker, Siani L.
University of Cambridge, Computer Laboratory
1994-02
en
Text
UCAM-CL-TR-327
ISSN 1476-2986
Generalisation is currently a major theorem-proving problem. This
paper proposes a new method of generalisation, involving the use of
explanation-based generalisation within a new domain, which may
succeed when other methods fail. The method has been implemented for
simple arithmetical examples.
The formal verification of the Fairisle ATM switching
element: an overview
Curzon, Paul
University of Cambridge, Computer Laboratory
1994-03
en
Text
UCAM-CL-TR-328
ISSN 1476-2986
We give an overview of the formal verification of an implementation
of a self routing ATM switching element. This verification was
performed using the HOL90 theorem proving system so is fully machine
checked. The switching element is in use in a real network,
switching real data. Thus, this work constitutes a realistic formal
verification case study. We give an informal overview of the switch
and element and give a tutorial on the methods used. We outline how
these techniques were applied to verify the switching element. We
then discuss the time spent on the verification. This was comparable
to the time spent designing and testing the element. Finally we
describe the errors discovered.
The formal verification of the Fairisle ATM switching
element
Curzon, Paul
University of Cambridge, Computer Laboratory
1994-03
en
Text
UCAM-CL-TR-329
ISSN 1476-2986
We describe the formal verification of an implementation of the
switching element of the fairisle ATM switch. This verification was
performed using the HOL90 theorem proving system so is fully
machine-checked. We give here all the definitions used in the
verification together with the main correctness theorems proved.
Fairisle switches are in use in a working network, switching real
data. Thus, this work constitutes a realistic formal verification
case study.
Interacting with paper on the DigitalDesk
Wellner, Pierre David
University of Cambridge, Computer Laboratory
1994-03
en
Text
UCAM-CL-TR-330
ISSN 1476-2986
In the 1970’s Xerox PARC developed the “desktop metaphor,” which
made computers easy to use by making them look and act like ordinary
desks and paper. This led visionaries to predict the “paperless
office” would dominate within a few years, but the trouble with this
prediction is that people like paper too much. It is portable,
tactile, universally accepted, and easier to read than a screen.
Today, we continue to use paper, and computers produce more of it
than they replace.
Instead of trying to use computers to replace paper, the DigitalDesk
takes the opposite approach. It keeps the paper, but uses computers
to make it more powerful. It provides a Computer Augmented
Environment for paper.
The DigitalDesk is built around an ordinary physical desk and can be
used as such, but it has extra capabilities. A video camera is
mounted above the desk, pointing down at the work surface. This
camera’s output is fed through a system that can detect where the
user is pointing, and it can read documents that are placed on the
desk. A computer-driven electronic projector is also mounted above
the desk, allowing the system to project electronic objects onto the
work surface and onto real paper documents — something that can’t be
done with flat display panels or rear-projection. The system is
called DigitalDesk because it allows pointing with the fingers.
Several applications have been prototyped on the DigitalDesk. The
first was a calculator where a sheet of paper such as an annual
report can be placed on the desk allowing the user to point at
numbers with a finger or pen. The camera reads the numbers off the
paper, recognizes them, and enters them into the display for further
calculations. Another is a translation system which allows users to
point at unfamiliar French words to get their English definitions
projected down next to the paper. A third is a paper-based paint
program (PaperPaint) that allows users to sketch on paper using
traditional tools, but also be able to select and paste these
sketches with the camera and projector to create merged paper and
electronic documents. A fourth application is the DoubleDigitalDesk,
which allows remote colleagues to “share” their desks, look at each
other’s paper documents and sketch on them remotely.
This dissertation introduces the concept of Computer Augmented
Environments, describes the DigitalDesk and applications for it, and
discusses some of the key implementation issues that need to be
addressed to make this system work. It describes a toolkit for
building DigitalDesk applications, and it concludes with some more
ideas for future work.
HPP: a hierarchical propagation protocol for large scale
replication in wide area networks
Adly, Noha
Kumar, Akhil
University of Cambridge, Computer Laboratory
1994-03
en
Text
UCAM-CL-TR-331
ISSN 1476-2986
This paper describes a fast, reliable, scalable and efficient
propagation protocol for weak-consistency replica management. This
protocol can be used to implement a bulletin board service such as
the Usenet news on the Internet. It is based on organizing the nodes
in a network into a logical hierarchy, and maintaining a limited
amount of state information at each node. It ensures that messages
are not lost due to failures or partitions once they are repaired
and minimizes redundancy. Further the protocol allows messages to be
diffused while nodes are down provided the parent and child nodes of
a failed node are alive. Moreover the protocol allows nodes to be
moved in the logical hierarchy, and the network to be restructured
dynamically in order to improve performance while still ensuring
that no messages are lost while the switch takes place and without
disturbing normal operation.
Distributed computing with objects
Evers, David Martin
University of Cambridge, Computer Laboratory
1994-03
en
Text
UCAM-CL-TR-332
ISSN 1476-2986
Distributed systems and object-based programming are now beginning
to enter the mainstream of computing practice. These developments
have the potential to simplify the distributed application
programmer’s task considerably, but current systems impose
unnecessary burdens. Distributed operating systems provide palatable
message passing between remote processes but leave the preparation
and interpretation of the messages to application code. Remote
procedure call systems use familiar language-level concepts to hide
distribution, but the awkwardness of service creation and binding
discourages the use of transient objects. Finally, object-based
programming languages which support distribution often ignore the
possibility of failures and do not efficiently accommodate
heterogeneity.
This dissertation discusses the design, implementation and
evaluation of a practical system for network objects which addresses
these problems for a representative programming language (Modula-3)
and distributed computing environment (the ANSA testbench). We
propose that language level objects should explicitly represent
bindings to potentially remote access points (interfaces), which are
sufficiently lightweight that they can be used as transient handles
for shared state. Our system uses local objects to stand for remote
services and local method call to cause remote operation invocation.
Within a process, concurrency control is provided by familiar
language-level facilities. The local programming language’s object
type system is made to represent the global service type system in a
natural way. We support dynamic creation of service interfaces and
the transmission of network object references in invocations. We
allow the dynamic types of network object references to propagate
between separate programs. Finally we provide automatic,
fault-tolerant and efficient distributed garbage collection of
network objects. In each case, we discuss the requirements of a
useful design and the tradeoffs necessary in a real implementation.
Our implementation runs on stock systems connected by standard local
and wide area networks and internetworking protocols. We believe our
approach would support additional library-level tools for security,
stable storage, distributed transactions and transparent service
replication, though we have not pursued this.
The dissertation demonstrates that it is practical to retain many
important amenities of modern programming languages when providing
support for the construction of applications in a heterogeneous and
evolving distributed system.
What is a categorical model of intuitionistic linear
logic?
Bierman, G.M.
University of Cambridge, Computer Laboratory
1994-04
en
Text
UCAM-CL-TR-333
ISSN 1476-2986
This paper re-addresses the old problem of providing a categorical
model for Intuitionistic Linear Logic (ILL). In particular we
compare the new standard model proposed by Seely to the lesser known
one proposed by Benton, Bierman, Hyland and de Paiva. Surprisingly
we find that Seely’s model is unsound in that it does not preserve
equality of proofs — we shall give some examples of equal proofs
which do not seem to be modelled as equal morphisms in the category.
We shall propose how to adapt Seely’s definition so as to correct
these problems and consider how this compares with the model due to
Benton et al.
A concrete final coalgebra theorem for ZF set
theory
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1994-05
en
Text
UCAM-CL-TR-334
ISSN 1476-2986
A special final coalgebra theorem, in the style of Aczel (1988), is
proved within standard Zermelo-Fraenkel set theory. Aczel’s
Anti-Foundation Axiom is replaced by a variant definition of
function that admits non-well-founded constructions. Variant ordered
pairs and tuples, of possibly infinite length, are special cases of
variant functions. Analogues of Aczel’s Solution and Substitution
Lemmas are proved in the style of Rutten and Turi (1993).
The approach is less general than Aczel’s; non-well-founded objects
can be modelled only using the variant tuples and functions. But the
treatment of non-well-founded objects is simple and concrete. The
final coalgebra of a functor is its greatest fixedpoint. The theory
is intended for machine implementation and a simple case of it is
already implemented using the theorem prover Isabelle.
Video mail retrieval using voice: report on keyword
definition and data collection (deliverable report on VMR task
No. 1)
Jones, G.J.F.
Foote, J.T.
Spärck Jones, K.
Young, S.J.
University of Cambridge, Computer Laboratory
1994-04
en
Text
UCAM-CL-TR-335
ISSN 1476-2986
This report describes the rationale, design, collection and basic
statistics of the initial training and test database for the
Cambridge Video Mail Retrieval (VMR) project. This database is
intended to support both training for the wordspotting processes and
testing for the document searching methods using these that are
being developed for the project’s message retrieval task.
Towards a proof theory of rewriting: the simply-typed 2-λ
calculus
Hilken, Barnaby P.
University of Cambridge, Computer Laboratory
1994-05
en
Text
UCAM-CL-TR-336
ISSN 1476-2986
This paper describes the simply typed 2-λ-calculus, a language with
three levels, types, terms and rewrites. The types and terms are
those of the simply typed λ-calculus, and the rewrites are
expressions denoting sequences of β-reductions and η-expansions. An
equational theory is imposed on the rewrites, based on 2-categorical
justifications, and the word problem for this theory is solved by
finding a canonical expression in each equivalence class.
The canonical form of rewrites allows us to prove several properties
of the calculus, including a strong form of confluence and a
classification of the long-β-η-normal forms in terms of their
rewrites. Finally we use these properties as the basic definitions
of a theory of categorical rewriting, and find that the expected
relationships between confluence, strong normalisation and normal
forms hold.
Efficiency in a fully-expansive theorem prover
Boulton, Richard John
University of Cambridge, Computer Laboratory
1994-05
en
Text
UCAM-CL-TR-337
ISSN 1476-2986
The HOL system is a fully-expansive theorem prover: Proofs generated
in the system are composed of applications of the primitive
inference rules of the underlying logic. This has two main
advantages. First, the soundness of the system depends only on the
implementations of the primitive rules. Second, users can be given
the freedom to write their own proof procedures without the risk of
making the system unsound. A full functional programming language is
provided for this purpose. The disadvantage with the approach is
that performance is compromised. This is partly due to the inherent
cost of fully expanding a proof but, as demonstrated in this thesis,
much of the observed inefficiency is due to the way the derived
proof procedures are written.
This thesis seeks to identify sources of non-inherent inefficiency
in the HOL system and proposes some general-purpose and some
specialised techniques for eliminating it. One area that seems to be
particularly amenable to optimisation is equational reasoning. This
is significant because equational reasoning constitutes large
portions of many proofs. A number of techniques are proposed that
transparently optimise equational reasoning. Existing programs in
the HOL system require little or no modification to work faster.
The other major contribution of this thesis is a framework in which
part of the computation involved in HOL proofs can be postponed.
This enables users to make better use of their time. The technique
exploits a form of lazy evaluation. The critical feature is the
separation of the code that generates the structure of a theorem
from the code that justifies it logically. Delaying the
justification allows some non-local optimisations to be performed in
equational reasoning. None of the techniques sacrifice the security
of the fully-expansive approach.
A decision procedure for a subset of the theory of linear arithmetic
is used to illustrate many of the techniques. Decision procedures
for this theory are commonplace in theorem provers due to the
importance of arithmetic reasoning. The techniques described in the
thesis have been implemented and execution times are given. The
implementation of the arithmetic procedure is a major contribution
in itself. For the first time, users of the HOL system are able to
prove many arithmetic lemmas automatically in a practical amount of
time (typically a second or two).
The applicability of the techniques to other fully-expansive theorem
provers and possible extensions of the ideas are considered.
A new approach to implementing atomic data types
Wu, Zhixue
University of Cambridge, Computer Laboratory
1994-05
en
Text
UCAM-CL-TR-338
ISSN 1476-2986
Many researchers have suggested the atomic data type approach to
maintaining data consistency in a system. In this approach atomicity
is ensured by the data objects that are shared by concurrent
activities. By using the semantics of the operations of the shared
objects, greater concurrency among activities can be permitted. In
addition, by encapsulating synchronisation and recovery in the
implementation of the shared objects, modularity can be enhanced.
Existing systems support user-defined atomic data types in an
explicit approach. They either permit limited semantics to be
presented thus providing less concurrency, or permit a high level of
semantics to be presented but in an encapsulated way, thus resulting
in a complicated implementation. This research was done to make the
implementation of user-defined atomic data types simple, efficient,
while still permitting great concurrency.
The research aims to lessen the programmer’s burden by supporting an
implicit approach for implementing atomic data types. It permits a
high level of semantics to be specified in a declarative way, which
makes the implementation of user defined atomic data types as simple
as in a sequential environment. A special concurrency control
mechanism is implemented by the system. By using type inheritance,
user-defined atomic data types can use the mechanism directly to
provide local atomicity for their objects. A language has been
developed for specifying the conflicts between object operations.
Since the concurrency control mechanism can take operation semantics
into account, the approach permits great concurrency.
To support the implicit approach, an appropriate concurrency control
protocol must be proposed which can take advantage of operation
semantics to increase concurrency and which can be implemented
independently from user-defined atomic data types. Such a protocol,
called the dual-level validation method, is presented and verified
in this thesis. The method can make use of the parameters and
results of object operations to achieve great concurrency. In
addition, it also provides great internal concurrency by permitting
operations to take place on an object concurrently.
The prototyping of the implicit approach in a persistent programming
language called PC++ is described. The feasibility of the approach
is shown by an application, namely a naming database for an active
badge system. Some related issues are also addressed in the thesis
such as remote object invocation, distributed transaction commitment
and data persistence.
Belief revision and dialogue management in information
retrieval
Logan, Brian
Reece, Steven
Cawsey, Alison
Galliers, Julia
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1994-05
en
Text
UCAM-CL-TR-339
ISSN 1476-2986
This report describes research to evaluate a theory of belief
revision proposed by Galliers in the context of information-seeking
interaction as modelled by Belkin, Brooks and Daniels and
illustrated by user-librarian dialogues. The work covered the
detailed assessment and development, and computational
implementation and testing, of both the belief revision theory and
the information retrieval model. Some features of the belief theory
presented problems, and the original ‘multiple expert’ retrieval
model had to be drastically modified to support rational dialogue
management. But the experimental results showed that the
characteristics of literature seeking interaction could be
successfully captured by the belief theory, exploiting important
elements of the retrieval model. Thus, though the system’s knowledge
and dialogue performance were very limited, it provides a useful
base for further research. The report presents all aspects of the
research in detail, with particular emphasis on the implementation
of belief and intention revision, and the integration of revision
with domain reasoning and dialogue interaction.
Operating system support for quality of service
Hyden, Eoin Andrew
University of Cambridge, Computer Laboratory
1994-06
en
Text
UCAM-CL-TR-340
ISSN 1476-2986
The deployment of high speed, multiservice networks within the local
area has meant that it has become possible to deliver continuous
media data to a general purpose workstation. This, in conjunction
with the increasing speed of modern microprocessors, means that it
is now possible to write application programs which manipulate
continuous media in real-time. Unfortunately, current operating
systems do not provide the resource management facilities which are
required to ensure the timely execution of such applications.
This dissertation presents a flexible resource management paradigm,
based on the notion of Quality of Service, with which it is possible
to provide the scheduling support required by continuous media
applications. The mechanisms which are required within an operating
system to support this paradigm are described, and the design and
implementation of a prototypical kernel which implements them is
presented.
It is shown that, by augmenting the interface between an application
and the operating system, the application can be informed of varying
resource availabilities, and can make use of this information to
vary the quality of its results. In particular an example decoder
application is presented, which makes use of such information and
exploits some of the fundamental properties of continuous media data
to trade video image quality for the amount of processor time which
it receives.
Presentation support for distributed multimedia
applications
Bates, John
University of Cambridge, Computer Laboratory
1994-06
en
Text
UCAM-CL-TR-341
ISSN 1476-2986
Distributed computing environments can now support digital
continuous media (such as audio and video) in addition to still
media (such as text and pictures). The work presented in this
dissertation is motivated by the desire of application developers to
create applications which utilise these multimedia environments.
Many important application areas are emerging such as Computer-Aided
Instruction (CAI) and Computer-Supported Cooperative Working (CSCW).
Building multimedia applications is currently a difficult and time
consuming process. At run-time, an application must manage
connections to a range of heterogeneous sevices to access data.
Building applications directly on top of environment specific
features roots them to those features. Continuous media introduces
new problems into application management such as control of Quality
of Service (QoS) and synchronisation of data items. An application
may also be required to analyse, process or display data. Some
multimedia applications are event-driven, i.e. they must perform
actions in response to asynchronous run-time occurrences. They may
also be required to control many workspaces and involve multiple
users.
The thesis of this dissertation is based on two principles. Firstly,
despite the heterogeneity between and within multimedia
environments, that their functionality should be provided in a
uniform way to application developers. By masking the control
differences with generic abstractions, applications can easily be
developed and ported. Secondly, that it is possible to develop such
abstractions to support a wide range of multimedia applications.
Extensible and configurable facilities can be provided to access,
and present multimedia data and to support event-diven applications
including cooperative ones.
The approach taken in this work is to provide a presentation support
platform. To application developers this platform offers an
authoring interface based on data modelling and specification using
a script language. Using these facilities, the parts of an
application involving interactive presentation of multimedia can be
specified. Services have been built to support the run-time
realisation of authored presentations on top of environments.
Experiments show that a wide range of applications can be supported.
An architecture for distributed user interfaces
Freeman, Stephen Martin Guy
University of Cambridge, Computer Laboratory
1994-07
en
Text
UCAM-CL-TR-342
ISSN 1476-2986
Computing systems have changed rapidly since the first graphical
user interfaces were developed. Hardware has become faster and
software architectures have become more flexible and more open; a
modern computing system consists of many communicating machines
rather than a central host. Understanding of human-computer
interaction has also become more sophisticated and places new
demands on interactive software; these include, in particular,
support for multi-user applications, continuous media, and
‘ubiquitous’ computing. The layer which binds user requirements and
computing systems together, the user interface, has not changed as
quickly; few user interface architectures can easily supportthe new
requirements placed on them and few take advantage of the facilities
offered by advanced computing systems.
Experiences of implementing systems with unusual user interfaces has
shown that current window system models are only a special case of
possible user interface architectures. These window systems are too
strongly tied to assumptions about how users and computers interact
to provide a suitable platform for further evolution. Users and
application builders may reasonably expect to be able to use
multiple input and output devices as their needs arise. Experimental
applications show that flexible user interface architectures, which
support multiple devices and users, can be built without excessive
implementation and processing costs.
This dissertation describes Gemma, a model for a new generation of
interactive systems that are not confined to virtual terminals but
allows collections of independent devices to be bound together for
the task at hand. It provides mediated shared access to basic
devices and higher-level virtual devices so that people can share
computational facilities in the real world, rather than in a virtual
world. An example window system shows how these features may be
exploited to provide a flexible, collaborative and mobile
interactive environment.
The contour tree image encoding technique and file
format
Turner, Martin John
University of Cambridge, Computer Laboratory
1994-07
en
Text
UCAM-CL-TR-344
ISSN 1476-2986
The process of contourization is presented which converts a raster
image into a discrete plateaux of contours. These contours can be
grouped into a hierarchical structure, defining total spacial
inclusion called a contour tree. A contour coder has been developed
which fully describes these contours in a compact and efficient
manner and is the basis for an image compression method.
Simplification of the contour tree has been undertaken by merging
contour tree nodes thus lowering the contour tree’s entropy. This
can be exploited by the contour coder to increase the image
compression ratio. By applying general and simple rules derived from
physiological experiments on the human vision system, lossy image
compression can be achieved which minimises noticable artifacts in
the simplified image.
The contour merging technique offers a complementary lossy
compression system to the QDCT (Quantised Discrete Cosine
Transform). The artifacts introduced by the two methods are very
different; QDCT produces a general blurring and adds extra
highlights in the form of overshoots, whereas contour merging
sharpens edges, reduces highlights and introduces a degree of false
contouring.
A format based on the contourization technique which caters for most
image types is defined, called the contour tree image format. Image
operations directly on this compressed format have been studied
which for certain manipulations can offer significant operational
speed increases over using a standard raster image format. A couple
of examples of operations specific to the contour tree format are
presented showing some of the features of the new format.
A proof environment for arithmetic with the Omega
rule
Baker, Siani L.
University of Cambridge, Computer Laboratory
1994-08
en
Text
UCAM-CL-TR-345
ISSN 1476-2986
An important technique for investigating the derivability in formal
systems of arithmetic has been to embed such systems into
semi-formal systems with the ω-rule. This paper exploits this notion
within the domain of automated theorem-proving and discusses the
implementation of such a proof environment, namely the CORE system
which implements a version of the primitive recursive ω-rule. This
involves providing an appropriate representation for infinite
proofs, and a means of verifying properties of such objects. By
means of the CORE system, from a finite number of instances a
conjecture of the proof of the universally quantified formula is
automatically derived by an inductive inference algorithm, and
checked for correctness. In addition, candidates for cut formulae
may be generated by an explanation-based learning algorithm. This is
an alternative approach to reasoning about inductively defined
domains from traditionas structural induction, which may sometimes
be more intuitive.
On intuitionistic linear logic
Bierman, G.M.
University of Cambridge, Computer Laboratory
1994-08
en
Text
UCAM-CL-TR-346
ISSN 1476-2986
In this thesis we carry out a detailed study of the (propositional)
intuitionistic fragment of Girard’s linear logic (ILL). Firstly we
give sequent calculus, natural deduction and axiomatic formulations
of ILL. In particular our natural deduction is different from others
and has important properties, such as closure under substitution,
which others lack. We also study the process of reduction in all
three local formulations, including a detailed proof of cut
elimination. Finally, we consider translations between
Instuitionistic Logic (IL) and ILL.
We then consider the linear term calculus, which arises from
applying the Curry-Howard correspondence to the natural deduction
formulation. We show how the various proof theoretic formulations
suggest reductions at the level of terms. The properties of strong
normalization and confluence are proved for these reduction rules.
We also consider mappings between the extended λ-calculus and the
linear term calculus.
Next we consider a categorical model for ILL. We show how by
considering the linear term calculus as an equational logic, we can
derive a model: a linear category. We consider two alternative
models: firstly, one due to Seely and then one due to Lafont.
Surprisingly, we find that Seely’s model is not sound, in that equal
terms are not modelled with equal morphisms. We show how after
adapting Seely’s model (by viewing it in a more abstract setting) it
becomes a particular instance of a linear category. We show how
Lafont’s model can also be seen as another particular instance of a
linear category. Finally we consider various categories of
coalgebras, whose construction can be seen as a categorical
equivalent of the translation of IL into ILL.
Reflections on TREC
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
1994-07
en
Text
UCAM-CL-TR-347
ISSN 1476-2986
This paper discusses the Text REtrieval Conferences (TREC) programme
as a major enterprise in information retrieval research. It reviews
its structure as an evaluation exercise, characterises the methods
of indexing and retrieval being tested within it in terms of the
approaches to system performance factors these represent; analyses
the test results for solid, overall conclusions that can be drawn
from them; and, in the light of the particular features of the test
data, assesses TREC both for generally-applicable findings that
emerge from it and for directions it offers for future research.
Integrated sound synchronisation for computer
animation
Hunter, Jane Louise
University of Cambridge, Computer Laboratory
1994-08
en
Text
UCAM-CL-TR-348
ISSN 1476-2986
A HOL interpretation of Noden
Graham, Brian
University of Cambridge, Computer Laboratory
1994-09
en
Text
UCAM-CL-TR-349
ISSN 1476-2986
Ten commandments of formal methods
Bowen, Jonathan P.
Hinchey, Michael G.
University of Cambridge, Computer Laboratory
1994-09
en
Text
UCAM-CL-TR-350
ISSN 1476-2986
The formal methods community is in general very good at undertaking
research into the mathematical aspects of formal methods, but not so
good at promulgating the use of formal methods in an engineering
environment and at an industrial scale. Technology transfer is an
extremely important part of the overall effort necessary in the
acceptance of formal techniques. This paper explores some of the
more informal aspects of applying formal methods and presents some
maxims with associated discussion that may help in the application
of formal methods in an industrial setting. A significant
bibliography is included providing pointers to more technical and
detailed aspects.
Handling realtime traffic in mobile networks
Biswas, Subir Kumar
University of Cambridge, Computer Laboratory
1994-09
en
Text
UCAM-CL-TR-351
ISSN 1476-2986
The rapidly advancing technology of cellular communication and
wireless LAN makes ubiquitous computing feasible where the mobile
users can have access to the location independent information and
the computing resources. Multimedia networking is another emerging
technological trend of the 1990s and there is an increasing demand
for supporting continuous media traffic in wireless personal
communication environment. In order to guarantee the strict
performance requirements of realtime traffic, the
connection-oriented approaches are proving to be more efficient
compared to the conventional datagram based networking. This
dissertation deals with a network architecture and its design issues
for implementing the connection-oriented services in a mobile radio
environment.
The wired backbone of the proposed wireless LAN comprises of high
speed ATM switching elements, connected in a modular fashion, where
the new switches and the user devices can be dynamically added and
reconnected for maintaining a desired topology. A dynamic
reconfiguration protocol, which can cope with these changing network
topologies, is proposed for the present network architecture. The
details about a prototype implementation of the protocol and a
simulation model for its performance evaluation are presented.
CSMA/AED, a single frequency and carrier sensing based protocol is
proposed for the radio medium access operations. A simulation model
is developed in order to investigate the feasibility of this
statistical and reliable access scheme for the proposed radio
network architecture. The effectiveness of a per-connection window
based flow control mechanism, for the proposed radio LAN, is also
investigated. A hybrid technique is used, where the medium access
and the radio data-link layers are modelled using the mentioned
simulator; an upper layer end-to-end queueing model, involving flow
dependent servers, is solved using an approximate Mean Value
Analysis technique which is augmented for faster iterative
convergence.
A distributed location server, for managing mobile users’ location
information and for aiding the mobile connection management tasks,
is proposed. In order to hide the effects of mobility from the
non-mobile network entities, the concept of a per-mobile software
entity, known as a “representative”, is introduced. A mobile
connection management scheme is also proposed for handling the
end-to-end network layer connections in the present mobile
environment. The scheme uses the representatives and a novel
connection caching technique for providing the necessary realtime
traffic support functionalities.
A prototype system, comprising of the proposed location and the
connection managers, has been built for demonstrating the
feasibility of the presented architecture for transporting
continuous media traffic. A set of experiments have been carried out
in order to investigate the impacts of various design decisions and
to identify the performance-critical parts of the design.
A mixed linear and non-linear logic: proofs, terms and
models
Benton, P.N.
University of Cambridge, Computer Laboratory
1994-10
en
Text
UCAM-CL-TR-352
ISSN 1476-2986
Intuitionistic linear logic regains the expressive power of
intuitionistic logic through the ! (‘of course’) modality. Benton,
Bierman, Hyland and de Paiva have given a term assignment system for
ILL and an associated notion of catagorical model in which the !
modality is modelled by a comonad satisfying certain extra
conditions. Ordinary intuitionistic logic is then modelled in a
cartesian closed category which arises as a full subcategory of the
category of coalgebras for the comonad.
This paper attempts to explain the connection between ILL and IL
more directly and symmetrically by giving a logic, term calculus and
categorical model for a system in which the linear and non-linear
worlds exist on an equal footing, with operations allowing one to
pass in both directions. We start from the categorical model of ILL
given by Benton, Bierman, Hyland and de Paiva and show that that
this is equivalent to having a symmetric monoidal adjunction between
a symmetric monoidal closed category and a cartesian closed
category. We then derive both a sequent calculus and a natural
deduction presentation of the logic corresponding to the new notion
of model.
Merging HOL with set theory
Gordon, Mike
University of Cambridge, Computer Laboratory
1994-11
en
Text
UCAM-CL-TR-353
ISSN 1476-2986
Set theory is the standard foundation for mathematics, but the
majority of general purpose mechanized proof assistants support
versions of type theory (higher order logic). Examples include Alf,
Automath, Coq, Ehdm, HOL, IMPS, Lambda, LEGO, Nuprl, PVS and
Veritas. For many applications type theory works well and provides
for specification the benefits of type-checking that are well known
in programming. However, there are areas where types get in the way
or seem unmotivated. Furthermore, most people with a scientific or
engineering background already know set theory, whereas type theory
may appear inaccessible and so be an obstacle to the uptake of proof
assistants based on it. This paper describes some experiments (using
HOL) in combining set theory and type theory; the aim is to get the
best of both worlds in a single system. Three approaches have been
tried, all based on an axiomatically specified type V of ZF-like
sets: (i) HOL is used without any additions besides V; (ii) an
embedding of the HOL logic into V is provided; (iii) HOL axiomatic
theories are automatically translated into set-theoretic
definitional theories. These approaches are illustrated with two
examples: the construction of lists and a simple lemma in group
theory.
Formalising a model of the λ-calculus in HOL-ST
Agerholm, Sten
University of Cambridge, Computer Laboratory
1994-11
en
Text
UCAM-CL-TR-354
ISSN 1476-2986
Many new theorem provers implement strong and complicated type
theories which eliminate some of the limitations of simple type
theories such as the HOL logic. A more accessible alternative might
be to use a combination of set theory and simple type theory as in
HOL-ST which is a version of the HOL system supporting a ZF-like set
theory in addition to higher order logic. This paper presents a case
study on the use of HOL-ST to build a model of the λ-calculus by
formalising the inverse limit construction of domain theory. This
construction is not possible in the HOL system itself, or in simple
type theories in general.
Two cryptographic notes
Wheeler, David
Needham, Roger
University of Cambridge, Computer Laboratory
1994-11
en
Text
UCAM-CL-TR-355
ISSN 1476-2986
A large block DES-like algorithm
DES was designed to be slow in software. We give here a DES type of
code which applies directly to single blocks comprising two or more
words of 32 bits. It is thought to be at least as secure as
performing DES separately on two word blocks, and has the added
advantage of not requiring chaining etc. It is about 8m/(12+2m)
times as fast as DES for an m word block and has a greater gain for
Feistel codes where the number of rounds is greater. We use the name
GDES for the codes we discuss. The principle can be used on any
Feistel code.
TEA, a Tiny Encryption Algorithm
We design a short program which will run on most machines and
encypher safely. It uses a large number of iterations rather than a
complicated program. It is hoped that it can easily be translated
into most languages in a compatible way. The first program is given
below. It uses little set up time and does a weak non linear
iteration enough rounds to make it secure. There are no preset
tables or long set up times. It assumes 32 bit words.
Simple, proven approaches to text retrieval
Robertson, S.E.
Spärck Jones, K.
University of Cambridge, Computer Laboratory
1994-12
en
Text
UCAM-CL-TR-356
ISSN 1476-2986
This technical note describes straightforward techniques for
document indexing and retrieval that have been solidly established
through extensive testing and are easy to apply. They are useful for
many different types of text material, are viable for very large
files, and have the advantage that they do not require special
skills or training for searching, but are easy for end users.
Seven more myths of formal methods
Bowen, Jonathan P.
Hinchey, Michael G.
University of Cambridge, Computer Laboratory
1994-12
en
Text
UCAM-CL-TR-357
ISSN 1476-2986
For whatever reason, formal methods remain one of the more
contentious techniques in industrial software engineering. Despite
great increases in the number of organizations and projects applying
formal methods, it is still the case that the vast majority of
potential users of formal methods fail to become actual users. A
paper by Hall in 1990 examined a number of ‘myths’ concerning formal
methods, assumed by some to be valid. This paper considers a few
more beliefs held by many and presents some counter examples.
Multithreaded processor design
Moore, Simon William
University of Cambridge, Computer Laboratory
1995-02
en
Text
UCAM-CL-TR-358
ISSN 1476-2986
Multithreaded processors aim to improve upon both control-flow and
data-flow processor models by forming some amalgam of the two. They
combine sequential behaviour from the control-flow model with
concurrent aspects from data-flow design.
Some multithreaded processor designs have added just a little
concurrency to control-flow or limited sequential execution to
data-flow. This thesis demonstrates that more significant benefits
may be obtained by a more radical amalgamation of the two models. A
data-driven microthread model is proposed where a microthread is a
short control flow code sequence. To demonstrate the efficiency of
this model, a suitable multithreaded processor called Anaconda is
designed and evaluated.
Anaconda incorporates a scalable temporally predictable memory tree
structure with distributed virtual address translation and memory
protection. A temporally predictable cached direct-mapped matching
store is provided to synchronise data to microthreads. Code is
prefetched into an instruction cache before execution commences.
Earliest-deadline-first or fixed-priority scheduling is supported
via a novel hardware priority queue. Control-flow execution is
performed by a modified Alpha 21064 styled pipeline which assists
comparison with commerical processors.
A case study of co-induction in Isabelle
Frost, Jacob
University of Cambridge, Computer Laboratory
1995-02
en
Text
UCAM-CL-TR-359
ISSN 1476-2986
The consistency of the dynamic and static semantics for a small
functional programming language was informally proved by R. Milner
and M. Tofte. The notions of co-inductive definitions and the
associated principle of co-induction played a pivotal role in the
proof. With emphasis on co-induction, the work presented here deals
with the formalisation of this result in the generic theorem prover
Isabelle.
On the calculation of explicit polymetres
Clocksin, W.F.
University of Cambridge, Computer Laboratory
1995-03
en
Text
UCAM-CL-TR-360
ISSN 1476-2986
Computer scientists take an interest in objects or events which can
be counted, grouped, timed and synchronised. The computational
problems involved with the interpretation and notation of musical
rhythm are therefore of particular interest, as the most complex
time-stamped structures yet devised by humankind are to be found in
music notation. These problems are brought into focus when
considering explicit polymetric notation, which is the concurrent
use of different time signatures in music notation. While not in
common use the notation can be used to specify complicated
cross-rhythms, simple versus compound metres, and unequal note
values without the need for tuplet notation. From a computational
point of view, explicit polymetric notation is a means of specifying
synchronisation relationships amongst multiple time-stamped streams.
Human readers of explicit polymetic notation use the time signatures
together with the layout of barlines and musical events as clues to
determine the performance. However, if the aim is to lay out the
notation (such as might be required by an automatic music notation
processor), the location of barlines and musical events will be
unknown, and it is necessary to calculate them given only the
information conveyed by the time signatures. Similar problems arise
when trying to perform the notation (i.e. animate the specification)
in real-time. Some problems in the interpretation of explicit
polymetric notation are identified and a solution is proposed. Two
different interpretations are distinguished, and methods for their
automatic calculation are given. The solution given may be applied
to problems which involve the synchronisation or phase adjustment of
multiple independent threads of time-stamped objects.
Explicit network scheduling
Black, Richard John
University of Cambridge, Computer Laboratory
1995-04
en
Text
UCAM-CL-TR-361
ISSN 1476-2986
This dissertation considers various problems associated with the
scheduling and network I/O organisation found in conventional
operating systems for effective support for multimedia applications
which require Quality of Service.
A solution for these problems is proposed in a micro-kernel
structure. The pivotal features of the proposed design are that the
processing of device interrupts is performed by user-space processes
which are scheduled by the system like any other, that events are
used for both inter- and intra-process synchronisation, and the use
of a specially developed high performance I/O buffer management
system.
An evaluation of an experimental implementation is included. In
addition to solving the scheduling and networking problems
addressed, the prototype is shown to out-perform the Wanda system (a
locally developed micro-kernel) on the same platform.
This dissertation concludes that it is possible to construct an
operating system where the kernel provides only the fundamental job
of fine grain sharing of the CPU between processes, and hence
synchronisation between those processes. This enables processes to
perform task specific optimisations; as a result system performance
is enhanced, both with respect to throughput and the meeting of soft
real-time guarantees.
W-learning: competition among selfish Q-learners
Humphrys, Mark
University of Cambridge, Computer Laboratory
1995-04
en
Text
UCAM-CL-TR-362
ISSN 1476-2986
W-learning is a self-organising action-selection scheme for systems
with multiple parallel goals, such as autonomous mobile robots. It
uses ideas drawn from the subsumption architecture for mobile robots
(Brooks), implementing them with the Q-learning algorithm from
reinforcement learning (Watkins). Brooks explores the idea of
multiple sensing-and-acting agents within a single robot, more than
one of which is capable of controlling the robot on its own if
allowed. I introduce a model where the agents are not only
autonomous, but are in fact engaged in direct competition with each
other for control of the robot. Interesting robots are ones where no
agent achieves total victory, but rather the state-space is
fragmented among different agents. Having the agents operate by
Q-learning proves to be a way to implement this, leading to a local,
incremental algorithm (W-learning) to resolve competition. I present
a sketch proof that this algorithm converges when the world is a
discrete, finite Markov decision process. For each state,
competition is resolved with the most likely winner of the state
being the agent that is most likely to suffer the most if it does
not win. In this way, W-learning can be viewed as ‘fair’ resolution
of competition. In the empirical section, I show how W-learning may
be used to define spaces of agent-collections whose action selection
is learnt rather than hand-designed. This is the kind of
solution-space that may be searched with a genetic algorithm.
Names and higher-order functions
Stark, Ian
University of Cambridge, Computer Laboratory
1995-04
en
Text
UCAM-CL-TR-363
ISSN 1476-2986
Many functional programming languages rely on the elimination of
‘impure’ features: assignment to variables, exceptions and even
input/output. But some of these are genuinely useful, and it is of
real interest to establish how they can be reintroducted in a
controlled way. This dissertation looks in detail at one example of
this: the addition to a functional language of dynamically generated
“names”. Names are created fresh, they can be compared with each
other and passed around, but that is all. As a very basic example of
“state”, they capture the graduation between private and public,
local and global, by their interaction with higher-order functions.
The vehicle for this study is the “nu-calculus”, an extension of the
simply-typed lambda-calculus. The nu-calculus is equivalent to a
certain fragment of Standard ML, omitting side-effects, exceptions,
datatypes and recursion. Even without all these features, the
interaction of name creation with higher-order functions can be
complex and subtle.
Various operational and denotational methods for reasoning about the
nu-calculus are developed. These include a computational
metalanguage in the style of Moggi, which distinguishes in the type
system between values and computations. This leads to categorical
models that use a strong monad, and examples are devised based on
functor categories.
The idea of “logical relations” is used to derive powerful reasoning
methods that capture some of the distinction between private and
public names. These techniques are shown to be complete for
establishing contextual equivalence between first-order expressions;
they are also used to construct a correspondingly abstract
categorical model.
All the work with the nu-calculus extends cleanly to Reduced ML, a
larger language that introduces integer references: mutable storage
cells that are dynamically allocated. It turns out that the step up
is quite simple, and both the computational metalanguage and the
sample categorical models can be reused.
The Church-Rosser theorem in Isabelle: a proof porting
experiment
Rasmussen, Ole
University of Cambridge, Computer Laboratory
1995-04
en
Text
UCAM-CL-TR-364
ISSN 1476-2986
This paper describes a proof of the Church-Rosser theorem for the
pure lambda-calculus formalised in the Isabelle theorem prover. The
initial version of the proof is ported from a similar proof done in
the Coq proof assistant by Girard Huet, but a number of
optimisations have been performed. The development involves the
introduction of several inductive and recursive definitions and thus
gives a good presentation of the inductive package of Isabelle.
Computational types from a logical perspective I
Benton, P.N.
Bierman, G.M.
de Paiva, V.C.V.
University of Cambridge, Computer Laboratory
1995-05
en
Text
UCAM-CL-TR-365
ISSN 1476-2986
Moggi’s computational lambda calculus is a metalanguage for
denotational semantics which arose from the observation that many
different notions of computation have the categorical structure of a
strong monad on a cartesian closed category. In this paper we show
that the computational lambda calculus also arises naturally as the
term calculus corresponding (by the Curry-Howard correspondence) to
a novel intuitionistic modal propositional logic. We give natural
deduction, sequent calculus and Hilbert-style presentations of this
logic and prove a strong normalisation result.
Retrieving spoken documents: VMR Project
experiments
Spärck Jones, K.
Jones, G.J.F.
Foote, J.T.
Young, S.J.
University of Cambridge, Computer Laboratory
1995-05
en
Text
UCAM-CL-TR-366
ISSN 1476-2986
This paper describes initial work on an application for the
retrieval of spoken documents in multimedia systems. Speech
documents pose a particular problem for retrieval since the contents
are unknown. The VMR project seeks to address this problem for a
video mail application by combining state of the art speech
recognition with established document retrieval technologies to
provide an effective and efficient retrieval tool. Experiments with
a small spoken message collection show that retrieval precision for
the spoken file can reach 90% of that obtained when the same file is
used, as a benchmark, in text transcription form.
Categorical logic
Pitts, Andrew M.
University of Cambridge, Computer Laboratory
1995-05
en
Text
UCAM-CL-TR-367
ISSN 1476-2986
This document provides an introduction to the interaction between
category theory and mathematical logic which is slanted towards
computer scientists. It will be a chapter in the forthcoming Volume
VI of: S. Abramsky, D. M. Gabbay, and T. S. E. Maibaum (eds),
“Handbook of Logic in Computer Science”, Oxford University Press.
CogPiT – configuration of protocols in TIP
Stiller, Burkhard
University of Cambridge, Computer Laboratory
1995-06
en
Text
UCAM-CL-TR-368
ISSN 1476-2986
The variety of upcoming applications in terms of their performance
and Quality-of-Service (QoS) requirements is increasing. Besides
almost well-known applications, such as teleconferencing, audio- and
video-transmissions, even more contemporary ones, such as medical
imaging, Video-on-Demand, and interactive tutoring systems, are
introduced and applied to existing networks. On the contrary,
traditionally data-oriented applications, such as file transfer and
remote login, are considerably different in terms of their QoS
requirements. Therefore, the consequences of this evolution effect
the architectures of end-systems, e.g., workstations that have to be
capable of maintaining all different kinds of multi-media data, and
intermediate-systems as well.
Therefore, a configuration approach of communication protocols has
been developed to support the variety of applications. This approach
offers the possibility to configure communication protocols
automatically depending on the application requirements expressed in
various QoS parameters. The result, an application-tailored
communication protocol, matches the requested application
requirements as far as possible. Additionally, network and system
resources (NSR) are taken into account for a well-suited
configuration.
The Configuration of Protocols in TIP is called CogPiT and is part
of the Transport and Internetworking Package (TIP). As an example,
in the TIP environment the transport protocol TEMPO is used for
configuration purposes.
A comparison of HOL-ST and Isabelle/ZF
Agerholm, Sten
University of Cambridge, Computer Laboratory
1995-07
en
Text
UCAM-CL-TR-369
ISSN 1476-2986
The use of higher order logic (simple type theory) is often limited
by its restrictive type system. Set theory allows many constructions
on sets that are not possible on types in higher order logic. This
paper presents a comparison of two theorem provers supporting set
theory, namely HOL-ST and Isabelle/ZF, based on a formalization of
the inverse limit construction of domain theory; this construction
cannot be formalized in higher order logic directly. We argue that
whilst the combination of higher order logic and set theory in
HOL-ST has advantages over the first order set theory in
Isabelle/ZF, the proof infrastructure of Isabelle/ZF has better
support for set theory proofs than HOL-ST. Proofs in Isabelle/ZF are
both considerably shorter and easier to write.
A package for non-primitive recursive function definitions
in HOL
Agerholm, Sten
University of Cambridge, Computer Laboratory
1995-07
en
Text
UCAM-CL-TR-370
ISSN 1476-2986
This paper provides an approach to the problem of introducing
non-primitive recursive function definitions in the HOL system. A
recursive specification is translated into a domain theory version,
where the recursive calls are treated as potentially
non-terminating. Once we have proved termination, the original
specification can be derived easity. Automated tools implemented in
HOL88 are provided to support the definition of both partial
recursive functions and total recursive functions which have well
founded recursive specifications. There are constructions for
building well-founded relations easily.
LIMINF convergence in Ω-categories
Wagner, Kim Ritter
University of Cambridge, Computer Laboratory
1995-06
en
Text
UCAM-CL-TR-371
ISSN 1476-2986
The aim of this article is twofold. From a mathematical perspective
we present a notion of convergence which is suitably general such as
to include the convergence of chains to their least upper bounds in
preordered sets, and the convergence of Cauchy sequences to their
metric limits in metric spaces. Rather than presenting this theory
from a purely mathematical perspective however, we will use it to
introduce a simple-minded domain theory based on a generic notion of
approximation. Although this is the use which motivated the
development of these concepts, it is hoped that this is not the only
one.
A brief history of mobile telephony
Hild, Stefan G.
University of Cambridge, Computer Laboratory
1995-01
en
Text
UCAM-CL-TR-372
ISSN 1476-2986
Mobile telephony has gone through a decade of tremendous change and
progress. Today, mobile phones are an indispensable tool to many
professionals, and have great potential to become vital components
in mobile data communication applications. In this survey we will
attempt to present some of the milestones from the route which
mobile telephony has taken over the past decades while developing
from an experimental system with limited capabilities with to a
mature technology (section 1), followd by a more detailed
introduction into the modern pan-European GSM standard (section 2).
Section 3 is devoted to the data communication services, covering
two packet-oriented data only networks as well as data services
planned for the GSM system. Section 4 covers some security issues
and section 5 gives an insight into the realities today with details
of some networks available in the UK. Finally, section 6 concludes
this overview with a brief look into the future.
Natural-language processing and requirements
specifications
Macías, Benjamín
Pulman, Stephen G.
University of Cambridge, Computer Laboratory
1995-07
en
Text
UCAM-CL-TR-373
ISSN 1476-2986
This document reports on our participation in the MORSE (“A Method
for Object Reuse in Safety-critical Environments”) project. Our
brief in the project was to investigate the role that
natural-language processing (NLP) techniques can play in improving
any of the aspects linking natural-language requirements
specifications and formal specifications. The contents are as
follows: We begin with a brief introduction to NLP in the context of
requirements tasks, followed by an examination of some strategies to
control the form of requirements specifications. We continue by
describing an interface designed to correct some of the problems
with known methods to control specifications, while employing
current NLP to maximum advantage. We then show how to build a
natural-langauge interface to a formal specification, and some
aspects of the problem of paraphrasing formal expressions. We finish
with the conclusions reached at the end of our participation in the
project.
A framework for QoS updates in a networking
environment
Stiller, Burkhard
University of Cambridge, Computer Laboratory
1995-07
en
Text
UCAM-CL-TR-374
ISSN 1476-2986
The support of sufficient Quality-of-Service (QoS) for applications
residing in a distributed environment and running on top of high
performance networks is a demanding issue. Currently, the areas to
provide this support adequately include communication protocols,
operating systems support, and offered network services. A
configurable approach of communication protocols offers the needed
protocol flexibility to react accordingly on various different
requirements.
Communication protocols and operating systems have to be
parametrized using internal configuration parameters, such as window
sizes, retry counters, or scheduling mechanisms, that rely closely
on requested application-oriented or network-dependent QoS, such as
bandwidth or delay. Moreover, these internal parameters have to be
recalculated from time to time due to network changes (such as
congestion or line break-down) or due to application-specific
alterations (such as enhanced bandwidth requirements or increased
reliability) to adjust a temporary or semi-permanent “out-of-tune”
service behavior.
Therefore, a rule-based evaluation and QoS updating framework for
configuration parameters in a networking environment has been
developed. The resulting “rulework” can be used within highly
dynamic environments in a communication subsystem that offers the
possibility to specify for every QoS parameter both a bounding
interval of values and an average value. As an example, the
framework has been integrated in the Function-based Communication
Subsystem (F-CSS). Especially, an enhanced application service
interface is offered, allowing for the specification of various
QoS-parameters that are used to configure a sufficient
application-tailored communication protocol.
Restructuring virtual memory to support distributed
computing environments
Huang, Feng
University of Cambridge, Computer Laboratory
1995-07
en
Text
UCAM-CL-TR-375
ISSN 1476-2986
This dissertation considers the limitations of conventional memory
and storage management approaches and proposes a coherent
memory-mapped object system architecture for emerging distributed
computing environments.
Conventionally, main memory and secondary storage management is
based on the two-level store architecture, which provides one
interface to access memory segments and another to access secondary
storage objects. The quality and productivity of software
development is impaired by two different views of volatile data and
persistent data. Operating system performance is compromised because
of mandatory data copying and unnecessary user/kernel boundary
crossings. This is exacerbated in microkernel architectures, in
which most of the user/kernel boundary crossings become context
switches. Double paging may cause resources to be used inefficiently
and the double paging anomaly may occur if a database system is
implemented on top of this architecture. The work presented here
seeks to tackle these problems by integrating main memory with
secondary storage by using memory-mapping techniques. The different
views of volatile and persistent data are unified; mandatory
information copying and unnecessary user/kernel boundary crossings
(or context switches in microkernels) are avoided; and double paging
is eliminated.
Distributed Shared Memory (DSM) has been proposed as an attractive
abstraction for constructing distributed applications bacause it is
easier to program than the message-passing abstraction. However, the
overhead for maintaining memory coherency in DSM systems is high.
Also, existing DSM systems typically provide only one coherence
protocol and there exists a potential mismatch between the supplied
protocol and some applications’ requirements. This work explores the
architectural support for a flexible coherence mechanism, through
which clients can choose the most suitable protocols for their
applications to avoid coherency mismatch. Also low-level coherency
control is integrated with high level concurrency control so that
system-wide object coherency and synchronisation are realised
without sacrificing performance.
In this dissertation, an architectural framework is proposed;
various design issues are discussed and the design of a flexible
coherence mechanism, which accommodates multiple coherence
protocols, is detailed. A prototype implementation and performance
measurements are then presented; and the use of the architecture is
illustrated.
The structure of a multi-service operating system
Roscoe, Timothy
University of Cambridge, Computer Laboratory
1995-08
en
Text
UCAM-CL-TR-376
ISSN 1476-2986
Increases in processor speed and network bandwidth have led to
workstations being used to process multimedia data in real time.
These applications have requirements not met by existing operating
systems, primarily in the area of resource control: there is a need
to reserve resources, in particular the processor, at a fine
granularity. Furthermore, guarantees need to be dynamically
renegotiated to allow users to reassign resources when the machine
is heavily loaded. There have been few attempts to provide the
necessary facilities in traditional operating systems, and the
internal structure of such systems makes the implementation of
useful resource control difficult.
This dissertation presents a way of structuring an operating system
to reduce crosstalk between applications sharing the machine, and
enable useful resource guarantees to be made: instead of system
services being located in the kernel or server processes, they are
placed as much as possible in client protection domains and
scheduled as part of the client, with communication between domains
only occurring when necessary to enforce protection and concurrency
control. This amounts to multiplexing the service at as low a level
of abstraction as possible. A mechanism for sharing processor time
between resources is also described. The prototype Nemesis operating
system is used to demonstrate the ideas in use in a practical
system, and to illustrate solutions to several implementation
problems that arise.
Firstly, structuring tools in the form of typed interfaces within a
single address space are used to reduce the complexity of the system
from the programmer’s viewpoint and enable rich sharing of text and
data between applications.
Secondly, a scheduler is presented which delivers useful Quality of
Service guarantees to applications in a highly efficient manner.
Integrated with the scheduler is an inter-domain communication
system which has minimal impact on resource guarantees, and a method
of decoupling hardware interrupts from the execution of device
drivers.
Finally, a framework for high-level inter-domain and inter-machine
communication is described, which goes beyond object-based RPC
systems to permit both Quality of Service negotiation when a
communication binding is established, and services to be implemented
straddling protection domain boundaries as well as locally and in
remote processes.
Mechanising set theory: cardinal arithmetic and the axiom of
choice
Paulson, Larry
Grabczewski, Krzysztof
University of Cambridge, Computer Laboratory
1995-07
en
Text
UCAM-CL-TR-377
ISSN 1476-2986
Fairly deep results of Zermelo-Fraenkel (ZF) set theory have been
mechanised using the proof assistant Isabelle. The results concern
cardinal arithmetic and the Axiom of Choice (AC). A key result about
cardinal multiplication is K*K=K, where K is any infinite cardinal.
Proving this result required developing theories of orders,
order-isomorphisms, order types, ordinal arithmetic, cardinals,
etc.; this covers most of Kunen, Set Theory, Chapter I. Furthermore,
we have proved the equivalence of 7 formulations of the
Well-ordering Theorem and 20 formulations of AC; this covers the
first two chapters of Rubin and Rubin, Equivalents of the Axiom of
Choice. The definitions used in the proofs are largely faithful in
style to the original mathematics.
Performance evaluation of HARP: a hierarchical asynchronous
replication protocol for large scale system
Adly, Noha
University of Cambridge, Computer Laboratory
1995-08
en
Text
UCAM-CL-TR-378
ISSN 1476-2986
This report evaluates the performance of HARP, a hierarchical
replication protocol based on nodes organised into a logical
hierarchy. The scheme is based on communication with nearby replicas
and scales well for thousands of replicas. It proposes a new service
interface that provides different levels of asynchrony, allowing
strong consistency and weak consistency to be integrated into the
same framework. Further, it provides the ability to offer different
levels of staleness, by querying from different levels of the
hierarchy. We present results from a detailed simulation analysis
evaluating the benefits and losses in performance resulting from
using synchronous versus asynchronous operation within HARP under
different system configurations and load mixes. Further, the
performance is evaluated on different network topologies. An
analytical solution based on the Open Queueing Network Model with
Multiple Job Classes is carried out for the verification of the
simulation model and the results are presented.
Proceedings of the First Isabelle Users Workshop
Paulson, Lawrence
University of Cambridge, Computer Laboratory
1995-09
en
Text
UCAM-CL-TR-379
ISSN 1476-2986
Quality-of-Service issues in networking
environments
Stiller, Burkhard
University of Cambridge, Computer Laboratory
1995-09
en
Text
UCAM-CL-TR-380
ISSN 1476-2986
Quality-of-Service (QoS) issues in networking environments cover
various separate areas and topics. They include at least the
specification of applications requirements, the definition of
network services, QoS models, resource reservation methods,
negotiation and transformation methods for QoS, and operating system
support for guaranteed services. An embracing approach for handling,
dealing with, and supporting QoS in different scenarios and
technical set-ups is required to manage sufficiently forthcoming
communication and networking tasks. Modern telecommunication systems
require an integrated architecture for applications, communication
subsystems, and network perspectives to overcome drawbacks of
traditional communication architectures, such as redundant protocol
functionality, weakly designed interfaces between the end-system and
a network adapter, or impossibility of specifying and guaranteeing
QoS parameter.
This work contains the discussion of a number of interconnected QoS
issues, e.g., QoS mapping, QoS negotiation, QoS-based configuration
of communication protocols, or QoS aspects in Asynchronous Transfer
Mode (ATM) signaling protocols, which have been dealt with during a
one-year research fellowship. This report is not intended to be a
complete description of every technical detail, but tries to provide
a brief overall picture of the emerging and explosively developing
QoS issues in telecommunication systems. Additionally,
investigations of some of these issues are undertaken in a more
closer detail. It is mainly focussed on QoS mapping, negotiation,
and updating in the communication protocol area.
Rendering for free form deformations
Nimscheck, Uwe Michael
University of Cambridge, Computer Laboratory
1995-10
en
Text
UCAM-CL-TR-381
ISSN 1476-2986
Sederberg’s Free Form Deformation (FFD) is an intuitive modelling
technique that lets users sculpt and deform objects without having
to worry about internal model representation issues. Unfortunately
displaying these deformed objects is problematic and there exist no
algorithms to display general FFD deformed polygonal models. Based
on deRose’s Bezier composition algorithms we develop geometrically
intuitive composition algorithms to find analytic expressions for
deformed objects, which can then be rendered using standard
rendering hardware. Alternatively, one can adaptively tessellate
deformed objects into a mesh of triangles and display this deformed
mesh. The finite element method provides us with a wealth of
algorithms to mesh all types of objects. We show how to adapt these
algorithms to computer graphics problems. The main problem is to
define curvature measures to vary the mesh density according to the
curvature of deformed objects. We find such measures and use them to
develop a new meshing scheme, based on Lo’s advancing front
algorithm, to mesh and render FFD deformed objects. Our algorithm is
superior to existing schemes both in the quality of the generated
meshes and in the variety of solids it can be applied to.
The major contributions of this dissertation are: Firstly, the
development of geometrically intuitive algorithms to determine
closed form expressions of FFD deformed surfaces. Secondly, the
transformation of tangent and normal vectors into deformed space.
Thirdly, development of a new advancing front meshing algorithm that
allows to mesh solids that have been deformed by non-uniform
B-spline volumes. Finally, systematic experiments have been
performed to assess the performance and limitations of the new
meshing algorithm.
Synthetic image generation for a multiple-view autostereo
display
Castle, Oliver M.
University of Cambridge, Computer Laboratory
1995-10
en
Text
UCAM-CL-TR-382
ISSN 1476-2986
Multiple-view autostereo displays provide true 3D images which offer
depth cues (such as binocular stereo and motion parallax) that are
not available with conventional monoscopic displays. This thesis
addresses some of the problems of synthetic image generation for a
multi-view autostereo display, with particular attention paid to the
question of how rendering costs may be reduced by taking advantage
of the similarities between the views.
A functional description of the prototype multi-view autostereo
display device developed separately at the University of Cambridge
sets the technological beckground of this research. The problems
faced by synthetic image generation in general are reviewed next, of
which visible surface determination is identified as the most
important for multi-view stereo. A viewing model for multi-view
stereo is then derived, building on experience with existing
monoscopic and two-view stereoscopic viewing models.
Using this multi-view autostereo viewing model as a framework, two
distinct approaches to multi-view stereo image synthesis are
investigated. The first is an extension of conventional Z-buffer
rendering methods, adapted to take advantage of the coherence
between the views to eliminate redundant processing and share
computational overheads whenever possible. The second, based on
approximate stereo reprojection techniques, shares visible surface
information between the views in an attempt to eliminate processing
those parts of the scene considered unlikely to be visible in the
final image, thus trading off image quality against rendering speed.
An experimental evaluation of these two techniques demonstrates that
both are capable of producing multi-view stereo images at a lower
cost per view than similar single-view rendering methods. The
results indicate that the performance improvements of both
algorithms increase with the number of views in the image, reaching
asymptotic levels as the shared processing costs become relatively
less significant compared with the overall rendering time. Of the
two, however, the approximate algorithm appears to offer the better
potential speedup, owing to the way in which it enables the
effective depth complexity of the scene to be reduced.
Management of replicated data in large scale
systems
Adly, Noha
University of Cambridge, Computer Laboratory
1995-11
en
Text
UCAM-CL-TR-383
ISSN 1476-2986
Data is replicated in distributed systems to improve system
availability and performance. In recent years, the growth of
internetworks and distributed applications has increased the need
for large scale replicated systems. However, existing replication
protocols do not address scale and autonomy issues adequately.
Further, current applications require different degrees of of
consistency, and therefore they should be given the ability to
choose the level of consistency that is appropriate for their
particular semantics. This dissertation presents a new scalable
replication protocol (HARP) that is based on organising the replicas
into a logical hierarchy. It is argues that adopting a hierarchical
structure allows for exploiting localised communication, which is
taken as the key to achieve scalability. Moreover it gives the
ability to provide different degrees of consistency.
HARP provides an efficient and scalable propagation scheme where
each node needs to communicate with a few nodes only while ensuring
reliable delivery. A new service interface is proposed that gives
the application the flexibility to choose between strong and weak
consistency. Further the scheme provides the ability to offer
different levels of staleness, depending on the needs of various
applications. Dynamic restructuring operations are presented which
allow the hierarchy to be built and reconfigured, including the
restarting of failed nodes and re-merging partitioned networks. The
operations produce low message traffic by exploiting localised
communication, and do not disturb normal operations. This is
achieved while ensuring no loss of messages.
Reconciliation methods based on delivery order mechanisms are
provided to resolve temporary inconsistencies and an application can
choose from them. A new algorithm that supports casual order
delivery is proposed. The desirable characteristic of the algorithm
is that, by relying on the hierarchical propagation of HARP, it cuts
down the size of the timestamp required to verify causality
significantly, and thus enhances scalability.
A detailed simulation study was carried out to evaluate the
performance of HARP and to quantify the benefits and losses
resulting from moving from strong consistency to weak consistency
under different system configurations and load mixes. Further, a
simulation study was conducted to compare the performance of HARP to
another weak consistency replication protocol, the Time Stamped Anti
Entropy.
An alternative hierarchical propagation protocol is proposed as an
optimisation of HARP, called HPP. The main difference between HPP
and HARP is that HPP avoids the exchange of global state information
when reconfiguration or failiures occur. Therefore HPP is more
scalable; however, it can tolerate only special patterns of
failiure. The protocol is presented in detail and its strengths and
limitations are analysed.
Securing ATM networks
Chuang, Shaw-Cheng
University of Cambridge, Computer Laboratory
1995-01
en
Text
UCAM-CL-TR-384
ISSN 1476-2986
This is an interim report on the investigations into securing
Asynchronous Transfer Mode (ATM) networks. We look at the challenge
in providing such a secure ATM network and identify the important
issues in achieving such goal. In this paper, we discuss the issues
and problems involved and outline some techniques to solving these
problems. The network environment is first examined and we also
consider the correct placement of security mechanism in such an
environment. Following the analysis of the security requirement, we
introduce and describe a key agile cryptographic device for ATM. The
protection of the ATM data plane is extremely important to provide
data confidentiality and data integrity. Techniques in providing
synchronisation, dynamic key change, dynamic initialisation vector
change and Message Authentication Code on ATM data, are also being
considered. Next, we discuss the corresponding control functions. A
few key exchange protocols are given as possible candidates for the
establishment of the session key. The impact of such key exchange
protocols on the design of an ATM signalling protocol has also been
examined and security extension to an existing signalling protocol
being discussed. We also talk about securing other control plane
functions such as NNI routing, Inter-Domain Policy Routing,
authorisation and auditing, firewall and intrusion detection,
Byzantine robustness. Management plane functions are also being
looked at, with discussions on bootstrapping, authenticated
neighbour discovery, ILMI Security, PVC security, VPI security and
ATM Forum management model.
Performance evaluation of the Delphi machine
Saraswat, Sanjay
University of Cambridge, Computer Laboratory
1995-12
en
Text
UCAM-CL-TR-385
ISSN 1476-2986
Bisimilarity for a first-order calculus of objects with
subtyping
Gordon, Andrew D.
Rees, Gareth D.
University of Cambridge, Computer Laboratory
1996-01
en
Text
UCAM-CL-TR-386
ISSN 1476-2986
Monitoring composite events in distributed
systems
Schwiderski, Scarlet
Herbert, Andrew
Moody, Ken
University of Cambridge, Computer Laboratory
1996-02
en
Text
UCAM-CL-TR-387
ISSN 1476-2986
One way of integrating heterogeneous, autonomous and distributed
systems is to monitor their behaviour in terms of global compostite
events. In specific applications, for example database, it is
essential that global composite events can take account of general
conditions such as the timing constraints on distributed system
behaviour. In this paper the use of global composite events
incorporating time events for expressing physical time is
investigated. The detection of global composite events is
complicated by the inherent features of distributed systems: lack of
global time, message delays between sites and independent failures.
Global event detectors are distributed to arbitrary sites. Relevant
constituent events occur on remote sites and are signalled to
corresponding global event detectors, where they are evaluated. Two
different algorithms for the detection of global composite events
are introduced which are based on the evaluation of trees:
asynchronous and synchronous evaluation. Asynchronous evaluation
provides fast but unreliable detection of global composite events,
whereas synchronous evaluation is characterized by reliability and
unpredictable delays.
A unified approach to strictness analysis and optimising
transformations
Benton, P.N.
University of Cambridge, Computer Laboratory
1996-02
en
Text
UCAM-CL-TR-388
ISSN 1476-2986
We present an inference system for translating programs in a
PCF-like source language into a variant of Moggi’s computational
lambda calculus. This translation combines a simple strictness
analysis with its associated optimising transformations into a
single system. The correctness of the translation is established
using a logical relation between the denotational semantics of the
source and target languages.
A proof checked for HOL
Wong, Wai
University of Cambridge, Computer Laboratory
1996-03
en
Text
UCAM-CL-TR-389
ISSN 1476-2986
Formal proofs generated mechanically by theorem provers are often
vary large and shallow, and the theorem provers themselves very
complex. Therefore, in certain application areas, such as
safety-critical systems, it is necessary to have an independent
means for ensuring the consistency of such formal proofs. This
report describes an efficient proof checker for the HOL theorem
prover. This proof checker has been tested with practical proofs
consisting of thousands of inference steps. It was implemented in
Standard ML of New Jersey.
The first part of the report gives an overview of the program. It
describes: the rationale of developing a proof checker; how to use
the checker; and, how the checker works.
The second part of the report describes the program in detail. The
complete source code is included in the description.
Syn: a single language for specifiying abstract syntax
tress, lexical analysis, parsing and pretty-printing
Boulton, Richard J.
University of Cambridge, Computer Laboratory
1996-03
en
Text
UCAM-CL-TR-390
ISSN 1476-2986
A language called Syn is described in which all aspects of
context-free syntax can be specified without redundancy. The
language is essentially an extended BNF grammar. Unusual features
include high-level constructs for specifying lexical aspects of a
language and specification of precedence by textual order. A system
has been implemented for generating lexers, parsers, pretty-printers
and abstract syntax tree representations from a Syn specification.
Programming languages and dimensions
Kennedy, Andrew John
University of Cambridge, Computer Laboratory
1996-04
en
Text
UCAM-CL-TR-391
ISSN 1476-2986
Scientists and engineers must ensure that the equations and formulae
which they use are dimensionally consistent, but existing
programming languages treat all numeric values as dimensionless.
This thesis investigates the extension of programming languages to
support the notion of physical dimension.
A type system is presented similar to that of the programming
language ML but extended with polymorphic dimension types. An
algorithm which infers most general dimension types automatically is
then described and proved correct.
The semantics of the language is given by a translation into an
explicitlytyped language in which dimensions are passed as arguments
to functions. The operational semantics of this language is
specified in the usual way by an evaluation relation defined by a
set of rules. This is used to show that if a program is well-typed
then no dimension errors can occur during its evaluation.
More abstract properties of the language are investigated using a
denotational semantics: these include a notion of invariance under
changes in the units of measure used, analogous to parametricity in
the polymorphic lambda calculus. Finally the dissertation is
summarised and many possible directions for future research in
dimension types and related type systems are described.
Decoding choice encodings
Nestmann, Uwe
Pierce, Benjamin C.
University of Cambridge, Computer Laboratory
1996-04
en
Text
UCAM-CL-TR-392
ISSN 1476-2986
We study two encodings of the asynchronous π-calculus with
input-guarded choice into its choice-free fragment. One encoding is
divergence-free, but refines the atomic commitment of choice into
gradual commitment. The other preserves atomicity, but introduces
divergence. The divergent encoding is fully abstract with respect to
weak bisumulation, but the more natural divergence-free encoding is
not. Instead we shot that it is fully abstract with respect to
coupled simulation, a slightly coarser -- but still coinductively
defined -- equivalence that does not require bisimilarity of
internal branching decisions. The correctness proofs for the two
choice encodings exploit the properties of decodings from
translations to source terms.
Performance management in ATM networks
Crosby, Simon Andrew
University of Cambridge, Computer Laboratory
1996-04
en
Text
UCAM-CL-TR-393
ISSN 1476-2986
The Asynchronous Transfer Mode (ATM) has been identified as the
technology of choice amongst high speed communication networks for
its potential to integrate services with disparate resource needs
and timing constraints. Before it can successfully deliver
integrated services, however, significant problems remain to be
solved. They centre around two major issues. First, there is a need
for a simple, powerful network service interface capable of meeting
the communications needs of new applications. Second, within the
network there is a need to dynamically control a mix of diverse
traffic types to ensure that they meet their performance criteria.
Addressing the first concern, this dissertation argues that a simple
network control interface offers significant advantages over the
traditional, heavyweight approach of the telecommunications
industry. A network control architecture based on a distributed
systems approach is presented which locates both the network control
functions and its services outside the network. The network service
interface uses the Remote Procedure Call (RPC) paradigm and enables
more complicated service offerings to be built from the basic
primitives. A formal specification and verification of the
user-network signalling protocol is presented. Implementations of
the architecture, both on Unix and the Wanda micro-kernel, used on
the Fairisle ATM switch, are described. The implementations
demonstrate the feasibility of the architecture, and feature a high
degree of experimental flexibility. This is exploited in the balance
of the dissertation, which presents the results of a practical study
of network performance under a range of dynamic control mechanisms.
Addressing the second concern, results are presented from a study of
the cell delay variation suffered by ATM connections when
multiplexed with real ATM traffic in an uncontrolled network, and
from an investigation of the expansion of bursts of ATM traffic as a
result of multiplexing. The results are compared with those of
analytical models. Finally, results from a study of the performance
delivered to delay sensitive traffic by priority and rate based cell
scheduling algorithms, and the loss experienced by different types
of traffic under several buffer allocation strategies are presented.
A simple formalization and proof for the mutilated chess
board
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1996-04
en
Text
UCAM-CL-TR-394
ISSN 1476-2986
The impossibility of tiling the mutilated chess board has been
formalized and verified using Isabelle. The formalization is concise
because it is expressed using inductive definitions. The proofs are
straightforward except for some lemmas concerning finite
cardinalities. This exercise is an object lesson in choosing a good
formalization. is applicable in a variety of domains.
Cut-elimination for full intuitionistic linear
logic
Bräuner, Torben
de Paiva, Valeria
University of Cambridge, Computer Laboratory
1996-05
en
Text
UCAM-CL-TR-395
ISSN 1476-2986
We describe in full detail a solution to the problem of proving the
cut elimination theorem for FILL, a variant of (multiplicative and
exponential-free) Linear Logic introduced by Hyland and de Paiva.
Hyland and de Paiva’s work used a term assignment system to describe
FILL and barely sketched the proof of cut elimination. In this
paper, as well as correcting a small mistake in their paper and
extending the system to deal with exponentials, we introduce a
different formal system describing the intuitionistic character of
FILL and we provide a full proof of the cut elimination theorem. The
formal system is based on a notion of dependency between formulae
within a given proof and seems of independent interest. The
procedure for cut elimination applies to (classical) multiplicative
Linear Logic, and we can (with care) restrict our attention to the
subsystem FILL. The proof, as usual with cut elimination proofs, is
a little involved and we have not seen it published anywhere.
Generic automatic proof tools
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1996-05
en
Text
UCAM-CL-TR-396
ISSN 1476-2986
This paper explores a synthesis between two distinct traditions in
automated reasoning: resolution and interaction. In particular it
discusses Isabelle, an interactive theorem prover based upon a form
of resolution. It aims to demonstrate the value of proof tools that,
compared with traditional resolution systems, seem absurdly limited.
Isabelle’s classical reasoner searches for proofs using a tableau
approach. The reasoner is generic: it accepts rules proved in
applied theories, involving defined connectives. New constants are
not reduced to first-order logic; the reasoner
Optimal routing in 2-jump circulant networks
Robič, Borut
University of Cambridge, Computer Laboratory
1996-06
en
Text
UCAM-CL-TR-397
ISSN 1476-2986
An algorithm for routing a message along the shortest path between a
pair of processors in 2-jump circulant (undirected double fixed
step) network is given. The algorithm requires O(d) time for
preprocessing, and l = O(d) routing steps, where l is the distance
between the processors and d is the diameter of the network.
Design and implementation of an autostereoscopic camera
system
Dodgson, N.A.
Moore, J.R.
University of Cambridge, Computer Laboratory
1996-06
en
Text
UCAM-CL-TR-398
ISSN 1476-2986
An autostereoscopic display provides the viewer with a
three-dimensional image without the need for special glasses, and
allows the user to look around objects in the image by moving the
head left-right. The time-multiplexed autostereo display developed
at the University of Cambridge has been in operation since late
1991.
An autostereoscopic camera system has been designed and implemented.
It is capable of taking video input from up to sixteen cameras, and
multiplexing these into a video output stream with a pixel rate an
order of magnitude faster than the individual input streams. Testing
of the system with eight cameras and a Cambridge Autostereo Display
has produced excellent live autostereoscopic video.
This report describes the design of this camera system which has
been successfully implemented and demonstrated. Problems which arose
during this process are discussed, and a comparison with similar
systems made.
OASIS: An open architecture for secure interworking
services
Hayton, Richard
University of Cambridge, Computer Laboratory
1996-06
en
Text
UCAM-CL-TR-399
ISSN 1476-2986
An emerging requirement is for applications and distributed services
to cooperate or inter-operate. Mechanisms have been devised to hide
the heterogeneity of the host operating systems and abstract the
issues of distribution and object location. However, in order for
systems to inter-operate securely there must also be mechanisms to
hide differences in security policy, or at least negotiate between
them.
This would suggest that a uniform model of access control is
required. Such a model must be extremely flexible with respect to
the specification of policy, as different applications have
radically different needs. In a widely distributed environment this
situation is exacerbated by the differing requirements of different
organisations, and in an open environment there is a need to
interwork with organisations using alternative security mechanisms.
Other proposals for the interworking of security mechanisms have
concentrated on the enforcement of access policy, and neglected the
concerns of freedom of expression of this policy. For example it is
common to associate each request with a user identity, and to use
this as the only parameter when performing access control. This work
describes an architectural approach to security. By reconsidering
the role of the client and the server, we may reformulate access
control issues in terms of client naming.
We think of a client as obtaining a name issued by a service; either
based on credentials already held by the client, or by delegation
from another client. A grammar has been devised that allows the
conditions under which a client may assume a name to be specified,
and the conditions under which use of the name will be revoked. This
allows complex security policies to be specified that define how
clients of a service may interact with each other (through election,
delegation and revocation), how clients interact with a service (by
invoking operations or receiving events) and how clients and
services may inter-operate. (For example, a client of a Login
service may become a client of a file service.)
This approach allows great flexibility when integrating a number of
services, and reduces the mismatch of policies common in
heterogeneous systems. A flexible security definition is meaningless
if not backed by a robust and efficient implementation. In this
thesis we present a systems architecture that can be implemented
efficiently, but that allows individual services to ‘fine tune’ the
trade-offs between security, efficiency and freedom of policy
expression. The architecture is inherently distributed and scalable,
and includes mechanisms for rapid and selective revocation of
privileges which may cascade between services and organisations.
Monitoring the behaviour of distributed systems
Schwiderski, Scarlet
University of Cambridge, Computer Laboratory
1996-07
en
Text
UCAM-CL-TR-400
ISSN 1476-2986
Monitoring the behaviour of computing systems is an important task.
In active database systems, a detected system behaviour leads to the
triggering of an ECA (event-condition-action) rule. ECA rules are
employed for supporting database management system functions as well
as external applications. Although distributed database systems are
becoming more commonplace, active database research has to date
focussed on centralised systems. In distributed debugging systems, a
detected system behaviour is compared with the expected system
behaviour. Differences illustrate erroneous behaviour. In both
application areas, system behaviours are specified in terms of
events: primitive events represent elementary occurrences and
composite events represent complex occurrence patterns. At system
runtime, specified primitive and composite events are monitored and
event occurrences are detected. However, in active database systems
events are monitored in terms of physical time and in distributed
debugging systems events are monitored in terms of logical time. The
notion of physical time is difficult in distributed systems because
of their special characteristics: no global time, network delays,
etc.
This dissertation is concerned with monitoring the behaviour of
distributed systems in terms of physical time, i.e. the syntax, the
semantics, the detection, and the implementation of events are
considered.
The syntax of primitive and composite events is derived from the
work of both active database systems and distributed debugging
systems; differences and necessities are highlighted.
The semantics of primitive and composite events establishes when and
where an event occurs; the semantics depends largely on the notion
of physical time in distributed systems. Based on the model for an
approximated global time base, the ordering of events in distributed
systems is considered, and the structure and handling of timestamps
are illustrated. In specific applications, a simplified version of
the semantics can be applied which is easier and therefore more
efficient to implement.
Algorithms for the detection of composite events at system runtime
are developed; event detectors are distributed to arbitrary sites
and composite events are evaluated concurrently. Two different
evaluation policies are examined: asynchronous evaluation and
synchronous evaluation. Asynchronous evaluation is characterised by
the ad hoc consumption of signalled event occurrences. However,
since the signalling of events involves variable delays, the events
may not be evaluated in the system-wide order of their occurrence.
On the other hand, synchronous evaluation enforces events to be
evaluated in the system-wide order of their occurrence. But, due to
site failures and network congestion, the evaluation may block on a
fairly long-term basis.
The prototype implementation realises the algorithms for the
detection of composite events with both asynchronous and synchronous
evaluation. For the purpose of testing, primitive event occurrences
are simulated by distributed event simulators. Several tests are
performed illustrating the differences between asynchronous and
synchronous evaluation: the first is ‘fast and unreliable’ whereas
the latter is ‘slow and reliable’.
A classical linear λ-calculus
Bierman, Gavin
University of Cambridge, Computer Laboratory
1996-07
en
Text
UCAM-CL-TR-401
ISSN 1476-2986
This paper proposes and studies a typed λ-calculus for classical
linear logic. I shall give an explanation of a multiple-conclusion
formulation for classical logic due to Parigot and compare it to
more traditional treatments by Prawitz and others. I shall use
Parigot’s method to devise a natural deduction fomulation of
classical linear logic. This formulation is compared in detail to
the sequent calculus formulation. In an appendix I shall also
demonstrate a somewhat hidden connection with the paradigm of
control operators for functional languages which gives a new
computational interpretation of Parigot’s techniques.
Video mail retrieval using voice: report on collection of
naturalistic requests and relevance assessments
Jones, G.J.F.
Foote, J.T.
Spärck Jones, K.
Young, S.J.
University of Cambridge, Computer Laboratory
1996-09
en
Text
UCAM-CL-TR-402
ISSN 1476-2986
This report discusses the rationale, design, collection and initial
statistics of a message request and retrieved document relevance
assessment set for the Cambridge Video Mail Rewtrieval (VMR)
Project. This data set is designed to complement the VMR Database 1
(VMR1) message set and was designed for the testing of document
searching methods being investigated in the VMR project. The
combined message and request set is referred to as VMR1b.
Devices in a multi-service operating system
Barham, Paul Ronald
University of Cambridge, Computer Laboratory
1996-10
en
Text
UCAM-CL-TR-403
ISSN 1476-2986
Increases in processor speed and network and device bandwidth have
led to general purpose workstations being called upon to process
continuous media data in real time. Conventional operating systems
are unable to cope with the high loads and strict timing constraints
introduced when such applications form part of a multi-tasking
workload. There is a need for the operating system to provide
fine-grained reservation of processor, memory and I/O resources and
the ability to redistribute these resources dynamically. A small
group of operating systems researchers have recently proposed a
“vertically-structured” architecture where the operating system
kernel provides minimal functionality and the majority of operating
system code executes within the application itself. This structure
greatly simplifies the task of accounting for processor usage by
applications. The prototype Nemesis operating system embodies these
principles and is used as the platform for this work.
This dissertation extends the provision of Quality of Service
guarantees to the I/O system by presenting an architecture for
device drivers which minimises crosstalk between applications. This
is achieved by clearly separating the data-path operations, which
require careful accounting and scheduling, and the infrequent
control-path operations, which require protection and concurrency
control. The approach taken is to abstract and multiplex the I/O
data-path at the lowest level possible so as to simplify accounting,
policing and scheduling of I/O resources and enable
application-specific use of I/O devices.
The architecture is applied to several representative classes of
device including network interfaces, network connected peripherals,
disk drives and framestores. Of these, disks and framestores are of
particular interest since they must be shared at a very fine
granularity but have traditionally been presented to the application
via a window system or file-system with a high-level and
coarse-grained interface.
A device driver for the framestore is presented which abstracts the
device at a low level and is therefore able to provide each client
with guaranteed bandwidth to the framebuffer. The design and
implementation of a novel client-rendering window system is then
presented which uses this driver to enable rendering code to be
safely migrated into a shared library within the client.
A low-level abstraction of a standard disk drive is also described
which efficiently supports a wide variety of file systems and other
applications requiring persistent storage, whilst providing
guaranteed rates of I/O to individual clients. An extent-based file
system is presented which can provide guaranteed rate file access
and enables clients to optimise for application-specific access
patterns.
Adaptive parallelism for computing on heterogeneous
clusters
Shum, Kam Hong
University of Cambridge, Computer Laboratory
1996-11
en
Text
UCAM-CL-TR-404
ISSN 1476-2986
Until recent years most parallel machines have been made up of
closely-coupled microprocessor-based computers. With the advent of
high-performance workstations and high speed networking, the
aggregate computational power and memory capacity of workstation
clusters have become attractive and indispensable resources for
parallel computing. Techniques to harness the power of workstation
cluster computing, however, require the development of practical
methods for controlling heterogeneous resources dynamically.
This dissertation proposes an integrated framework that comprises
two related parts. The first part of the framework is a software
structure that enables parallel applications to be adaptable to
workload imbalances at runtime. To realize the adaptation,
applications are partitioned into small components called tasks. The
tasks are then grouped into grains; each grain is an object that
facilitates execution of tasks on a workstation. An application can
therefore optimize its performance by the reconfiguration of
task-to-grain and grain-to-workstation mappings. Based on the
software structure, the implementation and evaluation of workload
distribution schemes for data-parallel and task-parallel
applications are presented. The second part of the framework is a
resource management system that allocates resources to parallel
applications through competition. The applications respond to
allocation decisions by dynamic reconfiguration. The objectives of
the system are to maximise the speedup of the parallel applications
and, at the same time, to allocate workstations fairly and
efficiently to the applications. A prototype implementation which
provides a testbed for studying the dynamics of competition is
structured.
In addition a new structure for organizing replicated parallel
applications is developed and an architecture for a multi-user,
multi-parallel program environment based on the proposed framework
is suggested. The effectiveness of the concept and the framework is
demonstrated by the results of experiments conducted on the testbed.
Ther parallel applications involved in the experiments consist of
block-matrix multiplication, cycle-searching of a non-linear
cryptographic function, and simulators of an ATM network.
A tool to support formal reasoning about computer
languages
Boulton, Richard J.
University of Cambridge, Computer Laboratory
1996-11
en
Text
UCAM-CL-TR-405
ISSN 1476-2986
A tool to support formal reasoning about computer languages and
specific language texts is described. The intention is to provide a
tool that can build a formal reasoning system in a mechanical
theorem prover from two specifications, one for the syntax of the
language and one for the semantics. A parser, pretty-printer and
internal representations are generated from the former. Logical
representations of syntax and semantics, and associated theorem
proving tools, are generated from the combination of the two
specifications. The main aim is to eliminate tedious work from the
task of prototyping a reasoning tool for a computer language, but
the abstract specifications of the language also assist the
automation of proof.
Tool support for logics of programs
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1996-11
en
Text
UCAM-CL-TR-406
ISSN 1476-2986
Proof tools must be well designed if they are to be more effective
than pen and paper. Isabelle supports a range of formalisms, two of
which are described (higher-order logic and set theory). Isabelle’s
representation of logic is influenced by logic programming: its
“logical variables” can be used to implement step-wise refinement.
Its automatic proof procedures are based on search primitives that
are directly available to users. While emphasizing basic concepts,
the article also discusses applications such as an approach to the
analysis of security protocols.
The L4 microkernel on Alpha : Design and
implementation
Schoenberg, Sebastian
University of Cambridge, Computer Laboratory
1996-09
en
Text
UCAM-CL-TR-407
ISSN 1476-2986
The purpose of a microkernel is to cover the lowest level of the
hardware and to provide a more general platform to operating systems
and applications than the hardware itself. This has made microkernel
development increasingly interesting. Different types of
microkernels have been developed, ranging from kernels which merely
deal with the hardware infterface (Windows NT HAL), kernels
especially for embedded systems (RTEMS), to kernels for multimedia
streams and real time support (Nemesis) and general purpose kernels
(L4, Mach).
The common opinion that microkernels lead to deterioration in system
performance has been disproved by recent research. L4 is an example
of a fast and small, multi address space, message-based microkernel,
developed originally for Intel systems only. Based on the L4
interface, which should be as similar as possible on different
platforms, the L4 Alpha version has been developed.
This work describes design decisions, implementation and interfaces
of the L4 version for 64-bit Alpha processors.
Theorem proving with the real numbers
Harrison, John Robert
University of Cambridge, Computer Laboratory
1996-11
en
Text
UCAM-CL-TR-408
ISSN 1476-2986
This thesis discusses the use of the real numbers in theorem
proving. Typically, theorem provers only support a few ‘discrete’
datatypes such as the natural numbers. However the availability of
the real numbers opens up many interesting and important application
areas, such as the verification of floating point hardware and
hybrid systems. It also allows the formalization of many more
branches of classical mathematics, which is particularly relevant
for attempts to inject more rigour into computer algebra systems.
Our work is conducted in a version of the HOL theorem prover. We
describe the rigorous definitional construction of the real numbers,
using a new version of Cantor’s method, and the formalization of a
significant portion of real analysis. We also describe an advanced
derived decision procedure for the ‘Tarski subset’ of real algebra
as well as some more modest but practically useful tools for
automating explicit calculations and routine linear arithmetic
reasoning.
Finally, we consider in more detail two interesting application
areas. We discuss the desirability of combining the rigour of
theorem provers with the power and convenience of computer algebra
systems, and explain a method we have used in practice to achieve
this. We then move on to the verification of floating point
hardware. After a careful discussion of possible correctness
specifications, we report on two case studies, one involving a
transcendental function.
We aim to show that a theory of real numbers is useful in practice
and interesting in theory, and that the ‘LCF style’ of theorem
proving is well suited to the kind of work we describe. We hope also
to convince the reader that the kind of mathematics needed for
applications is well within the abilities of current theorem proving
technology.
Proving properties of security protocols by
induction
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1996-12
en
Text
UCAM-CL-TR-409
ISSN 1476-2986
Security protocols are formally specified in terms of traces, which
may involve many interleaved protocol runs. Traces are defined
inductively. Protocol descriptions model accidental key losses as
well as attacks. The model spy can send spoof messages made up of
components decrypted from previous traffic.
Correctness properties are verified using the proof tool
Isabelle/HOL. Several symmetric-key protocols have been studied,
including Needham-Schroeder, Yahalom and Otway-Rees. A new attack
has been discovered in a variant of Otway-Rees (already broken by
Mao and Boyd). Assertions concerning secrecy and authenticity have
been proved.
The approach rests on a common theory of messages, with three
operators. The operator “parts” denotes the components of a set of
messages. The operator “analz” denotes those parts that can be
decrypted with known keys. The operator “synth” denotes those
messages that can be expressed in terms of given components. The
three operators enjoy many algebraic laws that are invaluable in
proofs.
Proof style
Harrison, John
University of Cambridge, Computer Laboratory
1997-01
en
Text
UCAM-CL-TR-410
ISSN 1476-2986
We are concerned with how to communicate a mathematical proof to a
computer theorem prover. This can be done in many ways, while
allowing the machine to generate a completely formal proof object.
The most obvious choice is the amount of guidance required from the
user, or from the machine perspective, the degree of automation
provided. But another important consideration, which we consider
particularly significant, is the bias towards a ‘procedural’ or
‘declarative’ proof style. We will explore this choice in depth, and
discuss the strengths and weaknesses of declarative and procedural
styles for proofs in pure mathematics and for verification
applications. We conclude with a brief summary of our own
experiments in trying to combine both approaches.
Formalising process calculi in Higher Order Logic
Nesi, Monica
University of Cambridge, Computer Laboratory
1997-01
en
Text
UCAM-CL-TR-411
ISSN 1476-2986
In the past few years, several methods and tools based on process
calculi have been developed for verifying properties of concurrent
and communicating systems. In this dissertation the interactive
theorem prover HOL is used as a framework for supporting reasoning
about process calculi based on all the various components of their
formal theory. The aim is to build a sound and effective tool to
allow both verification of process specifications and meta-theoretic
reasoning. In particular the process calculus CCS is embedded in HOL
logic. This is achieved by first addressing the pure subset of this
calculus (no value passing) and then extending it to its
value-passing version. The CCS theory is mechanised in HOL by
following a purely definitional approach. This means that new
objects are embedded in HOL using definition mechanisms which
guarantee that no inconsistencies are introduced in the logic, and
by deriving new facts from definitions and/or previously proved
theorems by formal proof.
Pure CCS agent expressions are encoded as a type in the HOL logic,
in which initially actions are represented as strings, agents with
infinite behaviour are given through the rec-notation and agent
summation is the usual binary operator. Recursive agents are then
allowed to be defined through systems of recursive equations and to
be parameterised. This makes the type of CCS expressions polymorphic
and parameterised on the parameters’ type. Operational and
behavioural semantics and modal logic are defined and their
properties and laws derived in HOL. Several proof tools, such as
inference rules, conversions and tactics, are developed to enable
users to carry out their proofs in an interactive way and to
automate them whenever possible. Properties of infinite state
systems, e.g. a counter which can expand indefinitely, can be
formally verified in the resulting proof environment.
Then value-passing CCS is mechanised in HOL by translating
value-passing expressions into pure ones. This entails a more
general polymorphic type for pure agent expressions that includes an
indexed summation operator. The translation is proved to be correct
with respect to the semantics of value-passing CCS and then used at
meta-level, together with the HOL formalisation for pure CCS, for
developing behavioural theories for the value-passing calculus. A
proof environment is thus derived, in which users will directly work
on the value-passing specifications. A verification example
illustrates how proofs about the data are neatly separated from
proofs about the process behaviour and how ω-data-rules can be used
in a practical way to reason about value-passing agents defined over
an infinite value domain.
Observations on a linear PCF (preliminary report)
Bierman, G.M.
University of Cambridge, Computer Laboratory
1997-01
en
Text
UCAM-CL-TR-412
ISSN 1476-2986
This paper considers some theoretical and practical issues
concerning the use of linear logic as a logical foundation of
functional programming languages such as Haskell and SML. First I
give an operational theory for a linear PCF: the (typed) linear
λ-calculus extended with booleans, conditional and non-termination.
An operational semantics is given which corresponds in a precise way
to the process of β-reduction which originates from proof theory.
Using this operational semantics I define notions of observational
equivalence (sometimes called contextual equivalence). Surprisingly,
the linearity of the language forces a reworking of the traditional
notion of a context (the details are given in an appendix). A
co-inductively defined notion, applicative bi-simularity, is
developed and compared with observational equivalence using a
variant of Howe’s method. Interestingly the equivalence of these two
notions is greatly complicated by the linearity of the language.
These equivalences are used to study a call-by-name translation of
PCF into linear PCF. It is shown that this translation is adequate
but not fully abstract. Finally I show how Landin’s SECD machine can
be adpacted to execute linear PCF programs.
Mechanized proofs of security protocols: Needham-Schroeder
with public keys
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1997-01
en
Text
UCAM-CL-TR-413
ISSN 1476-2986
The inductive approach to verifying security protocols, previously
applied to shared-key encryption, is here applied to the public key
version of the Needham-Schroeder protocol. As before, mechanized
proofs are performed using Isabelle/HOL. Both the original, flawed
version and Lowe’s improved version are studied; the properties
proved highlight the distinctions between the two versions. The
results are compared with previous analyses of the same protocol.
The analysis reported below required only 30 hours of the author’s
time. The proof scripts execute in under three minutes.
A calculus for cryptographic protocols : The SPI
calculus
Abadi, Martín
Gordon, Andrew D.
University of Cambridge, Computer Laboratory
1997-01
en
Text
UCAM-CL-TR-414
ISSN 1476-2986
We introduce the spi calculus, an extension of the pi calculus
designed for the description and analysis of cryptographic
protocols. We show how to use the spi calculus, particularly for
studying authentication protocols. The pi calculus (without
extension) suffices for some abstract protocols; the spi calculus
enables us to consider cryptographic issues in more detail. We
represent protocols as processes in the spi calculus and state their
security properties in terms of coarse-grained notions of protocol
equivalence.
Application support for mobile computing
Pope, Steven Leslie
University of Cambridge, Computer Laboratory
1997-02
en
Text
UCAM-CL-TR-415
ISSN 1476-2986
In recent years small, completely portable computers have become
available on the marketplace. There is demand for such computers,
termed walkstations, to access network services while retaining
their mobility, and to operate effectively in a range of conditions.
Future office environments are expected to support wireless networks
with bandwidths which are several orders of magnitude greater than
are available outdoors. In such environments there will be powerful
compute servers available for a walkstation’s use.
This dissertation describes a novel architecture called Notus and
its support for applications operating in a mobile environment. The
concept of the traded handoff is introduced where applications are
able to participate in the handoff process, rebuilding connections
to the most appropriate service. This is expected to benefit
walkstations which roam over large distances, where connections to
servers would otherwise be strained, and also between heterogeneous
networks where cooperation between the networks in performing a
handoff might be problematic. It is also proposed in this
dissertation that applications could benefit from the ability to
migrate onto compute servers as a walkstation moves into the office
environment. This enables both the walkstation to conserve its own
resources, and applications to improve the service provided to the
end user. Finally by interleaving a traded handoff with the
migration process it is possible for a migrating application to
easily rebuild its connections as it moves to a new host.
The Notus architecture has been implemented, including a traded
handoff service and a new application migration service. The new
application migration service was designed since existing
application migration services are unsuited to mobile environments
and it enables applications to migrate between heterogeneous hosts
with little disruption. Applications which use the service are
written in a standard compiled langauge, and normal running
applications suffer little overhead. A number of existing
applications which are representative of a walkstation’s interactive
desk-top environment have been adapted to use the Notus
architecture, and are evaluated.
In summary, this work describes how mobility awareness and the
support from appropriate tools, can enable walkstation applications
to better adapt to a changing mobile environment, particularly when
the walkstation is carried between different network types or over
great distances.
DECLARE: a prototype declarative proof system for higher
order logic
Syme, Donald
University of Cambridge, Computer Laboratory
1997-02
en
Text
UCAM-CL-TR-416
ISSN 1476-2986
This report describes DECLARE, a prototype implementation of a
declarative proof system for simple higher order logic. The purpose
of DECLARE is to explore mechanisms of specification and proof that
may be incorporated into other theorem provers. It has been
developed to aid with reasoning about operational descriptions of
systems and languages. Proofs in DECLARE are expressed as proof
outlines, in a language that approximates written mathematics. The
proof language includes specialised constructs for (co-)inductive
types and relations. The system includes an abstract/article
mechanism that provides a way of isolating the process of
formalisation from what results, and simultaneously allow the
efficient separate processing of work units. After describing the
system we discuss our approach on two subsidiary issues: automation
and the interactive environment provided to the user.
Selective mesh refinement for interactive terrain
rendering
Brown, Peter J.C.
University of Cambridge, Computer Laboratory
1997-02
en
Text
UCAM-CL-TR-417
ISSN 1476-2986
Terrain surfaces are often approximated by geometric meshes to
permit efficient rendering. This paper describes how the complexity
of an approximating irregular mesh can be varied across its domain
in order to minimise the number of displayed facets while ensuring
that the rendered surface meets pre-determined resolution
requirements. We first present a generalised scheme to represent a
mesh over a continuous range of resolutions using the output from
conventional single-resolution approximation methods. We then
describe an algorithm which extracts a surface from this
representation such that the resolution of the surface is enhanced
only in specific areas of interest. We prove that the extracted
surface is complete, minimal, satisfies the given resolution
constraints and meets the Delaunay triangulation criterion if
possible. In addition, we present a method of performing smooth
visual transitions between selectively-refined meshes to permit
efficient animation of a terrain scene.
A HTML version of that report is at
https://www.cl.cam.ac.uk/research/rainbow/publications/pjcb/tr417/
Mechanized proofs for a recursive authentication
protocol
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1997-03
en
Text
UCAM-CL-TR-418
ISSN 1476-2986
A novel protocol has been formally analyzed using the prover
Isabelle/HOL, following the inductive approach described in earlier
work. There is no limit on the length of a run, the nesting of
messages or the number of agents involved. A single run of the
protocol delivers session keys for all the agents, allowing
neighbours to perform mutual authentication. The basic security
theorem states that session keys are correctly delivered to adjacent
pairs of honest agents, regardless of whether other agents in the
chain are compromised. The protocol’s complexity caused some
difficulties in the specification and proofs, but its symmetry
reduced the number of theorems to prove.
Video-augmented environments
Stafford-Fraser, James Quentin
University of Cambridge, Computer Laboratory
1997-04
en
Text
UCAM-CL-TR-419
ISSN 1476-2986
In the future, the computer will be thought of more as an assistant
than as a tool, and users will increasingly expect machines to make
decisions on their behalf. As with a human assistant, a machine’s
ability to make informed choices will often depend on the extent of
its knowledge of activities in the world around it. Equipping
personal computers with a large number of sensors for monitoring
their environment is, however, expensive and inconvenient, and a
preferable solution would involve a small number of input devices
with a broad scope of application. Video cameras are ideally suited
to many realworld monitoring applications for this reason. In
addition, recent reductions in the manufacturing costs of simple
cameras will soon make their widespread deployment in the home and
office economically viable. The use of video as an input device also
allows the creation of new types of user-interface, more suitable in
some circumstances than those afforded by the conventional keyboard
and mouse.
This thesis examines some examples of these ‘Video-Augmented
Environments’ and related work, and then describes two applications
in detail. The first, a ‘software cameraman’, uses the analysis of
one video stream to control the display of another. The second,
‘BrightBoard’, allows a user to control a computer by making marks
on a conventional whiteboard, thus ‘augmenting’ the board with many
of the facilities common to electronic documents, including the
ability to fax, save, print and email the image of the board. The
techniques which were found to be useful in the construction of
these applications are common to many systems which monitor
real-world video, and so they were combined in a toolkit called
‘Vicar’. This provides an architecture for ‘video plumbing’, which
allows standard videoprocessing components to be connected together
under the control of a scripting language. It is a single
application which can be programmed to create a variety of simple
Video-Augmented Environments, such as those described above, without
the need for any recompilation, and so should simplify the
construction of such applications in the future. Finally,
opportunities for further exploration on this theme are discussed.
Managing complex models for computer graphics
Sewell, Jonathan Mark
University of Cambridge, Computer Laboratory
1997-04
en
Text
UCAM-CL-TR-420
ISSN 1476-2986
Three-dimensional computer graphics is becoming more common as
increasing computational power becomes more readily available.
Although the images that can be produced are becoming more complex,
users’ expectations continue to grow. This dissertation examines the
changes in computer graphics software that will be needed to support
continuing growth in complexity, and proposes techniques for
tackling the problems that emerge.
Increasingly complex models will involve longer rendering times,
higher memory requirements, longer data transfer periods and larger
storage capacities. Furthermore, even greater demands will be placed
on the constructors of such models. This dissertation aims to
describe how to construct scalable systems which can be used to
visualise models of any size without requiring dedicated hardware.
This is achieved by controlling the quality of the results, and
hence the costs incurred. In addition, the use of quality controls
can become a tool to help users handle the large volume of
information arising from complex models.
The underlying approach is to separate the model from the graphics
application which uses it, so that the model exists independently.
By doing this, an application is free to access only the data which
is required at any given time. For the application to function in
this manner, the data must be in an appropriate form. To achieve
this, approximation hierarchies are defined as a suitable new model
structure. These utilise multiple representations of both objects
and groups of objects at all levels in the model.
In order to support such a structure, a novel method is proposed for
rapidly constructing simplified representations of groups of complex
objects. By calculating a few geometrical attributes, it is possible
to generate replacement objects that preserve important aspects of
the originals. Such objects, once placed into an approximation
hierarchy, allow rapid loading and rendering of large portions of a
model. Extensions to rendering algorithms are described that take
advantage of this structure.
The use of multiple representations encompasses not only different
quality levels, but also different storage formats and types of
objects. It provides a framework within which such aspects are
hidden from the user, facilitating the sharing and re-use of
objects. A model manager is proposed as a means of encapsulating
these mechanisms. This software gives, as far as possible, the
illusion of direct access to the whole complex model, while at the
same time making the best use of the limited resources available.
An abstract dynamic semantics for C
Norrish, Michael
University of Cambridge, Computer Laboratory
1997-05
en
Text
UCAM-CL-TR-421
ISSN 1476-2986
This report is a presentation of a formal semantics for the C
programming language. The semantics has been defined operationally
in a structured semantics style and covers the bulk of the core of
the language. The semantics has been developed in a theorem prover
(HOL), where some expected consequences of the language definition
Using the BONITA primitives: a case study
Rowstron, Antony
University of Cambridge, Computer Laboratory
1997-05
en
Text
UCAM-CL-TR-422
ISSN 1476-2986
The co-ordination language Linda has been used for parallel
processing for many years. Linda uses a shared tuple space and a
number of primitives to provide access to the tuple space and
thereby enabling communication between processes executing
concurrently. Linda provides asynchronous communication between
processes, but synchronous access between the processes and the
tuple spaces. The Bonita primitives are a different set of
primitives that provide asynchronous access to the tuple spaces. The
Bonita priomitives can emulate the primary Linda primitives and
therefore provides both asynchronous access and synchronous access
to tuple spaces. It has been previously claimed that asynchronous
tuple space access primitives are required to provide new
co-ordination constructs and to improve performance for
geographically distributed processes which are required to
co-ordinate distributed processes (or agents).
In this paper a talk program is used as an example to demonstrate
that the concept of tuple spaces are well suited for process
co-ordination for distributed processes (or agents), and to provide
a comparison between the use of Linda primitives and the Bonita
primitives. It is shown that asynchronous tuple space access is
essential for such process co-ordination.
Symbol grounding: Learning categorical and sensorimotor
predictions for coordination in autonomous robots
MacDorman, Karl F.
University of Cambridge, Computer Laboratory
1997-05
en
Text
UCAM-CL-TR-423
ISSN 1476-2986
To act intelligently, agents must be able to adapt to changing
behavioural possibilities. This dissertation proposes a model that
enables them to do this. An agent learns sensorimotor predictions
from spatiotemporal correlations in sensory projections, motor
signals, and physiological variables. Currently elicited predictions
constitute its model of the world.
Agents learn predictions for mapping between different sensory
modalities. In one example a robot records sensory projections as
points in a multidimensional space. It coordinates hand-eye
movements by using closest-point approximations to map between
vision and proprioception. Thus, one modality elicits predictions
more closely identifiable with another. In a different example, an
agent generalizes about a car’s sensorimotor relations by weighting
sensorimotor variables according to their mutual influence: it
learns to navigate without any a priori model of the car’s dynamics.
With feedback from miscategorization, an agent can develop links
between categorical representations and the relevant objects they
distinguish. Wavelet analysis provides a neurologically plausible
means of accentuating invariance that can subserve categorization.
In some experiments, categorical representations, derived from
inter-category invariance after wavelet analysis, proved to be
efficient and accurate at distinguishing different species of
mushrooms.
In a simulation of fish chemoreception, agents learn sensorimotor
predictions that uncover salient invariance in their environment.
Predictions are formed by quantizing a sensory subspace after each
dimension has been weighted according to its impact on physiological
variables. As these predictions also map from motor signals to
likely changes in sensory projections, the agent can chain backwards
from desired outcomes to form plans for their attainment.
Simplification with renaming: a general proof technique for
tableau and sequent-based provers
Massacci, Fabio
University of Cambridge, Computer Laboratory
1997-05
en
Text
UCAM-CL-TR-424
ISSN 1476-2986
Tableau and sequent calculi are the basis for most popular
interactive theorem provers for hardware and software verification.
Yet, when it comes to decision procedures or automatic proof search,
tableaux are orders of magnitude slower than Davis-Putnam, SAT based
procedures or other techniques based on resolution.
To meet this challenge, this paper proposes a theoretical
innovation: the rule of simplification, which plays the same role
for tableaux as subsumption does for resolution, and unit for
Davis-Putman.
This technique gives a unifying view of a number of tableaux-like
calculi such as DPLL, KE, HARP, hyper-tableaux etc. For instance the
stand-alone nature of the first-order Davis-Putnam-Longeman-Loveland
procedure can be explained away as a case of Smullyan tableau with
propositional simplification.
Besides its computational effectiveness, the simplicity and
generality of simplification make its extension possible in a
uniform way. We define it for propositional and first order logic
and a wide range of modal logics. For a full-fledged first order
simplification we combine it with another technique, renaming, which
subsumes the use of free universal variables in sequent and tableau
calculi.
New experimental results are given for random SAT and the IFIP
benchmarks for hardware verification.
Should your specification language be typed?
Lamport, Leslie
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1997-05
en
Text
UCAM-CL-TR-425
ISSN 1476-2986
Most specification languages have a type system. Type systems are
hard to get right, and getting them wrong can lead to
inconsistencies. Set theory can serve as the basis for a
specification language without types. This possibility, which has
been widely overlooked, offers many advantages. Untyped set theory
is simple and is more flexible than any simple typed formalism.
Polymorphism, overloading, and subtyping can make a type system more
powerful, but at the cost of increased complexity, and such
refinements can never attain the flexibility of having no types at
all. Typed formalisms have advantages too, stemming from the power
of mechanical type checking. While types serve little purpose in
hand proofs, they do help with mechanized proofs. In the absence of
verification, type checking can catch errors in specifications. It
may be possible to have the best of both worlds by adding typing
annotations to an untyped specification language.
We consider only specification languages, not programming languages.
Action selection methods using reinforcement
learning
Humphrys, Mark
University of Cambridge, Computer Laboratory
1997-06
en
Text
UCAM-CL-TR-426
ISSN 1476-2986
The Action Selection problem is the problem of run-time choice
between conflicting and heterogenous goals, a central problem in the
simulation of whole creatures (as opposed to the solution of
isolated uninterrupted tasks). This thesis argues that Reinforcement
Learning has been overlooked in the solution of the Action Selection
problem. Considering a decentralised model of mind, with internal
tension and competition between selfish behaviors, this thesis
introduces an algorithm called “W-learning”, whereby different parts
of the mind modify their behavior based on whether or not they are
succeeding in getting the body to execute their actions. This thesis
sets W-learning in context among the different ways of exploiting
Reinforcement Learning numbers for the purposes of Action Selection.
It is a ‘Minimize the Worst Unhappiness’ strategy. The different
methods are tested and their strengths and weaknesses analysed in an
artificial world.
Proving Java type soundness
Syme, Don
University of Cambridge, Computer Laboratory
1997-06
en
Text
UCAM-CL-TR-427
ISSN 1476-2986
This technical report describes a machine checked proof of the type
soundness of a subset of the Java language called Java_s. A formal
semantics for this subset has been developed by Drossopoulou and
Eisenbach, and they have sketched an outline of the type soundness
proof. The formulation developed here complements their written
semantics and proof by correcting and clarifying significant
details; and it demonstrates the utility of formal, machine checking
when exploring a large and detailed proof based on operational
semantics. The development also serves as a case study in the
application of ‘declarative’ proof techniques to a major property of
an operational system.
Floating point verification in HOL Light: the exponential
function
Harrison, John
University of Cambridge, Computer Laboratory
1997-06
en
Text
UCAM-CL-TR-428
ISSN 1476-2986
In that they often embody compact but mathematically sophisticated
algorithms, operations for computing the common transcendental
functions in floating point arithmetic seem good targets for formal
verification using a mechanical theorem prover. We discuss some of
the general issues that arise in verifications of this class, and
then present a machine-checked verification of an algorithm for
computing the exponential function in IEEE-754 standard binary
floating point arithmetic. We confirm (indeed strengthen) the main
result of a previously published error analysis, though we uncover a
minor error in the hand proof and are forced to confront several
subtle issues that might easily be overlooked informally.
Our main theorem connects the floating point exponential to its
abstract mathematical counterpart. The specification we prove is
that the function has the correct overflow behaviour and, in the
absence of overflow, the error in the result is less than 0.54 units
in the last place (0.77 if the answer is denormalized) compared
against the exact mathematical exponential function. The algorithm
is expressed in a simple formalized programming language, intended
to be a subset of real programming and hardware description
languages. It uses underlying floating point operations (addition,
multiplication etc.) that are assumed to conform to the IEEE-754
standard for binary floating point arithmetic.
The development described here includes, apart from the proof
itself, a formalization of IEEE arithmetic, a mathematical semantics
for the programming language in which the algorithm is expressed,
and the body of pure mathematics needed. All this is developed
logically from first principles using the HOL Light prover, which
guarantees strict adherence to simple rules of inference while
allowing the user to perform proofs using higher-level derived
rules. We first present the main ideas and conclusions, and then
collect some technical details about the prover and the underlying
mathematical theories in appendices.
Compilation and equivalence of imperative objects
Gordon, Andrew D.
Hankin, Paul D.
Lassen, Søren B.
University of Cambridge, Computer Laboratory
1997-06
en
Text
UCAM-CL-TR-429
ISSN 1476-2986
We adopt the untyped imperative object calculus of Abadi and
Cardelli as a minimal setting in which to study problems of
compilation and program equivalence that arise when compiling
object-oriented languages. We present both a big-step and a
small-step substitution-based operational semantics for the
calculus. Our first two results are theorems asserting the
equivalence of our substitution-based semantics with a closure-based
semantics like that given by Abadi and Cardelli. Our third result is
a direct proof of the correctness of compilation to a stack-based
abstract machine via a small-step decompilation algorithm. Our
fourth result is that contextual equivalence of objects coincides
with a form of Mason and Talcott’s CIU equivalence; the latter
provides a tractable means of establishing operational equivalences.
Finally, we prove correct an algorithm, used in our prototype
compiler, for statically resolving method offsets. This is the first
study of correctness of an object-oriented abstract machine, and of
operational equivalence for the imperative object calculus.
Video mail retrieval using voice: Report on topic spotting
(Deliverable report on VMR task no. 6)
Jones, G.J.F.
Foote, J.T.
Sparck Jones, K.
Young, S.J.
University of Cambridge, Computer Laboratory
1997-07
en
Text
UCAM-CL-TR-430
ISSN 1476-2986
This report describes research on topic spotting in audio document
retrieval carried out in years 2 and 3 of the Cambridge Video Mail
Retrieval (VMR) project. Topic spotting within VMR was concerned
with ad-hoc querying of a message archive using classical
information retrieval techniques developed from experience with text
archives. The report describes experiments using three approaches to
document indexing: fixed-vocabulary keyword spotting,
open-vocabulary search term indexing using phone lattices, and
message transcription using large vocabulary speech recognition.
Additional experiments investigate the combination of these
techniques for improved retrieval effectiveness.
The MCPL programming manual and user guide
Richards, Martin
University of Cambridge, Computer Laboratory
1997-07
en
Text
UCAM-CL-TR-431
ISSN 1476-2986
MCPL is a programming language that has been derived from BCPL by
the inclusion of features found in ML, C and Prolog. Like BCPL, it
is typeless, uses a contiguous runtime stack and has no builtin
garbage collector, but it does make extensive use of ML-like pattern
matching. The low level aspects of the language resemble those of
BCPL and C. MCPL uses its own function calling sequence, however it
is designed to allow MCPL and C functions to call each other.
Notable features of MCPL are its pattern matching facilities and the
simple way in which data structures are handled.
This document gives the definition of the language, its library and
how to obtain and install the system.
On two formal analyses of the Yahalom protocol
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1997-07
en
Text
UCAM-CL-TR-432
ISSN 1476-2986
The Yahalom protocol is one of those analyzed by Burrows et al. in
the BAN paper. Based upon their analysis, they have proposed
modifications to make the protocol easier to understand and analyze.
Both versions of Yahalom have now been proved, using Isabelle/HOL,
to satisfy strong security goals. The mathematical reasoning behind
these machine proofs is presented informally.
The new proofs do not rely on a belief logic; they use an entirely
different formal model, the inductive method. They confirm the BAN
analysis and the advantages of the proposed modifications. The new
proof methods detect more flaws than BAN and analyze protocols in
finer detail, while remaining broadly consistent with the BAN
principles. In particular, the proofs confirm the explicitness
principle of Abadi and Needham.
Backtracking algorithms in MCPL using bit patterns and
recursion
Richards, Martin
University of Cambridge, Computer Laboratory
1997-07
en
Text
UCAM-CL-TR-433
ISSN 1476-2986
This paper presents example programs, implemented in MCPL, that use
bit pattern techniques and recursion for the efficient solution of
various tree search problems.
Demonstration programs for CTL and μ-calculus symbolic model
checking
Richards, Martin
University of Cambridge, Computer Laboratory
1997-08
en
Text
UCAM-CL-TR-434
ISSN 1476-2986
This paper presents very simple implementations of Symbolic Model
Checkers for both Computational Tree Logic (CTL) and μ-calculus.
They are intended to be educational rather than practical. The first
program discovers, for a given non-deterministic finite state
machine (NFSM), the states for which a given CTL formula holds. The
second program does the same job for μ-calculus formulae.
For simplicity the number of states in the NFSM has been limited to
32 and a bit pattern representation is used to represent the boolean
functions involved. It would be easy to extend both programs to use
ordered binary decision diagrams more normally used in symbolic
model checking.
The programs include lexical and syntax analysers for the formulae,
the model checking algorithms and drivers to exercise them with
respect to various simple machines. The programs are implemented in
MCPL. A brief summary of MCPL is given at the end.
Global/local subtyping for a distributed
π-calculus
Sewell, Peter
University of Cambridge, Computer Laboratory
1997-08
en
Text
UCAM-CL-TR-435
ISSN 1476-2986
In the design of mobile agent programming languages there is a
tension between the implementation cost and the expressiveness of
the communication mechanisms provided. This paper gives a static
type system for a distributed π-calculus in which the input and
output of channels may be either global or local. This allows
compile-time optimization where possible but retains the
expressiveness of channel communication. Subtyping allows all
communications to be invoked uniformly. Recursive types and products
are included. The distributed π-calculus used integrates location
and migration primitives from the Distributed Join Calculus with
asynchronous π communication, taking a simple reduction semantics.
Some alternative calculi are discussed.
A new method for estimating optical flow
Clocksin, W.F.
University of Cambridge, Computer Laboratory
1997-11
en
Text
UCAM-CL-TR-436
ISSN 1476-2986
Accurate and high density estimation of optical flow vectors in an
image sequence is accomplished by a method that estimates the
velocity distribution function for small overlapping regions of the
image. Because the distribution is multimodal, the method can
accurately estimate the change in velocity near motion contrast
borders. Large spatiotemporal support without sacrificing spatial
resolution is a feature of the method, so it is not necessary to
smooth the resulting flow vectors in a subsequent operation, and
there is a certain degree of resistance to aperture and aliasing
effects. Spatial support also provides for the accurate estimation
of long-range displacements, and subpixel accuracy is achieved by a
simple weighted mean near the mode of the velocity distribution
function.
The method is demonstrated using image sequences obtained from the
analysis of ceramic and metal materials under stress. The
performance of the system under degenerate conditions is also
analysed to provide insight into the behaviour of optical flow
methods in general.
Trusting in computer systems
Harbison, William S.
University of Cambridge, Computer Laboratory
1997-12
en
Text
UCAM-CL-TR-437
ISSN 1476-2986
We need to be able to reason about large systems, and not just about
their components. For this we need new conceptual tools, and this
dissertation therefore indicates the need for a new methodology
which will allow us to better identify areas of possible conflict or
lack of knowledge in a system.
In particular, it examines at the concept of trust, and how this can
help us to understand the basic security aspects of a system. The
main proposal of this present work is that systems are viewed in a
manner which analyses the conditions under which they have been
designed to perform, and the circumstances under which they have
been implemented, and then compares the two. This problem is then
examined from the point of what is being trusted in a system, or
what it is being trusted for.
Starting from an approach developed in a military context, we
demonstrate how this can lead to unanticipated risks when applied
inappropriately. We further suggest that ‘trust’ be considered a
relative concept, in contast to the more usual usage, and that it is
not the result of knowledge but a substitute for it. The utility of
these concepts is in their ability to quantify the risks associated
with a specific participant, whether these are explicitly accepted
by them, or not.
We finally propose a distinction between ‘trust’ and ‘trustworthy’
and demonstrate that most current uses of the term ‘trust’ are more
appropriately viewed as statements of ‘trustworthiness’. Ultimately,
therefore, we suggest that the traditional “Orange Book” concept of
trust resulting from knowledge can violate the security policy of a
system.
An architecture for scalable and deterministic video
servers
Shi, Feng
University of Cambridge, Computer Laboratory
1997-11
en
Text
UCAM-CL-TR-438
ISSN 1476-2986
A video server is a storage system that can provide a repository for
continuous media (CM) data and sustain CM stream delivery (playback
or recording) through networks. The voluminous nature of CM data
demands a video server to be scalable in order to serve a large
number of concurrent client requests. In addition, deterministic
services can be provided by a video server for playback because the
characteristics of variable bit rate (VBR) video can be analysed in
advance and used in run-time admission control (AC) and data
retrieval.
Recent research has made gigabit switches a reality, and the
cost/performance ratio of microprocessors and standard PCs is
dropping steadily. It would be more cost effective and flexible to
use off-the-shelf components inside a video server with a scalable
switched network as the primary interconnect than to make a special
purpose or massively parallel multiprocessor based video server.
This work advocates and assumes such a scalable video server
structure in which data is striped to multiple peripherals attached
directly to a switched network.
However, most contemporary distributed file systems do not support
data distribution across multiple networked nodes, let alone
providing quality of service (QoS) to CM applications at the same
time. It is the observation of this dissertation that the software
system framework for network striped video servers is as important
as the scalable hardware architecture itself. This leads to the
development of a new system architecture, which is scalable,
flexible and QoS aware, for scalable and deterministic video
servers. The resulting srchitecture is called Cadmus from sCAlable
and Deterministic MUlitmedia Servers.
Cadmus also provides integrated solutions to AC and actual QoS
enforcement in storage nodes. This is achieved by considering
resources such as CPU buffer, disk, and network, simultaneously but
not independently and by including both real-time (RT) and
non-real-time (NRT) activities, In addition, the potential to smooth
the variability of VBR videos using read-ahead under client buffer
constraints is identified. A new smoothing algorithm is presented,
analysed, and incorporated into the Cadmus architecture.
A prototype implementation of Cadmus has been constructed based on
distributed object computing and hardware modules directly connected
to an Asynchronous Transfer Mode (ATM) network. Experiments were
performed to evaluate the implementation and demonstrate the utility
and feasibility of the architecture and its AC criteria.
Applying mobile code to distributed systems
Halls, David A.
University of Cambridge, Computer Laboratory
1997-12
en
Text
UCAM-CL-TR-439
ISSN 1476-2986
Use of mobile code can make distributed systems and the abstractions
they provide more flexible to build and use.
Richer functionality can be given to the interaction between
processes by allowing code to be sent between them. More convenient,
application level operations can be made over a network. By making
higher order language features transmissible, distributed components
can be tightly bound together when they communicate. At the same
time familiar distributed systems can be built using mobile code.
Mobile code can make distributed systems adaptable to application
needs. Rather than fixing the interface to a resource and the
pattern of interaction with it, a minimal interface can be defined
and code implementing higher level interfaces placed alongside it as
and when required. These higher level interfaces can be application
specific, allowing for interaction patterns that were unknownat the
time the resource was made available. Sending code close to a
resource can also reduce network usage because the point of
interaction with it moves.
The combination of document markup supporting hypertext and a
language supporting state-saving allows for stateful client-server
sessions with stateless servers and lightweight clients. Putting
dormant mobile code in documents provides an alternative to holding
knowledge of application functionality on a server machine or
running arbitrary code on a client machine.
Mobile code helps to support user mobility. Personalised
environments that support state saving can follow a user between
computers. Heterogeneous state-saving allows a user’s programs to be
relocated between computers. By using a mobile code system with
language support for state-saving, applications can direct arbitrary
component migration without priming program servers with specific
support.
In summary, this dissertation supports the thesis that mobile code
can be used to enhance distributed systems.
Inductive analysis of the internet protocol TLS
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1997-12
en
Text
UCAM-CL-TR-440
ISSN 1476-2986
Internet browsers use security protocols to protect confidential
messages. An inductive analysis of TLS (a descendant of SSL 3.0) has
been performed using the theorem prover Isabelle. Proofs are based
on higher-order logic and make no assumptions concerning beliefs or
finiteness. All the obvious security goals can be proved; session
resumption appears to be secure even if old session keys have been
compromised. The analysis suggests modest changes to simplify the
protocol.
TLS, even at an abstract level, is much more complicated than most
protocols that researchers have verified. Session keys are
negotiated rather than distributed, and the protocol has many
optional parts. Nevertheless, the resources needed to verify TLS are
modest. The inductive approach scales up.
A generic tableau prover and its integration with
Isabelle
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1998-01
en
Text
UCAM-CL-TR-441
ISSN 1476-2986
A generic tableau prover has been implemented and integrated with
Isabelle. It is based on leantap but is much more complicated, with
numerous modifications to allow it to reason with any supplied set
of tableau rules. It has a higher-order syntax in order to support
the binding operators of set theory; unification is first-order
(extended for bound variables in obvious ways) instead of
higher-order, for simplicity.
When a proof is found, it is returned to Isabelle as a list of
tactics. Because Isabelle verifies the proof, the prover can cut
corners for efficiency’s sake without compromising soundness. For
example, it knows almost nothing about types.
A combination of nonstandard analysis and geometry theorem
proving, with application to Newton’s Principia
Fleuriot, Jacques
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1998-01
en
Text
UCAM-CL-TR-442
ISSN 1476-2986
The theorem prover Isabelle is used to formalise and reproduce some
of the styles of reasoning used by Newton in his Principia. The
Principia’s reasoning is resolutely geometric in nature but contains
“infinitesimal” elements and the presence of motion that take it
beyond the traditional boundaries of Euclidean Geometry. These
present difficulties that prevent Newton’s proofs from being
mechanised using only the existing geometry theorem proving (GTP)
techniques.
Using concepts from Robinson’s Nonstandard Analysis (NSA) and a
powerful geometric theory, we introduce the concept of an
infinitesimal geometry in which quantities can be infinitely small
or infinitesimal. We reveal and prove new properties of this
geometry that only hold because infinitesimal elements are allowed
and use them to prove lemmas and theorems from the Principia.
The inductive approach to verifying cryptographic
protocols
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1998-02
en
Text
UCAM-CL-TR-443
ISSN 1476-2986
Informal arguments that cryptographic protocols are secure can be
made rigorous using inductive definitions. The approach is based on
ordinary predicate calculus and copes with infinite-state systems.
Proofs are generated using Isabelle/HOL. The human effort required
to analyze a protocol can be as little as a week or two, yielding a
proof script that takes a few minutes to run.
Protocols are inductively defined as sets of traces. A trace is a
list of communication events, perhaps comprising many interleaved
protocol runs. Protocol descriptions incorporate attacks and
accidental losses. The model spy knows some private keys and can
forge messages using components decrypted from previous traffic.
Three protocols are analyzed below: Otway-Rees (which uses
shared-key encryption), Needham-Schroeder (which uses public-key
encryption), and a recursive protocol (which is of variable length).
One can prove that event ev always precedes event ev′ or that
property P holds provided X remains secret. Properties can be proved
from the viewpoint of the various principals: say, if A receives a
final message from B then the session key it conveys is good.
From rewrite rules to bisimulation congruences
Sewell, Peter
University of Cambridge, Computer Laboratory
1998-05
en
Text
UCAM-CL-TR-444
ISSN 1476-2986
The dynamics of many calculi can be most clearly defined by
reduction semantics. To work with a calculus, however, an
understanding of operational congruences is fundamental; these can
often be given tractable definitions or characterisations using a
labelled transition semantics. This paper considers calculi with
arbitary reduction semantics of three simple classes, firstly ground
term rewriting, then left-linear term rewriting, and then a class
which is esentially the action calculi lacking substantive name
binding. General definitions of labelled transitions are given in
each case, uniformly in the set of rewrite rules, and without
requiring the prescription of additional notions of observation.
They give rise to bisimulation congruences. As a test of the theory
it is shown that bisimulation for a fragment of CCS is recovered.
The transitions generated for a fragment of the Ambient Calculus of
Cardelli and Gordon, and for SKI combinators, are also discussed
briefly.
Secure sessions from weak secrets
Roe, Michael
Christianson, Bruce
Wheeler, David
University of Cambridge, Computer Laboratory
1998-07
en
Text
UCAM-CL-TR-445
ISSN 1476-2986
Sometimes two parties who share a weak secret k (such as a password)
wish to share a strong secret s (such as a session key) without
revealing information about k to a (possibly active) attacker. We
assume that both parties can generate strong random numbers and
forget secrets, and present three protocols for secure strong secret
sharing, based on RSA, Diffie-Hellman and El-Gamal. As well as being
simpler and quicker than their predecessors, our protocols also have
slightly stronger security properties: in particular, they make no
cryptographic use of s and so impose no subtle restrictions upon the
use which is made of s by other protocols.
A probabilistic model of information and retrieval:
development and status
Spärck Jones, K.
Walker, S.
Robertson, S.E.
University of Cambridge, Computer Laboratory
1998-08
en
Text
UCAM-CL-TR-446
ISSN 1476-2986
The paper combines a comprehensive account of the probabilistic
model of retrieval with new systematic experiments on TREC Programme
material. It presents the model from its foundations through its
logical development to cover more aspects of retrieval data and a
wider range of system functions. Each step in the argument is
matched by comparative retrieval tests, to provide a single coherent
account of a major line of research. The experiments demonstrate,
for a large test collection, that the probabilistic model is
effective and robust, and that it responds appropriately, with major
improvements in performance, to key features of retrieval
situations.
Are timestamps worth the effort? A formal
treatment
Bella, Giampaolo
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1998-09
en
Text
UCAM-CL-TR-447
ISSN 1476-2986
Theorem proving provides formal and detailed support to the claim
that timestamps can give better freshness guarantees than nonces do,
and can simplify the design of crypto-protocols. However, since they
rely on synchronised clocks, their benefits are still debatable. The
debate should gain from our formal analysis, which is achieved
through the comparison of a nonce-based crypto-protocol,
Needham-Schroeder, with its natural modification by timestamps,
Kerberos.
A computational interpretation of the λμ calculus
Bierman, G.M.
University of Cambridge, Computer Laboratory
1998-09
en
Text
UCAM-CL-TR-448
ISSN 1476-2986
This paper proposes a simple computational interpretation of
Parigot’s λμ-calculus. The λμ-calculus is an extension of the typed
λ-calculus which corresponds via the Curry-Howard correspondence to
classical logic. Whereas other work has given computational
interpretations by translating the λμ-calculus into other calculi, I
wish to propose here that the λμ-calculus itself has a simple
computational interpretation: it is a typed λ-calculus which is able
to save and restore the runtime environment. This interpretation is
best given as a single-step semantics which, in particular, leads to
a relatively simple, but powerful, operational theory.
Locales : A sectioning concept for Isabelle
Kammüller, Florian
Wenzel, Markus
University of Cambridge, Computer Laboratory
1998-10
en
Text
UCAM-CL-TR-449
ISSN 1476-2986
Locales are a means to define local scopes for the interactive
proving process of the theorem prover Isabelle. They delimit a range
in which fixed assumptions are made, and theorems are proved which
depend on these assumptions. A locale may also contain constants and
associated with pretty printing syntax.
Locales can be seen as a simple form of modules. They are similar to
sections as in Automath or Coq. Locales are used to enhance abstract
reasoning. It also discusses some implementation issues.
Open service support for ATM
van der Merwe, Jacobus Erasmus
University of Cambridge, Computer Laboratory
1998-11
en
Text
UCAM-CL-TR-450
ISSN 1476-2986
Asynchronous Transfer Mode (ATM) technology provides superior
transfer capabilities in an environment in which multiple services
are provided and carried by a single network. Fully exploiting this
potential is hampered by the assumption by standards bodies that a
single control architecture, which was derived from a mono-service
network, will filfil the needs of all applications in such a multi
service environment.
While this weakness has been widely recognised, previous efforts to
address it have met with limited success. This can be largely
attributed to the fact that such attempts have often been proposed
to replace one monolithic system with another. Avoiding this
“one-size-fits-all” approach, this dissertation presents an Open
Service Support Architecture (OSSA), in which multiple control
architectures can be operational simultaneously in the same physical
network. In this manner different control architectures, which
provide diverse functionality and were designed to different models,
can be accommodated.
A key concept of the OSSA is the partitioning of switch resources by
a software entity called a Divider. The subset of switch resources
is callled a switchlet, and the Divider allows each switchlet to be
controlled by a separate control architecture. The divider polices
the invocations of a control architecture to contain it in its
allocated switchlet. Switchlets are combined into virtual networks,
and a software entity called the Network Builder automates this
process. The Network Builder allows virtual networks of arbitrary
topology to by dynamically created and modified, and each virtual
network is therefore controlled by a separate instance of a control
architecture. The dissertation presents a proof of concept
implementation of the OSSA, and reports on the efficiency of various
implementations of crucial components.
The dynamic creation of virtual networks in the OSSA means that the
usage of resources in an ATM network now needs to be considered on
three time scales: short time scales for cell switching, longer time
scales for connection creation, and even longer time scales for
virtual network creation. The use of measurement based estimates of
effective bandwidth to effect resource management at the two longer
time scales of interest is investigated and the results presented.
Finally, the flexibility offered by the OSSA enables the use of
service specific control architectures (SSCAs). An SSCA is a control
architecture which utilises service specific knowledge in its
manipulation of network resources, thereby providing a more
efficient service than would be possible with a general purpose
control architecture. The design and implementation of an SCCA for
continuous media conferencing is presented.
The structure of open ATM control architectures
Rooney, Sean
University of Cambridge, Computer Laboratory
1998-11
en
Text
UCAM-CL-TR-451
ISSN 1476-2986
The design of networks capable of supporting a large number of
different services is one of the principal areas of network
research. ATM, by virtue of its ability to give resource guarantees
to arbitrary services, is likely to become the transport protocol
for high-speed service-independent networks. The ATM control plane —
handling as it does the needs of many distinct services with diverse
constraints — is necessarily complex. The approach adopted by
industry has been to try and adopt the techniques used to control
the telephony network to ATM.
This dissertation argues that current monolithic ATM signalling
standards reduce the service support flexibility that was the
principal motivating factor behind the introduction of ATM. It
argues that a more open approach is required if ATM is to be able to
meet the demands of a decentralised, deregulated service provision
market.
A natural approach in handling complex systems is to divide them
into simpler elements. This dissertation considers two types of
separation. Firstly it shows how a clean separation can be made
between the ATM control plane and the switch, allowing them to be
implemented and evolve independently. Secondly, as a consequence of
the clear separation of the controller from the switch, it
demonstrates how several distinct control architectures can coexist
simultaneously on the same physical network, removing the need for
one single monolithic control architecture and allowing network
operators to choose the control architecture most appropriate for
their purposes.
The utility and practicality of this approach are demonstrated
through the description of the structure of a switch-independent
control architecture which efficiently implements a complete range
of ATM control operations. Such a control architecture is more
versatile than conventional signalling systems, while the
environment in which it executes allows both standard and
proprietary signalling systems to coexist.
Network robustness is of primary importance for large scale
commercial networks. This dissertation shows how management of an
open control network can be made less centralised and more adaptive.
These qualities are particularly important in an environment in
which there may be many network operators managing distinct networks
simultaneously.
A formal proof of Sylow’s theorem : An experiment in
abstract algebra with Isabelle Hol
Kammüller, Florian
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1998-11
en
Text
UCAM-CL-TR-452
ISSN 1476-2986
The theorem of Sylow is proved in Isabelle HOL. We follow the proof
by Wielandt that is more general than the original and uses a
non-trivial combinatorial identity. The mathematical proof is
explained in some detail leading on to the mechanization of group
theory and the necessary combinatorics in Isabelle. We present the
mechanization of the proof in detail giving reference to theorems
contained in an appendix. Some weak points of the experiment with
respect to a natural treatment of abstract algebraic reasoning give
rise to a discussion of the use of module systems to represent
abstract algebra in theorem provers. Drawing from that, we present
tentative ideas for further research into a section concept for
Isabelle.
C formalised in HOL
Norrish, Michael
University of Cambridge, Computer Laboratory
1998-12
en
Text
UCAM-CL-TR-453
ISSN 1476-2986
We present a formal semantics of the C programming language,
covering both the type system and the dynamic behaviour of programs.
The semantics is wide-ranging, covering most of the language, with
its most significant omission being the C library. Using a
structural operational semantics we specify transition relations for
C’s expressions, statements and declarations in higher order logic.
The consistency of our definition is assured by its specification in
the HOL theorem prover. With the theorem prover, we have used the
semantics as the basis for a set of proofs of interesting theorems
about C. We investigate properties of expressions and statements
separately.
In our chapter of results about expressions, we begin with two
results about the interaction between the type system and the
dynamic semantics. We have both type preservation, that the values
produced by expressions conform to the type predicted for them; and
type safety, that typed expressions will not block, but will either
evaluate to a value, or cause undefined behaviour. We then also show
that two broad classes of expression are deterministic. This last
result is of considerable practical value as it makes later
verification proofs significantly easier.
In our chapter of results about statements, we prove a series of
derived rules that provide C with Floyd-Hoare style “axiomatic”
rules for verifying properties of programs. These rules are
consequences of the original semantics, not independently stated
axioms, so we can be sure of their soundness. This chapter also
proves the correctness of an automatic tool for constructing
post-conditions for loops with break and return statements.
Finally, we perform some simple verification case studies, going
some way towards demonstrating practical utility for the semantics
and accompanying tools.
This technical report is substantially the same as the PhD thesis I
submitted in August 1998. The minor differences between that
document and this are principally improvements suggested by my
examiners Andy Gordon and Tom Melham, whom I thank for their help
and careful reading.
Parametric polymorphism and operational
equivalence
Pitts, Andrew M.
University of Cambridge, Computer Laboratory
1998-12
en
Text
UCAM-CL-TR-454
ISSN 1476-2986
Studies of the mathematical properties of impredicative polymorphic
types have for the most part focused on the polymorphic lambda
calculus of Girard-Reynolds, which is a calculus of total
polymorphic functions. This paper considers polymorphic types from a
functional programming perspective, where the partialness arising
from the presence of fixpoint recursion complicates the nature of
potentially infinite (‘lazy’) datatypes. An approach to Reynolds’
notion of relational parametricity is developed that works directly
on the syntax of a programming language, using a novel closure
operator to relate operational behaviour to parametricity properties
of types. Working with an extension of Plotkin’s PCF with ∀-types,
lazy lists and existential types, we show by example how the
resulting logical relation can be used to prove properties of
polymorphic types up to operational equivalence.
Multiple modalities
Bierman, G.M.
University of Cambridge, Computer Laboratory
1998-12
en
Text
UCAM-CL-TR-455
ISSN 1476-2986
Linear logic removes the structural rules of weakening and
contraction and adds an S4-like modality (written !). Only formulae
of the form !φ can be weakened or contracted. An interesting
question is whether these two capabilities can be separated using
two different modalities. This question was studied semantically in
a comprehensive paper by Jacobs. This paper considers the question
proof-theoretically, giving sequent calculus, natural deduction and
axiomatic formulations.
An evaluation based approach to process calculi
Ross, Joshua Robert Xavier
University of Cambridge, Computer Laboratory
1999-01
en
Text
UCAM-CL-TR-456
ISSN 1476-2986
Process calculi have, starting with Milner’s CCS, traditionally been
expressed by specifying the operational semantics in terms of
action-labelled transition relations between process expressions.
Normally this has been done using transitions that are inductively
defined by rules following the structure of the process expressions.
This approach has been very successful but has suffered from certain
problems. One of these is that the construction of weak,
branching-time congruences has not been as simple as one might wish.
In particular the natural weak bisimulations are not congruences,
typically shown up by the introduction of summation. Secondly this
method has not lent itself to the development of congruences for
calculi that combine features of concurrency and higher-order
functional languages. Another problem is more aesthetic. It is that
in order to write these transition relations we need to use silent
(τ) actions which are supposed to be unobservable. However, we need
to represent them explicitly and make explicit reference to them in
defining the congruence relations.
In this thesis, an approach to process calculi based on evaluation
to committed forms is presented. In particular two process calculi
are given. The first is a first-order CCS-like calculus, NCCS. This
demonstrates the possibility of giving natural weak branching-time
congruences, with such features as summation, without the use of
explicit silent silent actions. Various bisumulations are defined on
NCCS, and these are related to existing equivalences for CCS. The
second is a higher order calculus, based on CML; a higher-order
functional language extended with concurrent features. Again it is
shown that a natural weak branching-time congruence exists. In both
cases a transition relation is also given and the relationship
between evaluation and transition is shown.
A concurrent object calculus: reduction and
typing
Gordon, Andrew D.
Hankin, Paul D.
University of Cambridge, Computer Laboratory
1999-02
en
Text
UCAM-CL-TR-457
ISSN 1476-2986
We obtain a new formalism for concurrent object-oriented languages
by extending Abadi abd Cardelli’s imperative object calculus with
operators for concurrency from the μ-calculus and with operators for
synchronisation based on mutexes. Our syntax of terms is extremely
expressive; in a precise sense it unifies notions of expression,
process, store, thread and configuration. We present a
chemical-style reduction semantics, and prove it equivalent to a
structural operational semantics. We identify a deterministic
fragment that is closed under reduction and show that it includes
the imperative object calculus. A collection of type systems for
object oriented constructs is at the heart of Abadi and Cardelli’s
work. We recast one of Abadi and Cardelli’s first-order type systems
with object types and subtyping in the setting of our calculus and
prove subject reduction. Since our syntax of terms includes both
stores and running expressions, we avoid the need to separate store
typing from typing of expressions. We translate communication
channels and choice-free asynchronous μ-calculus into our calculus
to illustrate its expressiveness; the types of read-only and
write-only channels are supertypes of read-write channels.
Final coalgebras as greatest fixed points in ZF set
theory
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1999-03
en
Text
UCAM-CL-TR-458
ISSN 1476-2986
A special final coalgebra theorem, in the style of Aczel (1988), is
proved within standard Zermelo-Fraenkel set theory. Aczel’s
Anti-Foundation Axiom is replaced by a variant definition of
function that admits non-well-founded constructions. Variant ordered
pairs and tuples, of possibly infinite length, are special cases of
variant functions. Analogues of Aczel’s solution and substitution
lemmas are proved in the style of Rutten and Turi (1993). The
approach is less general than Aczel’s, but the treatment of
non-well-founded objects is simple and concrete. The final coalgebra
of a functor is its greatest fixedpoint. Compared with previous work
(Paulson, 1995a), iterated substitutions and solutions are
considered, as well as final coalgebras defined with respect to
parameters. The disjoint sum construction is replaced by a smoother
treatment of urelements that simplifies many of the derivations. The
theory facilitates machine implementation of recursive definitions
by letting both inductive and coinductive definitions be represented
as fixedpoints. It has already been applied to the theorem prover
Isabelle (Paulson, 1994).
An open parallel architecture for data-intensive
applications
Afshar, Mohamad
University of Cambridge, Computer Laboratory
1999-07
en
Text
UCAM-CL-TR-459
ISSN 1476-2986
Data-intensive applications consist of both declarative
data-processing parts and imperative computational parts. For
applications such as climate modelling, scale hits both the
computational aspects which are typically handled in a procedural
programming language, and the data-processing aspects which are
handled in a database query language. Although parallelism has been
successfully exploited in the data-processing parts by parallel
evaluation of database queries associated with the application,
current database query languages are poor at expressing the
computational aspects, which are also subject to scale.
This thesis proposes an open architecture that delivers parallelism
shared between the database, system and application, thus enabling
the integration of the conventionally separated query and non-query
components of a data-intensive application. The architecture is
data-model independent and can be used in a variety of different
application areas including decision-support applications, which are
query based, and complex applications, which comprise procedural
language statements with embedded queries. The architecture
encompasses a unified model of parallelism and the realisation of
this model in the form of a language within which it is possible to
describe both the query and non-query components of data-intensive
applications. The language enables the construction of parallel
applications by the hierarchical composition of platform-independent
parallel forms, each of which implements a form of task or data
parallelism. These forms may be used to determine both query and
non-query actions.
Queries are expressed in a declarative language based on “monoid
comprehensions”. The approach of using monoids to model data types
and monoid homomorphisms to iterate over collection types enables
mathematically provable compile-time optimisations whilst also
facilitating multiple collection types and data type extensibility.
Monoid comprehension programs are automatically transformed into
parallel programs composed of applications of the parallel forms,
one of which is the “monoid homomorphism”. This process involves
identifying the parts of a query where task and data parallelism are
available and mapping that parallelism onto the most suitable form.
Data parallelism in queries is mapped onto a form that implements
combining tree parallelism for query evaluation and dividing tree
parallelism to realise data partitioning. Task parallelism is mapped
onto two separate forms that implement pipeline and independent
parallelism. This translation process is applied to all
comprehension queries including those in complex applications. The
result is a skeleton program in which both the query and non-query
parts are expressed within a single language. Expressions in this
language are amenable to the application of optimising skeleton
rewrite rules.
A complete prototype of the decision-support architecture has been
constructed on a 128-cell MIMD parallel computer. A demonstration of
the utility of the query framework is performed by modelling some of
OQL and a substantial subset of SQL. The system is evaluated for
query speedup with a number of hardware configurations using a large
music catalogue database. The results obtained show that the
implementation delivers the performance gains expected while
offering a convenient definition of the parallel environment.
Message reception in the inductive approach
Bella, Giampaolo
University of Cambridge, Computer Laboratory
1999-03
en
Text
UCAM-CL-TR-460
ISSN 1476-2986
Cryptographic protocols can be formally analysed in great detail by
means of Paulson’s Inductive Approach, which is mechanised by the
theorem prover Isabelle. The approach only relied on message sending
(and noting) in order to keep the models simple. We introduce a new
event, message reception, and show that the price paid in terms of
runtime is negligible because old proofs can be reused. On the other
hand, the new event enhances the global expressiveness, and makes it
possible to define an accurate notion of agents’ knowledge, which
extends and replaces Paulson’s notion of spy’s knowledge. We have
designed new guarantees to assure each agent that the peer does not
know the crucial message items of the session. This work thus
extends the scope of the Inductive approach. Finally, we provide
general guidance on updating the protocols analysed so far, and give
examples for some cases.
Integrating Gandalf and HOL
Hurd, Joe
University of Cambridge, Computer Laboratory
1999-03
en
Text
UCAM-CL-TR-461
ISSN 1476-2986
Gandalf is a first-order resolution theorem-prover, optimized for
speed and specializing in manipulations of large clauses. In this
paper I describe GANDALF TAC, a HOL tactic that proves goals by
calling Gandalf and mirroring the resulting proofs in HOL. This call
can occur over a network, and a Gandalf server may be set up
servicing multiple HOL clients. In addition, the translation of the
Gandalf proof into HOL fits in with the LCF model and guarantees
logical consistency.
Location-independent communication for mobile agents: a
two-level architecture
Sewell, Peter
Wojciechowski, Paweł T.
Pierce, Benjamin C.
University of Cambridge, Computer Laboratory
1999-04
en
Text
UCAM-CL-TR-462
ISSN 1476-2986
We study communication primitives for interaction between mobile
agents. They can be classified into two groups. At a low level there
are location dependent primitives that require a programmer to know
the current site of a mobile agent in order to communicate with it.
At a high level there are location independent primitives that allow
communication with a mobile agent irrespective of its current site
and of any migrations. Implementation of these requires delicate
distributed infrastructure. We propose a simple calculus of agents
that allows implementation of such distributed infrastructure
algorithms to be expressed as encodings, or compilations, of the
whole calculus into the fragment with only location dependent
communication. These encodings give executable descriptions of the
algorithms, providing a clean implementation strategy for prototype
languages. The calculus is equipped with a precise semantics,
providing a solid basis for understanding the algorithms and
reasoning about their correctness and robustness. Two sample
infrastructure algorithms are presented as encodings.
Secure composition of insecure components
Sewell, Peter
Vitek, Jan
University of Cambridge, Computer Laboratory
1999-04
en
Text
UCAM-CL-TR-463
ISSN 1476-2986
Software systems are becoming heterogeneous: instead of a small
number of large programs from well-established sources, a user’s
desktop may now consist of many smaller components that interact in
intricate ways. Some components will be downloaded from the network
from sources that are only partially trusted. A user would like to
know that a number of security properties hold, e.g. that personal
data is not leaked to the net, but it is typically infaesible to
verify that such components are well-behaved. Instead they must be
executed in a secure environment, or wrapper, that provides
fine-grain control of the allowable interactions between them, and
between components and other system resources.
In this paper we study such wrappers, focussing on how they can be
expressed in a way that enables their security properties to be
stated and proved rigorously. We introduce a model programming
language, the box-π calculus, that supports composition of software
components and the enforcement of security policies. Several example
wrappers are expressed using the calculus; we explore the delicate
security properties they guarantee.
Feature representation for the automatic analysis of
fluorescence in-situ hybridization images
Lerner, Boaz
Clocksin, William
Dhanjal, Seema
Hultén, Maj
Bishop, Christipher
University of Cambridge, Computer Laboratory
1999-05
en
Text
UCAM-CL-TR-464
ISSN 1476-2986
Fast and accurate analysis of flourescence in-situ hybridization
(FISH) images will depend mainly upon two components: a classifier
to discriminate between artifacts and valid signal data, and well
discriminating features to represent the signals. Our previous work
has focused on the first component. To investigate the second
component, we evaluate candidate feature sets by illustrating the
probability density functions and scatter plots for the features.
This analysis provides insight into dependencies between features,
indicates the relative importancce of members of a feature set, and
helps in identifying sources of potential classification errors. The
analysis recommends several intensity and hue-based features for
representing FISH signals. The recommendation is confirmed by the
probability of misclassification using a two-layer neural network
(NN), and also by a feature selection technique making use of a
class separability criterion. Represented by these intensity and
hue-based features, 90% of valid signals and artifacts are corrently
classified using the NN.
Gelfish – graphical environment for labelling FISH
images
Lerner, Boaz
Dhanjal, Seema
Hultén, Maj
University of Cambridge, Computer Laboratory
1999-05
en
Text
UCAM-CL-TR-465
ISSN 1476-2986
Dot counting in flourescence in-situ hybridization (FISH) images
that relies on an automatic focusing method for obtaining clearly
defined images is prone to errors. Our recently developed system has
dispensed with automatic focusing, and insteaqd relies on a larger
statistical sample of the specimen at a fixed focal plane. The
system is based on well-discriminating features to represent the
signals and a neural network classifier to discriminate between
artifacts and valid signal data. Results showed that nearly 90% of
valid signals and artifacts of two flourophores within 400 FISH
images were correctly classified. To train the classifier, accurate
labelling of the image is required. GELFISH is a Graphical
Environment for Labelling FISH images that enables the labelling of
FISH signals and the rejection of unanalysable nuclei simply and
rapidly. Feedback provided by the environment allows the user to
correct the results of labelling effortlessly by clicking GELFISH
buttons using the mouse. Furthermore, GELFISH is flexible and can be
modified easily for additional FISH applications. Implemented using
popular software, the environment can be employed on any computer by
any user.
Automatic signal classification in fluorescence in-situ
hybridization images
Lerner, Boaz
Clocksin, William
Dhanjal, Seema
Hultén, Maj
Bishop, Christipher
University of Cambridge, Computer Laboratory
1999-05
en
Text
UCAM-CL-TR-466
ISSN 1476-2986
Previous systems for dot counting in fluorescence in-situ
hybridization (FISH) images have relied on an automatic focusing
method for obtaining a clearly defined image. Because signals are
distributed in three dimensions within the nucleus and artifacts
such as debris and background fluorescence can attract the focusing
method , valid signals can be left unfocused or unseen. This leads
to dot counting errors, which increase with the number of probes.
The approach described here dispenses with automatic focusing, and
instead relies on a larger statistical sample of the specimen at a
fixed focal plane. Images across the specimen can be obtained in
significantly less time if a fixed focal plane is used. A trainable
classifier based on a neural network is used to discriminate between
valid and artifact signals represented by a set of features. This
improves on previous classification schemes that are based on
non-adaptable decision boundaries and are trained using only
examples of valid signals. Trained by examples of valid and artifact
signals, three NN classifiers, two of them hierarchical, each
achieve between 83% and 87% classification accuracy on unseen data.
When data is pre-discriminated in this way, errors in dot counting
can be significantly reduced.
Mechanizing UNITY in Isabelle
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
1999-06
en
Text
UCAM-CL-TR-467
ISSN 1476-2986
UNITY is an abstract formalism for proving properties of concurrent
systems, which typically are expressed using guarded assignments
[Chandy and Misra 1988]. UNITY has been mechanized in higher-order
logic using Isabelle, a proof assistant. Safety and progress
primitives, their weak forms (for the substitution axiom) and the
program composition operator (union) have been formalized. To give a
feel for the concrete syntax, the paper presents a few extracts from
the Isabelle definitions and proofs. It discusses a small example,
two-process mutual exclusion. A mechanical theory of unions of
programs supports a degree of compositional reasoning. Original work
on extending program states is presented and then illustrated
through a simple example involving an array of processes.
Synthesis of asynchronous circuits
Wilcox, Stephen Paul
University of Cambridge, Computer Laboratory
1999-07
en
Text
UCAM-CL-TR-468
ISSN 1476-2986
The majority of integrated circuits today are synchronous: every
part of the chip times its operation with reference to a single
global clock. As circuits become larger and faster, it becomes
progressively more difficult to coordinate all actions of the chip
to the clock. Asynchronous circuits do not suffer from this problem,
because they do not require global synchronization; they also offer
other benefits, such as modularity, lower power and automatic
adaptation to physical conditions.
The main disadvantage of asynchronous circuits is that there are few
tools to help with design. This thesis describes a new synthesis
tool for asynchronous modules, which combines a number of novel
ideas with existing methods for finite state machine synthesis.
Connections between modules are assumed to have unbounded finite
delays on all wires, but fundamental mode is used inside modules,
rather than the pessimistic speed-independent or
quasi-delay-insensitive models. Accurate technology-specific
verification is performed to check that circuits work correctly.
Circuits are described using a language based upon the Signal
Transition Graph, which is a well-known method for specifying
asynchronous circuits. Concurrency reduction techniques are used to
produce a large number of circuits that conform to a given
specification. Circuits are verified using a simulation algorithm
derived from the work of Brzozowski and Seger, and then performance
estimations are obtained by a gate-level simulator utilising a new
estimation of waveform slopes. Circuits can be ranked in terms of
high speed, low power dissipation or small size, and then the best
circuit for a particular task chosen.
Results are presented that show significant improvements over most
circuits produced by other synthesis tools. Some circuits are twice
as fast and dissipate half the power of equivalent speed-independent
circuits. Specification examples are provided which show that the
front-end specification is easier to use than current specification
approaches. The price that must be paid for the improved performance
is decreased reliability and technology dependence of the circuits
produced; the proposed tool can also can a very long time to produce
a result.
A combination of geometry theorem proving and nonstandard
analysis, with application to Newton’s Principia
Fleuriot, Jacques Désiré
University of Cambridge, Computer Laboratory
1999-08
en
Text
UCAM-CL-TR-469
ISSN 1476-2986
Sir Isaac Newton’s Philosophiae Naturalis Principia Mathematica (the
Principia) was first published in 1687 and set much of the
foundations that led to profound changes in modern science. Despite
the influence of the work, the elegance of the geometrical
techniques used by Newton is little known since the demonstrations
of most of the theorems set out in it are usually done using
calculus. Newton’s reasoning also goes beyond the traditional
boundaries of Euclidian geometry with the presence of both motion
and infinitesimals.
This thesis describes the mechanization of lemmas and propositions
from the Principia using formal tools developed in the generic
theorem prover Isabelle. We discuss the formalisation of a geometry
theory based on existing methods from automated geometry theorem
proving. The theory contains extra geometric notions, including the
definition of the ellipse and its tangent, that enables us to deal
with the motion of bodies and other physical aspects.
We introduce the formalization of a theory of filters and
ultrafilters, and the purely definitional construction of the
hyperreal numbers of Nonstandard Analysis (NSA). The hyperreals form
a proper field extension of the reals that contains new types of
numbers including infinitesimals and infinite numbers.
By combining notions from NSA and geometry theorem proving, we
propose an “infinitesimal” geometry in which quantities can be
infinitely small. This approach then reveals the the new properties
of the geometry that only hold because infinitesimal elements are
allowed. We also mechanize some analytic geometry and use it to
verify the geometry theories of Isabelle.
We then report on the main application of this framework. We discuss
the formalization of several results from the Principia and give a
detailed case study of one of its most important propositions: the
Propositio Kepleriana. An anomaly is revealed in Newton’s reasoning
through our rigorous mechanization.
Finally we present the formalization of a portion of mathematical
analysis using the nonstandard approach. We mechanize both standard
and nonstandard definitions of familiar concepts, prove their
equivalence, and use nonstandard arguments to provide intuitive yet
rigorous proofs of many of their properties.
Modular reasoning in Isabelle
Kammüller, Florian
University of Cambridge, Computer Laboratory
1999-08
en
Text
UCAM-CL-TR-470
ISSN 1476-2986
This work is concerned with modules for higher order logic theorem
provers, in particular Isabelle. Modules may be used to represent
abstract mathematical structures. This is typical for applications
in abstract algebra. In Chapter 1, we set out with the hypothesis
that for an adequate representation of abstract structures we need
modules that have a representation in the logic. We identify the
aspects of locality and adequacy that are connected to the idea of
modules in theorem provers.
In Chapter 2, we compare systems of interactive theorem provers and
their applicability to abstract algebra. Furthermore we investigate
a different family of proof systems based on type theory in Section
2.4.
We validate our hypothesis by performing a large case study in group
theory: a mechanization of Sylow’s theorem in Chapter 3.
Drawing from the experience gained by this large case study, we
develop a concept of locales in Chapter 4 that captures local
definitions, pretty printing syntax and local assumptions. This
concept is implemented and released with Iasbelle version 98-1.
However, this concept alone is not sufficient to describe abstract
structures. For example, structures like groups and rings need a
more explicit representation as objects in the logic. A
mechanization of dependent Σ-types and Π-types as typed sets in
higher order logic is produced in Chapter 5 to represent structures
adequately.
In Chapter 6, we test our results by applying the two concepts we
developed in combination. First, we reconsider the Sylow case study.
Furthermore, we demonstrate more algebraic examples. Factorization
of groups, direct product of groups, and ring automorphisms are
constructions that form themselves groups, which is formally proved.
We also discuss the proof of the full version of Tarski’s fixed
point theorem. Finally we consider how operations on modules can be
realized by structures as dependent types. Locales are used in
addition; we illustrate the reuse of theorems proved in a locale and
the construction of a union of structures.
Murphy’s law, the fitness of evolving species, and the
limits of software reliability
Brady, Robert M.
Anderson, Ross J.
Ball, Robin C.
University of Cambridge, Computer Laboratory
1999-09
en
Text
UCAM-CL-TR-471
ISSN 1476-2986
We tackle two problems of interest to the software assurance
community. Firstly, existing models of software development (such as
the waterfall and spiral models) are oriented towards one-off
software development projects, while the growth of mass market
computing has led to a world in which most software consists of
packages which follow an evolutionary development model. This leads
us to ask whether anything interesting and useful may be said about
evolutionary development. We answer in the affirmative. Secondly,
existing reliability growth models emphasise the Poisson
distribution of individual software bugs, while the empirically
observed reliability growth for large systems is asymptotically
slower than this. We provide a rigorous explanation of this
phenomenon. Our reliability growth model is inspired by statistical
thermodynamics, but also applies to biological evolution. It is in
close agreement with experimental measurements of the fitness of an
evolving species and the reliability of commercial software
products. However, it shows that there are significant differences
between the evolution of software and the evolution of species. In
particular, we establish maximisation properties corresponding to
Murphy’s law which work to the advantage of a biological species,
but to the detriment of software reliability.
Simulating music learning with autonomous listening agents:
entropy, ambiguity and context
Reis, Ben Y.
University of Cambridge, Computer Laboratory
1999-09
en
Text
UCAM-CL-TR-472
ISSN 1476-2986
Music learning describes the gradual process of acculturation
through which listeners in different cultures develop diverse sets
of musical preferences and intuitions. This dissertation describes
Maestro, a system designed over the course of this research to
simulate certain aspects of music listening and learning.
In order to maintain the unbaised flexibility necessary for handling
music from different styles, Maestro does not incorporate any a
priori style-specific knowledge into its design. Instead, Maestro is
based on a bottom up approach that maximises the use of perceptual
information present in a performance.
Maestro’s operation involves four stages: it first segments a
musical performance on-line according to perceptual cues
(segmentation) and constructs an appropriate model of the
performance (modelling), based on the context modelling paradign.
This model is simultaneously used to generate expectations about
upcoming events (prediction) and to interpret events once they have
arrived (parsing).
Ambiguity is an essential part of music listening, especially in the
context of learning, and can cause multiple hypotheses of
interpretation to arise. A novel multi-agent methodology is
developed and incorporated into Maestro for generating, maintaining,
and reconciling these hypotheses. An information theoretic approach,
based on measuring two types of entropy, is used to objectively
evaluate the system’s relative prediction performance. It is also
found that entropy, along with a measure of agent activation, is
useful for identifying and classifying different types of ambiguity.
Experiments performed with a collection of 100 Bach chorale melodies
provides a basis for comparison with previous machine modelling
reseasrch and with data from human subjects. A much larger
collection of roughly 8,000 folk songs from different cultures
enables significant large scale and panstylistic music learning
experiments to be performed. Perceptually guided segmentation is
argued to yield more cognitively realistic context models than other
methods, and it is also empirically shown to yield more efficient
models for prediction. Additionally, an adaptive modelling strategy
allows appropriate multiple-step-ahead predictions to be generated.
Finally a distributed, agent-based parsing methodology is developed
and implemented.
The system provides insights into what implications certain theories
from cognitive musicology have when put into practice. Maestro’s
flexible design together with the range of experiments performed and
the diverse corpus of musical data enable a thorough and systematic
machine-simulated study of key aspects of musical learning to be
carried out.
Computer algebra and theorem proving
Ballarin, Clemens
University of Cambridge, Computer Laboratory
1999-10
en
Text
UCAM-CL-TR-473
ISSN 1476-2986
Is the use of computer algebra technology beneficial for mechanised
reasoning in and about mathematical domains? Usually it is assumed
that it is. Many works in this area, however, either have little
reasoning content, or use symbolic computation only to simplify
expressions. In work that has achieved more, the methods used do not
scale up. They trust the computer algebra system either too much or
too little.
Computer algebra systems are not as rigorous as many provers. They
are not logically sound reasoning systems, but collections of
algorithms. We classify soundness problems that occur in computer
algebra systems. While many algorithms and their implementations are
perfectly trustworthy the semantics of symbols is often unclear and
leads to errors. On the other hand, more robust approaches to
interface external reasoners to provers are not always practical
because the mathematical depth of proof algorithms in computer
algebra can be enormous.
Our own approach takes both trustworthiness of the overall system
and efficiency into account. It relies on using only reliable parts
of a computer algebra system which can be achieved by using a
suitable library, and deriving specifications for these algorithms
from their literature.
We design and implement an interface between the prover Isabelle and
the computer algebra library Sumit and use it to prove non-trivial
theorems from coding theory. This is based on mechanisation of the
algebraic theories of rings and polynomials. Coding theory is an
area where proofs do have a substantial amount of computational
content. Also it is realistic to assume that the verification of an
encoding or decoding device cound be undertaken in, and indeed, be
simplified by, such a system.
The reason why semantics of symbols is often unclear in current
computer algebra systems is not mathematical difficulty, but the
design of those systems. For Gaussian elimination we show how the
soundness problem can be fixed by a small extention, and without
using efficiency. This is a prerequisite for the efficient use of
the algorithm in a prover.
A Bayesian methodology and probability density estimation
for fluorescence in-situ hybridization signal
classification
Lerner, Boaz
University of Cambridge, Computer Laboratory
1999-10
en
Text
UCAM-CL-TR-474
ISSN 1476-2986
Previous research has indicated the significance of accurate
classification of flourescence in-situ hybridization (FISH) signals
when images are captured in a fixed focal plane without relying on
an auto-focusing mechanism. Based on well-discriminating features
and a trainable neural network (NN) classifier, a previous system
enabled highly-accurate classification of valid signals and
artifacts of two fluorophores. However, since training and
optimisation of an NN require extensive resources and
experimentation, we investigate in this work a simpler alternative
for the NN classifier – the naive Bayesian classifier (NBC). The
Bayesian methodology together with an independence assumption allow
the NBC to predict the a posteriori probability of class membership
using estimated class-conditional densities. Densities measured by
three methods: single Gaussian estimation (SGE; parametric method),
Gaussian mixture model (GMM; semi-parametic method) and kernel
density estimation (KDE; non-parametric method) are evaluated for
this purpose. The accuracy of the NBC employing data modelled by SGE
is found to be similar to that based on GMM, slightly inferior to
that based on KDE but widely inferior to that of the NN. Therefore,
when supporting the two classifiers, the system enables a trade-off
between the NN performance and the NBC simplicity. Finally, the
evaluation of the NBC accuracy provides a mechanism for both model
and feature selection.
A comparison of state-of-the-art classification techniques
with application to cytogenetics
Lerner, Boaz
Lawrence, Neil D.
University of Cambridge, Computer Laboratory
1999-10
en
Text
UCAM-CL-TR-475
ISSN 1476-2986
Several state of the art techniques: a neural network, Bayesian
neural network, support vector machine and naive Bayesian classifier
are experimentally evaluated in discriminating flourescence in-situ
hybridization (FISH) signals. Highly-accurate classification of
signals from real data and artifacts of two cytogenetic probes
(colours) is required for detecting abnormalities in the data. More
than 3100 FISH signals are classified by the techniques into colour
and as real or artifact with accuracies of around 98% and 88%,
respectively. The results of the comparison also show a trade-off
between simplicity represented by the naive Bayesian classifier and
high classification performance represented by the other techniques.
Linking ACL2 and HOL
Staples, Mark
University of Cambridge, Computer Laboratory
1999-11
en
Text
UCAM-CL-TR-476
ISSN 1476-2986
This report describes ACL2PII, a system which dynamically links the
theorem provers ACL2 and Hol, using the PROSPER project’s Plug-In
interface to Hol. The focus of the system is on making ACL2 theorems
available from within Hol. In a motivating example we show how to
transfer results from ACL2’s ‘small machine’ theory. This theory
highlights two of ACL2’s strengths: symbolic simulation and the fast
execution of operationally defined functions. This allows ACL2
specifications to be readily validated against real world
requirements. The ACL2PII system allows Hol users to capitalise on
results about such ACL2 specifications. ACL2 and Hol are both
general purpose theorem provers, but Hol is slightly more
expressive, and has growing infrastructure for interoperability with
other systems. This report assumes a passing knowledge of both ACL2
and Hol.
Presheaf models for CCS-like languages
Cattani, Gian Luca
Winskel, Glynn
University of Cambridge, Computer Laboratory
1999-11
en
Text
UCAM-CL-TR-477
ISSN 1476-2986
The aim of this paper is to harness the mathematical machinery
around presheaves for the purposes of process calculi. Joyal,
Nielsen and Winskel proposed a general definition of bisimulation
from open maps. Here we show that open-map bisimulations within a
range of presheaf models are congruences for a general process
language, in which CCS and related languages are easily encoded. The
results are then transferred to traditional models for processes. By
first establishing the congruence results for presheaf models,
abstract, general proofs of congruence properties can be provided
and the awkwardness caused through traditional models not always
possessing the cartesian liftings, used in the break-down of process
operations, are side-stepped. The abstract results are applied to
show that hereditary history-preserving bisimulation is a congruence
for CCS-like languages to which is added a refinement operator on
event structures as proposed by van Glabbeek and Goltz.
Secure composition of untrusted code: wrappers and causality
types
Sewell, Peter
Vitek, Jan
University of Cambridge, Computer Laboratory
1999-11
en
Text
UCAM-CL-TR-478
ISSN 1476-2986
We consider the problem of assembling concurrent software systems
from untrusted or partially trusted off-the-shelf components, using
wrapper programs to encapsulate components and enforce security
policies. In previous work we introduced the box-π process calculus
with constrained interaction to express wrappers and discussed the
rigorous formulation of their security properties. This paper
addresses the verification of wrapper information flow properties.
We present a novel causal type system that statically captures the
allowed flows between wrapped possibly-badly-typed components; we
use it to prove that a unidirectional-flow wrapper enforces a causal
flow property.
The interaction between fault tolerance and
security
Price, Geraint
University of Cambridge, Computer Laboratory
1999-12
en
Text
UCAM-CL-TR-479
ISSN 1476-2986
This dissertation studies the effects on system design when
including fault tolerance design principles within security
services.
We start by looking at the changes made to the trust model within
protocol design, and how moving away from trusted server design
principles affects the structure of the protocol. Taking the primary
results from this work, we move on to study how control in protocol
execution can be used to increase assurances in the actions of
legitimate participants. We study some examples, defining two new
classes of attack, and note that by increasing client control in
areas of protocol execution, it is possible to overcome certain
vulnerabilities.
We then look at different models in fault tolerance, and how their
adoption into a secure environment can change the design principles
and assumptions made when applying the models.
We next look at the application of timing checks in protocols. There
are some classes of timing attack that are difficult to thwart using
existing techniques, because of the inherent unreliability of
networked communication. We develop a method of converting the
Quality of Service mechanisms built into ATM networks in order to
achieve another layer of protection against timing attacks.
We then study the use of primary-backup mechanisms within server
design, as previous work on server replication in security centres
on the use of the state machine approach for replication, which
provides a higher degree of assurance in system design, but adds
complexity.
We then provide a design for a server to reliably and securely store
objects across a loosely coupled, distributed environment. The main
goal behind this design was to realise the ability for a client to
exert control over the fault tolerance inherent in the service.
The main conclusions we draw from our research are that fault
tolerance has a wider application within security than current
practices, which are primarily based on replicating servers, and
clients can exert control over the protocols and mechanisms to
achieve resilience against differing classes of attack. We promote
some new ideas on how, by challenging the prevailing model for
client-server architectures in a secure environment, legitimate
clients can have greater control over the services they use. We
believe this to be a useful goal, given that the client stands to
lose if the security of the server is undermined.
Programming combinations of deduction and BDD-based symbolic
calculation
Gordon, Mike
University of Cambridge, Computer Laboratory
1999-12
en
Text
UCAM-CL-TR-480
ISSN 1476-2986
Theorem provers descended from LCF allow their users to write
complex proof tools that provide high assurance that false theorems
will not be proved. This paper describes some experiments in
extending the ‘LCF approach’ to enable BDD-based symbolic algorithms
to be programmed with a similar assurance. The deduction is supplied
by the HOL system and the BDD algorithms by the BuDDy package.
Combining the Hol98 proof assistant with the BuDDy BDD
package
Gordon, Mike
Larsen, Ken Friis
University of Cambridge, Computer Laboratory
1999-12
en
Text
UCAM-CL-TR-481
ISSN 1476-2986
Theorem provers descended from LCF allow their users to write
complex proof tools with high assurance that false theorems will not
be proved. This report describes an experimental system that extends
the LCF approach to enable combinations of deduction and BDD-based
symbolic calculation to be programmed with a similar assurance. The
deduction is supplied by the Hol98 system and the BDD algorithms by
Jørn Lind-Nielsen’s BuDDy package.
The main idea is to provide LCF-style support to a set of inference
rules for judgements ρ t ↦ b, where ρ is an order-inducing map from
HOL variables to BDD variables, t is a HOL term and b is a BDD. A
single oracle rule allows a HOL theorem ⊢ t to be deduced from ρ t ↦
TRUE.
This report is intended to serve as documentation for the Hol98
library HolBddLib. It is partly an exposition of standard results,
partly tutorial and partly an account of research in combining
deduction and symbolic state enumeration.
Biometric decision landscapes
Daugman, John
University of Cambridge, Computer Laboratory
2000-01
en
Text
UCAM-CL-TR-482
ISSN 1476-2986
This report investigates the “decision landscapes” that characterize
several forms of biometric decision making. The issues discussed
include: (i) Estimating the degrees-of-freedom associated with
different biometrics, as a way of measuring the randomness and
complexity (and therefore the uniqueness) of their templates. (ii)
The consequences of combining more than one biometric test to arrive
at a decision. (iii) The requirements for performing identification
by large-scale exhaustive database search, as opposed to mere
verification by comparison against a single template. (iv) Scenarios
for Biometric Key Cryptography (the use of biometrics for encryption
of messages). These issues are considered here in abstract form, but
where appropriate, the particular example of iris recognition is
used as an illustration. A unifying theme of all four sets of issues
is the role of combinatorial complexity, and its measurement, in
determining the potential decisiveness of biometric decision making.
Elastic network control
Bos, Hendrik Jaap
University of Cambridge, Computer Laboratory
2000-01
en
Text
UCAM-CL-TR-483
ISSN 1476-2986
Connection-oriented network technologies such as Asynchronous
Transfer Mode are capable, in principle, of supporting many
different services. Control and management of these networks,
however, are often rooted in the monolithic and inflexible design of
a traditional telephone network. This is unfortunate as the speed at
which new services can be introduced depends on the flexibility of
the control and management system.
Recent attemps at opening up network control and management have
achieved promising results. Using non-proprietary interfaces and
strict partitioning of network resources, multiple control systems
are allowed to be active simultaneously in the same physical
network. Each control system controls a virtual network, i.e. a
subset of the network resources. Success of this approach has been
limited, however, due to the inflexibility of its software
components. The way in which resources are partitioned, or virtual
networks built, is determined once and for all at implementation
time. Similarly, the control systems themselves are rigid. Building
and running a specialised control system in a separate virtual
network for each application area, although possible in principle,
is too heavy-weight for many applications.
This dissertation presents a solution for these problems, the
implementation of which is called the Haboob. It represents the next
step in opening up the network, by permitting customisation of all
aspects of network control, including the software components. For
this purpose, an agent environment, called the Sandbox, was
developed, which is both language and implementation independent,
and general enough to be used for purposes other than network
control as well. It includes a simple uniform way for agents on
different nodes to interact. Various mechanisms enforce protection
and access control.
Sandboxes have been successfully introduced to all components that
make up the network control and management system. Code running in
Sandboxes is able to extend or modify the functionality of the
components. This is called elastic behaviour. The customisability of
all aspects of network control and management eases the development
of new services. It is shown how recursive repartitioning of
resources allows for application-specific control at a very low
level and even enables clients to differentiate the traffic policing
associated with these partitions. Such low-level control by
dynamically loadable code may lead to significant performance
improvements. Elasticity has also been introduced to generic
services, such as traders, and components on the datapath. Elastic
behaviour allows network control and management to be completely
open.
When multiple control systems are active, interoperability becomes
extremely important. Existing solutions suffer from problems to do
with translation of control messages from one domain into those of
an incompatible neighbouring domain. These mappings are fixed and
suffer from loss of information at the domain boundaries, leading to
functionality degredation. This is solved by making the mappings
between domains programmable and by establishing inter-domain
signalling channels across control domains with only limited
functionality. In other words, the interoperability between control
domains has been made elastic as well.
It is concluded that elastic network control and management eases
the introduction of new functionality into the network.
Automatic summarising and the CLASP system
Tucker, Richard
University of Cambridge, Computer Laboratory
2000-01
en
Text
UCAM-CL-TR-484
ISSN 1476-2986
This dissertation discusses summarisers and summarising in general,
and presents CLASP, a new summarising system that uses a shallow
semantic representation of the source text called a “predication
cohesion graph”.
Nodes in the graph are “simple predications” corresponding to
events, states and entities mentioned in the text; edges indicate
related or similar nodes. Summary content is chosen by selecting
some of these predications according to criteria of “importance”,
“representativeness” and “cohesiveness”. These criteria are
expressed as functions on the nodes of a weighted graph. Summary
text is produced either by extracting whole sentences from the
source text, or by generating short, indicative “summary phrases”
from the selected predications.
CLASP uses linguistic processing but no domain knowledge, and
therefore does not restrict the subject matter of the source text.
It is intended to deal robustly with complex texts that it cannot
analyse completely accurately or in full. Experiments in summarising
stories from the Wall Street Journal suggest there may be a benefit
in identifying important material in a semantic representation
rather than a surface one, but that, despite the robustness of the
source representation, inaccuracies in CLASP’s linguistic analysis
can dramatically affect the readability of its summaries. I discuss
ways in which this and other problems might be overcome.
Three notes on the interpretation of Verilog
Stewart, Daryl
VanInwegen, Myra
University of Cambridge, Computer Laboratory
2000-01
en
Text
UCAM-CL-TR-485
ISSN 1476-2986
In order to simplify the many constructs available in the Verilog
Hardware Description Language two methods were used to normalise
code before analysis, scalarisation and hierarchy flattening.
A method for scalarising Verilog expressions is described and the
replacement of expressions with scalarised versions is considered.
This then forms the basis of an implementation of Verilog expression
evaluation and normalization.
The organisation of hierarchical designs is described and an
algorithm for flattening designs is derived from this.
Stretching a point: aspect and temporal discourse
Thomas, James Richard
University of Cambridge, Computer Laboratory
2000-02
en
Text
UCAM-CL-TR-486
ISSN 1476-2986
Sequential program composition in UNITY
Vos, Tanja
Swierstra, Doaitse
University of Cambridge, Computer Laboratory
2000-03
en
Text
UCAM-CL-TR-487
ISSN 1476-2986
Large distributed applications are composed of basic blocks by using
composition operators. In an ideal situation, one should be able to
develop and verify each of these basic components by itself using
compositionality theorems of the respective composition operators
stating that properties of a composite program can be proved by
proving properties of its components.
Generally two forms of distributed program composition can be
distinguished: parallel composition and sequential composition.
Parallel composition is standard in UNITY and is used when two
distributed component-programs need to cooperate in one way or
another. Sequential composition of UNITY programs is not part of
core UNITY. It can however be very useful when we want a program to
work with the results of another program. In this technical report
we shall formally define and model sequential program composition
within the HOL-UNITY embedding.
Formal verification of card-holder registration in
SET
Bella, Giampaolo
Massacci, Fabio
Paulson, Lawrence
Tramontano, Piero
University of Cambridge, Computer Laboratory
2000-03
en
Text
UCAM-CL-TR-488
ISSN 1476-2986
The first phase of the SET protocol, namely card-holder
registration, has been modelled inductively. This phase is presented
in outline and its formal model is described. A simple theorem has
been proved using Isabelle/HOL, stating that a certification
authority will certify a given key at most once. Many ambiguities,
contradictions and omissions were noted when formalizing the
protocol.
Designing a reliable publishing framework
Lee, Jong-Hyeon
University of Cambridge, Computer Laboratory
2000-04
en
Text
UCAM-CL-TR-489
ISSN 1476-2986
Due to the growth of the Internet and the widespread adoption of
easy-to use web browsers, the web provides a new environment for
conventional as well as new businesses. Publishing on the web is a
fundamental and important means of supporting various activities on
the Internet such as commercial transactions, personal home page
publishing, medical information distribution, public key
certification and academic scholarly publishing. Along with the
dramatic growth of the web, the number of reported frauds is
increasing sharply. Since the Internet was not originally designed
for web publishing, it has some weaknesses that undermine its
reliability.
How can we rely on web publishing? In order to resolve this
question, we need to examine what makes people confident when
reading conventional publications printed on paper, to investigate
what attacks can erode confidence in web publishing, and to
understand the nature of publishing in general.
In this dissertation, we examine security properties and policy
models, and their applicability to publishing. We then investigate
the nature of publishing so that we can extract its technical
requirements. To help us understand the practical mechanisms which
might satisfy these requirements, some applications of electronic
publishing are discussed and some example mechanisms are presented.
We conclude that guaranteed integrity, verifiable authenticity and
persistent availability of publications are required to make web
publishing more reliable. Hence we design a framework that can
support these properties. To analyse the framework, we define a
security policy for web publishing that focuses on the guaranteed
integrity and authenticity of web publications, and then describe
some technical primitives that enable us to achieve our
requirements. Finally, the Jikzi publishing system—an implementation
of our framework—is presented with descriptions of its architecture
and possible applications.
Selective mesh refinement for rendering
Brown, Peter John Cameron
University of Cambridge, Computer Laboratory
2000-04
en
Text
UCAM-CL-TR-490
ISSN 1476-2986
A key task in computer graphics is the rendering of complex models.
As a result, there exist a large number of schemes for improving the
speed of the rendering process, many of which involve displaying
only a simplified version of a model. When such a simplification is
generated selectively, i.e. detail is only removed in specific
regions of a model, we term this selective mesh refinement.
Selective mesh refinement can potentially produce a model
approximation which can be displayed at greatly reduced cost while
remaining perceptually equivalent to a rendering of the original.
For this reason, the field of selective mesh refinement has been the
subject of dramatically increased interest recently. The resulting
selective refinement methods, though, are restricted in both the
types of model which they can handle and the form of output meshes
which they can generate.
Our primary thesis is that a selectively refined mesh can be
produced by combining fragments of approximations to a model without
regard to the underlying approximation method. Thus we can utilise
existing approximation techniques to produce selectively refined
meshes in n-dimensions. This means that the capabilities and
characteristics of standard approximation methods can be retained in
our selectively refined models.
We also show that a selectively refined approximation produced in
this manner can be smoothly geometrically morphed into another
selective refinement in order to satisfy modified refinement
criteria. This geometric morphing is necessary to ensure that detail
can be added and removed from models which are selectively refined
with respect to their impact on the current view frustum. For
example, if a model is selectively refined in this manner and the
viewer approaches the model then more detail may have to be
introduced to the displayed mesh in order to ensure that it
satisfies the new refinement criteria. By geometrically morphing
this introduction of detail we can ensure that the viewer is not
distracted by “popping” artifacts.
We have developed a novel framework within which these proposals
have been verified. This framework consists of a generalised
resolution-based model representation, a means of specifying
refinement criteria and algorithms which can perform the selective
refinement and geometric morphing tasks. The framework has allowed
us to demonstrate that these twin tasks can be performed both on the
output of existing approximation techniques and with respect to a
variety of refinement criteria.
A HTML version of this thesis is at
https://www.cl.cam.ac.uk/research/rainbow/publications/pjcb/thesis/
Is hypothesis testing useful for subcategorization
acquisition?
Korhonen, Anna
Gorrell, Genevive
McCarthy, Diana
University of Cambridge, Computer Laboratory
2000-05
en
Text
UCAM-CL-TR-491
ISSN 1476-2986
Statistical filtering is often used to remove noise from
automatically acquired subcategorization frames. In this paper, we
compare three different approaches to filtering out spurious
hypotheses. Two hypothesis tests perfrom poorly, compared to
filtering frames on the basis of relative frequency. We discuss
reasons for this and consider directions for future research.
Nomadic Pict: language and infrastructure design for mobile
computation
Wojciechowski, Paweł Tomasz
University of Cambridge, Computer Laboratory
2000-06
en
Text
UCAM-CL-TR-492
ISSN 1476-2986
Mobile agents – units of executing computation that can migrate
between machines – are likely to become an important enabling
technology for future distributed systems. We study the distributed
infrastructures required for location-independent communication
between migrating agents. These infrastructures are problematic: the
choice or design of an infrastructure must be somewhat
application-specific – any given algorithm will only have
satisfactory performance for some range of migration and
communication behaviour; the algorithms must be matched to the
expected properties (and robustness demands) of applications and the
failure characteristic of the communication medium. To study this
problem we introduce an agent programming language – Nomadic Pict.
It is designed to allow infrastructure algorithms to be expressed
clearly, as translations from a high-level language to a lower
level. The levels are based on rigorously-defined process calculi,
which provide sharp levels of abstraction. In this dissertation we
describe the language and use it to develop a distributed
infrastructure for an example application. The language and examples
have been implemented; we conclude with a description of the
compiler and runtime system.
Inductive verification of cryptographic protocols
Bella, Giampaolo
University of Cambridge, Computer Laboratory
2000-07
en
Text
UCAM-CL-TR-493
ISSN 1476-2986
The dissertation aims at tailoring Paulson’s Inductive Approach for
the analysis of classical cryptographic protocols towards real-world
protocols. The aim is pursued by extending the approach with new
elements (e.g. timestamps and smart cards), new network events (e.g.
message reception) and more expressive functions (e.g. agents’
knowledge). Hence, the aim is achieved by analysing large protocols
(Kerberos IV and Shoup-Rubin), and by studying how to specify and
verify their goals.
More precisely, the modelling of timestamps and of a discrete time
are first developed on BAN Kerberos, while comparing the outcomes
with those of the BAN logic. The machinery is then applied to
Kerberos IV, whose complicated use of session keys requires a
dedicated treatment. Three new guarantees limiting the spy’s
abilities in case of compromise of a specific session key are
established. Also, it is discovered that Kerberos IV is subject to
an attack due to the weak guarantees of confidentiality for the
protocol responder.
We develop general strategies to investigate the goals of
authenticity, key distribution and non-injective agreement, which is
a strong form of authentication. These strategies require
formalising the agents’ knowledge of messages. Two approaches are
implemented. If an agent creates a message, then he knows all
components of the message, including the cryptographic key that
encrypts it. Alternatively, a broad definition of agents’ knowledge
can be developed if a new network event, message reception, is
formalised.
The concept of smart card as a secure device that can store
long-term secrets and perform easy computations is introduced. The
model cards can be stolen and/or cloned by the spy. The kernel of
their built-in algorithm works correctly, so they spy cannot acquire
unlimited knowledge from their use. However, their functional
interface is unreliable, so they send correct outputs in an
unspecified order. The provably secure protocol based on smart cards
designed by Shoup & Rubin is mechanised. Some design weaknesses
(unknown to the authors’ treatment by Bellare & Rogaway’s
approach) are unveiled, while feasible corrections are suggested and
verified.
We realise that the evidence that a protocol achieves its goals must
be available to the peers. In consequence, we develop a new a
principle of prudent protocol design, goal availability, which holds
of a protocol when suitable guarantees confirming its goals exist on
assumptions that both peers can verify. Failure to observe our
principle raises the risk of attacks, as is the case, for example,
of the attack on Kerberos IV.
An architecture for the notification, storage and retrieval
of events
Spiteri, Mark David
University of Cambridge, Computer Laboratory
2000-07
en
Text
UCAM-CL-TR-494
ISSN 1476-2986
Event-driven and messaging infrastructures are emerging as the most
flexible and feasible solution to enable rapid and dynamic
integration and legacy and monolithic software applications into
distributed systems. They also support deployment and enhancement of
traditionally difficult-to-build active systems such as large scale
collaborative environments and mobility aware architectures.
However, complex systems issues like mobility, scalability,
fedaration and persistence indicate a requirement for more advanced
services within these infrastructures. The event notification
paradigm is also applicable in emerging research areas such as
modelling of business information flow within organisations, as well
as workplace empowering through enchanced awareness of work
practices relating to communication and interaction between
individuals. In these areas further developments require complex
interpretation and correlation of event information, highlighting
the need for an event storage and retrieval service that provides
the required groundwork.
It is the thesis of this this dissertation that the lack of a
generic model for event representation and notification has
restricted evolution within event driven applications. Furthermore,
in order to empower existing applications and enable novel
solutions, a crucial, and so-far-missing, service within event
systems is capture, persistent storage, and meaningful retrieval of
the messaging information driving these systems.
In order to address these issues, this dissertation defines a
generic event model and presents a powerful event notification
infrastructure that, amongst other structural contributions, embeds
event storage functionality. An event repository architecture will
then be presented that can capture and store events, as well as
inject them back into distributed application components to simulate
replay of sequences of activity. The general-purpose architecture
presented is designed on the thesis that events are temporal
indexing points for computing activities. Changes in the state of a
distributed system can be captured in events, and replayed or
reviewed at a later stage, supporting fault-tolerance, systems
management, disconnected operation and mobility. The architecture
delivers powerful querying of event histories, enabling extraction
of simple and composite event patterns. This addresses the business
requirements in several industries (such as finance, travel, news,
retail and manufacturing) to locate temporal patterns of activity,
as well as support applications like memory prosthesis tools and
capture of collaboration. The repository offers a selective
store-and-forward functionality that enables messaging environments
to scale and provide enhanced brokering and federation services.
In addition to enabling novel applications, the general-purpose
infrastructure presented provides a more flexible approach to event
notification, storage and retrieval, in areas where bespoke
solutions had to be provided previously. The theoretical concepts
illustrated in this dissertation are demonstrated through a working
distributed implementation and deployment in several application
scenarios.
Automatic recognition of words in Arabic
manuscripts
Khorsheed, Mohammad S.M.
University of Cambridge, Computer Laboratory
2000-07
en
Text
UCAM-CL-TR-495
ISSN 1476-2986
The need to transliterate large numbers of historic Arabic documents
into machine-readable form has motivated new work on offline
recognition of Arabic script. Arabic script presents two challenges:
orthography is cursive and letter shape is context sensitive.
This dissertation presents two techniques to achieve high word
recognition rates: the segmentation-free technique and the
segmentation-based technique. The segmentation-free technique treats
the word as a whole. The word image is first transformed into a
normalised polar image. The two-dimensional Fourier transform is
then applied to the polar image. This results in a Fourier spectrum
that is invariant to dilation, translation, and rotation. The
Fourier spectrum is used to form the word template, or train the
word model in the template-based and the multiple hidden Markov
model (HMM) recognition systems, respectively. The recognition of an
input word image is based on the minimum distance measure from the
word templates and the maximum likelihood probability for the word
models.
The segmentation-based technique uses a single hidden Markov model,
which is composed of multiple character-models. The technique
implements the analytic approach in which words are segmented into
smaller units, not necessarily characters. The word skeleton is
decomposed into a number of links in orthographic order, it is then
transferred into a sequence of discrete symbols using vector
quantisation. the training of each character-model is performed
using either: state assignment in the lexicon-driven configuration
or the Baum-Welch method in the lexicon-free configuration. The
observation sequence of the input word is given to the hidden Markov
model and the Viterbi algorithm is applied to provide an ordered
list of the candidate recognitions.
Contexts and embeddings for closed shallow action
graphs
Cattani, Gian Luca
Leifer, James J.
Milner, Robin
University of Cambridge, Computer Laboratory
2000-07
en
Text
UCAM-CL-TR-496
ISSN 1476-2986
Action calculi, which have a graphical presentation, were introduced
to develop a theory shared among different calculi for interactive
systems. The π-calculus, the λ-calculus, Petri nets, the Ambient
calculus and others may all be represented as action calculi. This
paper develops a part of the shared theory.
A recent paper by two of the authors was concerned with the notion
of reactive system, essentially a category of process contexts whose
behaviour is presented as a reduction relation. It was shown that
one can, for any reactive system, uniformly derive a labelled
transition system whose associated behavioural equivalence relations
(e.g. trace equivalence or bisimilarity) will be congruential, under
the condition that certain relative pushouts exist in the reactive
system. In the present paper we treat closed, shallow action calculi
(those with no free names and no nested actions) as a generic
application of these results. We define a category of action graphs
and embeddings, closely linked to a category of contexts which forms
a reactive system. This connection is of independent interest; it
also serves our present purpose, as it enables us to demonstrate
that appropriate relative pushouts exist.
Complemented by work to be reported elsewhere, this demonstration
yields labelled transition systems with behavioural congruences for
a substantial class of action calculi. We regard this work as a step
towards comparable results for the full class.
Towards a formal type system for ODMG OQL
Bierman, G.M.
Trigoni, A.
University of Cambridge, Computer Laboratory
2000-09
en
Text
UCAM-CL-TR-497
ISSN 1476-2986
In this paper we consider in detail the type system of the
object-oriented database query language, OQL, as defined by the
ODMG. Our main technical contribution is a formal definition of the
typing relation for OQL—surprisingly we could not find a complete
definition in the literature. We have also uncovered a number of
inaccuracies in the ODMG proposal, and other work.
Applied π – a brief tutorial
Sewell, Peter
University of Cambridge, Computer Laboratory
2000-07
en
Text
UCAM-CL-TR-498
ISSN 1476-2986
This note provides a brief introduction to π-calculi and their
application to concurrent and distributed programming. Chapter 1
introduces a simple π-calculus and discusses the choice of
primitives, operational semantics (in terms of reductions and of
indexed early labelled transitions), operational equivalences,
Pict-style programming and typing. Chapter 2 goes on to discuss the
application of these ideas to distributed systems, looking
informally at the design of distributed π-calculi with grouping and
interaction primitives. Chapter 3 returns to typing, giving precise
definitions for a simple type system and soundness results for the
labelled transition semantics. Finally, Chapters 4 and 5 provide a
model development of the metatheory, giving first an outline and
then detailed proofs of the results stated earlier. The note can be
read in the partial order 1.(2+3+4.5).
Enhancing spatial deformation for virtual
sculpting
Gain, James Edward
University of Cambridge, Computer Laboratory
2000-08
en
Text
UCAM-CL-TR-499
ISSN 1476-2986
The task of computer-based free-form shape design is fraught with
practical and conceptual difficulties. Incorporating elements of
traditional clay sculpting has long been recognised as a means of
shielding a user from the complexities inherent in this form of
modelling. The premise is to deform a mathematically-defined solid
in a fashion that loosely simulates the physical moulding of an
inelastic substance, such as modelling clay or silicone putty.
Virtual sculpting combines this emulation of clay sculpting with
interactive feedback.
Spatial deformations are a class of powerful modelling techniques
well suited to virtual sculpting. They indirectly reshape an object
by warping the surrounding space. This is analogous to embedding a
flexible shape within a lump of jelly and then causing distortions
by flexing the jelly. The user controls spatial deformations by
manipulating points, curves or a volumetric hyperpatch. Directly
Manipulated Free-Form Deformation (DMFFD), in particular, merges the
hyperpatch- and point-based approaches and allows the user to pick
and drag object points directly.
This thesis embodies four enhancements to the versatility and
validity of spatial deformation:
1. We enable users to specify deformations by manipulating the
normal vector and tangent plane at a point. A first derivative frame
can be tilted, twisted and scaled to cause a corresponding
distortion in both the ambient space and inset object. This enhanced
control is accomplished by extending previous work on bivariate
surfaces to trivariate hyperpatches.
2. We extend DMFFD to enable curve manipulation by exploiting
functional composition and degree reduction. Although the resulting
curve-composed DMFFD introduces some modest and bounded
approximation, it is superior to previous curve-based schemes in
other respects. Our technique combines all three forms of spatial
deformation (hyperpatch, point and curve), can maintain any desired
degree of derivative continuity, is amenable to the automatic
detection and prevention of self-intersection, and achieves
interactive update rates over the entire deformation cycle.
3. The approximation quality of a polygon-mesh object frequently
degrades under spatial deformation to become either oversaturated or
undersaturated with polygons. We have devised an efficient adaptive
mesh refinement and decimation scheme. Our novel contributions
include: incorporating fully symmetrical decimation, reducing the
computation cost of the refinement/decimation trigger, catering for
boundary and crease edges, and dealing with sampling problems.
4. The potential self-intersection of an object is a serious
weakness in spatial deformation. We have developed a variant of
DMFFD which guards against self-intersection by subdividing
manipulations into injective (one-to-one) mappings. This depends on
three novel contributions: analytic conditions for identifying
self-intersection, and two injectivity tests (one exact but
computationally costly and the other approximate but efficient).
The memorability and security of passwords – some empirical
results
Yan, Jianxin
Blackwell, Alan
Anderson, Ross
Grant, Alasdair
University of Cambridge, Computer Laboratory
2000-09
en
Text
UCAM-CL-TR-500
ISSN 1476-2986
There are many things that are ‘well known’ about passwords, such as
that uers can’t remember strong passwords and that the passwords
they can remember are easy to guess. However, there seems to be a
distinct lack of research on the subject that would pass muster by
the standards of applied psychology.
Here we report a controlled trial in which, of four sample groups of
about 100 first-year students, three were recruited to a formal
experiment and of these two were given specific advice about
password selection. The incidence of weak passwords was determined
by cracking the password file, and the number of password resets was
measured from system logs. We observed a number of phenomena which
run counter to the established wisdom. For example, passwords based
on mnemonic phrases are just as hard to crack as random passwords
yet just as easy to remember as naive user selections.
Integrated quality of service management
Ingram, David
University of Cambridge, Computer Laboratory
2000-09
en
Text
UCAM-CL-TR-501
ISSN 1476-2986
It has recently become standard practice to run soft real time
applications, such as multimedia and games programs, on general
purpose desktop systems. The operating systems in use on these
platforms employ scheduling algorithms derived from the traditional
multi-user timesharing world, which are unsuitable for real time
purposes. The scheduler is viewed as a “black-box” whose behaviour
depends in an unpredictable way on the entire system load. Many hard
real time systems use static priorities instead, but these are not
suitable for the dynamic task sets encountered in a general purpose
computing environment.
A large number of prototype systems with improved real time
schedulers have been created in the past. Unfortunately, designers
of these systems placed constraints on the operating system
structure which are incompatible with ubiquitous monolithic kernels,
client-server architectures, existing standards and applications.
This has prevented their adoption in a production desktop system.
Furthermore little regard has been given to making real time
capabilities convenient to use. An integrated user interface and
automated quality of service management are necessary in a desktop
environment.
This dissertation makes three main contributions which combine to
overcome the difficulties just described:
(a) Scheduling. We present a conventionally structured, general
purpose platform which provides effective soft real time scheduling.
Binary compatibility with a large application software base has been
preserved by extending an existing operating system; the modified
platform is called Linux-SRT.
A basic design premise is that scheduling is separated from
functionality, which allows quality of service to be associated with
unmodified Linux applications and permits centralised control. We
have developed a named reserve abstraction to share quality of
service between threads and take advantage of application-specific
knowledge where appropriate. Reserves and processes are handled by
the same kernel scheduler without a separate real time mode or
hierarchy of schedulers.
(b) Servers and IPS. Techniques for scheduling real time servers
accurately are discussed, and a solution presented. This allows
server processes to utilise their clients’ quality of service
without restructuring. Multi-threaded servers are handled by
allocating a single reserve to a set of threads. Single-threaded
servers, including the X window system, are addressed with a
retrospective accounting mechanism.
The implementation makes use of a novel IPC mechanism. This
distributes kernel events to servers so they can synchronise with
other activities, and is also used to integrate with window
management and desktop control functions. In addition we have
improved the normal socket abstraction by adding authentication and
resource propagation, so that priority inheritance can take place
between real time processes.
(c) Quality of service management. Linux-SRT applies quality of
service parameters automatically to real time applications. These
are described using a new kind of dual policy specification. Tools
and user interface components which allow ordinary users to interact
with the quality of service management system are demonstrated.
These are tightly integrated with window management functions,
avoiding cumbersome control programs.
We also evaluate methods for determining scheduling parameter values
without user intervention. Processor time slices can be determined
by empirical adaptation. Where statistical multiplexing is used,
overrun probabilities are considered explicitly. Simple forms of
mode-change support such as automatic idle state detection are also
possible. Access control to real-time service classes has been
defined in a flexible capability based manner, so programs do not
need administrator rights to use them. Limits prevent
over-committing of resources, starvation of lower priority processes
and denial of service.
The resulting system has the following properties which have not
been achieved before: soft real time scheduling on a desktop
operating system, binary application compatibility, real time
support for single-threaded servers, and a simple user interface for
quality of service management.
Formalizing basic number theory
Rasmussen, Thomas Marthedal
University of Cambridge, Computer Laboratory
2000-09
en
Text
UCAM-CL-TR-502
ISSN 1476-2986
This document describes a formalization of basic number theory
including two theorems of Fermat and Wilson.
Most of this has (in some context) been formalized before but we
present a new generalized approach for handling some central parts,
based on concepts which seem closer to the original mathematical
intuition and likely to be useful in other (similar) developments.
Our formulation has been mechanized in the Isabelle/HOL system.
Hardware/software co-design using functional
languages
Mycroft, Alan
Sharp, Richard
University of Cambridge, Computer Laboratory
2000-09
en
Text
UCAM-CL-TR-503
ISSN 1476-2986
In previous work we have developed and prototyped a silicon compiler
which translates a functional language (SAFL) into hardware. Here we
present a SAFL-level program transformation which: (i) partitions a
specification into hardware and software parts and (ii) generates a
specialised architecture to execute the software part. The
architecture consists of a number of interconnected heterogeneous
processors. Our method allows a large design space to be explored by
systematically transforming a single SAFL specification to
investigate different points on the area-time spectrum.
Word sense selection in texts: an integrated
model
Kwong, Oi Yee
University of Cambridge, Computer Laboratory
2000-09
en
Text
UCAM-CL-TR-504
ISSN 1476-2986
Early systems for word sense disambiguation (WSD) often depended on
individual tailor-made lexical resources, hand-coded with as much
lexical information as needed, but of severely limited vocabulary
size. Recent studies tend to extract lexical information from a
variety of existing resources (e.g. machine-readable dictionaries,
corpora) for broad coverage. However, this raises the issue of how
to combine the information from different resources.
Thus while different types of resource could make different
contribution to WSD, studies to date have not shown what
contribution they make, how they should be combined, and whether
they are equally relevant to all words to be disambiguated. This
thesis proposes an Integrated Model as a framework to study the
inter-relatedness of three major parameters in WSD: Lexical
Resource, Contextual Information, and Nature of Target Words. We
argue that it is their interaction which shapes the effectiveness of
any WSD system.
A generalised, structurally-based sense-mapping algorithm was
designed to combine various types of lexical resource. This enables
information from these resources to be used simultaneously and
compatibly, while respecting their distinctive structures. In
studying the effect of context on WSD, different semantic relations
available from the combined resources were used, and a recursive
filtering algorithm was designed to overcome combinatorial
explosion. We then investigated, from two directions, how the target
words themselves could affect the usefulness of different types of
knowledge. In particular, we modelled WSD with the cloze test
format, i.e. as texts with blanks and all senses for one specific
word as alternative choices for filling the blank.
A full-scale combination of WordNet and Roget’s Thesaurus was done,
linking more than 30,000 senses. Using these two resources in
combination, a range of disambiguation tests was done on more than
60,000 noun instances from corpus texts of different types, and 60
blanks from real cloze texts. Results show that combining resources
is useful for enriching lexical information, and hence making WSD
more effective though not completely. Also, different target words
make different demand on contextual information, and this
interaction is closely related to text types. Future work is
suggested for expanding the analysis on target nature and making the
combination of disambiguation evidence sensitive to the requirements
of the word being disambiguated.
Models for name-passing processes: interleaving and
causal
Cattani, Gian Luca
Sewell, Peter
University of Cambridge, Computer Laboratory
2000-09
en
Text
UCAM-CL-TR-505
ISSN 1476-2986
We study syntax-free models for name-passing processes. For
interleaving semantics, we identify the indexing structure required
of an early labelled transition system to support the usual
π-calculus operations, defining Indexed Labelled Transition Systems.
For noninterleaving causal semantics we define Indexed Labelled
Asynchronous Transition Systems, smoothly generalizing both our
interleaving model and the standard Asynchronous Transition Systems
model for CCS-like calculi. In each case we relate a denotational
semantics to an operational view, for bisimulation and causal
bisimulation respectively. We establish completeness properties of,
and adjunctions between, categories of the two models. Alternative
indexing structures and possible applications are also discussed.
These are first steps towards a uniform understanding of the
semantics and operations of name-passing calculi.
Modules, abstract types, and distributed
versioning
Sewell, Peter
University of Cambridge, Computer Laboratory
2000-09
en
Text
UCAM-CL-TR-506
ISSN 1476-2986
In a wide-area distributed system it is often impractical to
synchronise software updates, so one must deal with many coexisting
versions. We study static typing support for modular wide-area
programming, modelling separate compilation/linking and execution of
programs that interact along typed channels. Interaction may involve
communication of values of abstract types; we provide the developer
with fine-grain versioning control of these types to support
interoperation of old and new code. The system makes use of a
second-class module system with singleton kinds; we give a novel
operational semantics for separate compilation/linking and execution
and prove soundness.
Mechanizing a theory of program composition for
UNITY
Paulson, Lawrence
University of Cambridge, Computer Laboratory
2000-11
en
Text
UCAM-CL-TR-507
ISSN 1476-2986
Compositional reasoning must be better understood if non-trivial
concurrent programs are to be verified. Chandy and Sanders [2000]
have proposed a new approach to reasoning about composition, which
Charpentier and Chandy [1999] have illustrated by developing a large
example in the UNITY formalism. The present paper describes
extensive experiments on mechanizing the compositionality theory and
the example, using the proof tool Isabelle. Broader issues are
discussed, in particular, the formalization of program states. The
usual representation based upon maps from variables to values is
contrasted with the alternatives, such as a signature of typed
variables. Properties need to be transferred from one program
component’s signature to the common signature of the system. Safety
properties can be so transferred, but progress properties cannot be.
Using polymorphism, this problem can be circumvented by making
signatures sufficiently flexible. Finally the proof of the example
itself is outlined.
Shallow linear action graphs and their embeddings
Leifer, James
Milner, Robin
University of Cambridge, Computer Laboratory
2000-10
en
Text
UCAM-CL-TR-508
ISSN 1476-2986
In previous work, action calculus has been presented in terms of
action graphs. Many calculi, or at least their salient features, can
be expressed as specific action calculi; examples are Petri nets,
λ-calculus, π-calculus, fusion calculus, ambient calculus and spi
calculus.
We here offer linear action graphs as a primitive basis for action
calculi. Linear action graphs have a simpler theory than the
non-linear variety. This paper presents the category of embeddings
of shallow linear action graphs (those without nesting), using a
novel form of graphical reasoning which simplifies some otherwise
complex manipulations in regular algebra. The work is done for
undirected graphs, and adapted in a few lines to directed graphs.
The graphical reasoning used here will be applied in future work to
develop behavioural congruences for action calculi.
Proximity visualisation of abstract data
Basalaj, Wojciech
University of Cambridge, Computer Laboratory
2001-01
en
Text
UCAM-CL-TR-509
ISSN 1476-2986
Data visualisation is an established technique for exploration,
analysis and presentation of data. A graphical presentation is
generated from the data content, and viewed by an observer, engaging
vision – the human sense with the greatest bandwidth, and the
ability to recognise patterns subconciously. For instance, a
correlation present between two variables can be elucidated with a
scatter plot. An effective visualisation can be difficult to achieve
for an abstract collection of objects, e.g. a database table with
many attributes, or a set of multimedia documents, since there is no
immediately obvious way of arranging the objects based on their
content. Thankfully, similarity between pairs of elements of such a
collection can be measured, and a good overview picture should
respect this proximity information, by positioning similar elements
close to one another, and far from dissimilar objects. The resulting
proximity visualisation is a topology preserving map of the
underlying data collection, and this work investigates various
methods for generating such maps. A number of algorithms are
devised, evaluated quantitatively by means of statistical inference,
and qualitatively in a case study for each type of data collection.
Other graphical representations for abstract data are surveyed and
compared to proximity visualisation.
A standard method for modelling prximity relations is
multidimensional scaling (MDS) analysis. The result is usually a
two- or three-dimensional configuration of points – each
representing a single element from a collection., with inter-point
distances approximating the corresponding proximities. The quality
of this approximation can be expressed as a loss function, and the
optimal arrangement can be found by minimising it numerically – a
procedure known as least-squares metric MDS. This work presents a
number of algorithmic instances of this problem, using established
function optimisation heuristics: Newton-Raphson, Tabu Search,
Genetic Algorithm, Iterative Majorization, and Stimulated annealing.
Their effectiveness at minimising the loss function is measured for
a representative sample of data collections, and the relative
ranking established. The popular classical scaling method serves as
a benchmark for this study.
The computational cost of conventional MDS makes it unsuitable for
visualising a large data collection. Incremental multidimensional
scaling solves this problem by considering only a carefully chosen
subset of all pairwise proximities. Elements that make up cluster
diameters at a certain level of the single link cluster hierarchy
are identified, and are subject to standard MDS, in order to
establish the overall shape of the configuration. The remaining
elements are positioned independently of one another with respect to
this skeleton configuration. For very large collections the skeleton
configuration can itself be built up incrementally. The incremental
method is analysed for the compromise between solution quality and
the proportion of proximities used, and compared to Principal
Components Analysis on a number of large database tables.
In some applications it is convenient to represent individual
objects by compact icons of fixed size, for example the use of
thumbnails when visualising a set of images. Because the MDS
analysis only takes the position of icons into account, and not
their size, its direct use for visualisation may lead to partial or
complete overlap of icons. Proximity grid – an analogue of MDS in a
discrete domain – is proposed to overcome this deficiency. Each
element of an abstract data collection is represented within a
single cell of the grid, and thus considerable detail can be shown
without overlap. The proximity relationships are preserved by
clustering similar elements in the grid, and keeping dissimilar ones
apart. Algorithms for generating such an arrangement are presented
and compared in terms of output quality to one another as well as
standard MDS.
Switchlets and resource-assured MPLS networks
Mortier, Richard
Isaacs, Rebecca
Fraser, Keir
University of Cambridge, Computer Laboratory
2000-05
en
Text
UCAM-CL-TR-510
ISSN 1476-2986
MPLS (Multi-Protocol Label Switching) is a technology with the
potential to support multiple control systems, each with guaranteed
QoS (Quality of Service), on connectionless best-effort networks.
However, it does not provide all the capabilities required of a
multi-service network. In particular, although resource-assured VPNs
(Virtual Private Networks) can be created, there is no provision for
inter-VPN resource management. Control flexibility is limited
because resources must be pinned down to be guaranteed, and
best-effort flows in different VPNs compete for the same resources,
leading to QoS crosstalk.
The contribution of this paper is an implementation on MPLS of a
network control framework that supports inter-VPN resource
management. Using resource partitions known as switchlets, it allows
the creation of multiple VPNs with guaranteed resource allocations,
and maintains isolation between these VPNs. Devolved control
techniques permit each VPN a customised control system.
We motivate our work by discussing related efforts and example
scenarios of effective deployment of our system. The implementation
is described and evaluated, and we address interoperability with
external IP control systems, in addition to interoperability of data
across different layer 2 technologies.
Software visualization in Prolog
Grant, Calum
University of Cambridge, Computer Laboratory
1999-12
en
Text
UCAM-CL-TR-511
ISSN 1476-2986
Software visualization (SV) uses computer graphics to communicate
the structure and behaviour of complex software and algorithms. One
of the important issues in this field is how to specify SV, because
existing systems are very cumbersome to specify and implement, which
limits their effectiveness and hinders SV from being integrated into
professional software development tools.
In this dissertation the visualization process is decomposed into a
series of formal mappings, which provides a formal foundation, and
allows separate aspects of visualization to be specified
independently. The first mapping specifies the information content
of each view. The second mapping specifies a graphical
representation of the information, and a third mapping specifies the
graphical components that make up the graphical representation. By
combining different mappings, completely different views can be
generated.
The approach has been implemented in Prolog to provide a very high
level specification language for information visualization, and a
knowledge engineering environment that allows data queries to tailor
the information in a view. The output is generated by a graphical
constraint solver that assembles the graphical components into a
scene.
This system provides a framework for SV called Vmax. Source code and
run-time data are analyzed by Prolog to provide access to
information about the program structure and run-time data for a wide
range of highly interconnected browsable views. Different views and
means of visualization can be selected from menus. An automatic
legend describes each view, and can be interactively modified to
customize how data is presented. A text window for editing source
code is synchronized with the graphical view. Vmax is a complete
Java development environment and end user SV system.
Vmax compares favourably to existing SV systems in many taxonometric
criteria, including automation, scope, information content,
graphical output form, specification, tailorability, navigation,
granularity and elision control. The performance and scalability of
the new approach is very reasonable.
We conclude that Prolog provides a formal and high level
specification language that is suitable for specifying all aspects
of a SV system.
An algebraic framework for modelling and verifying
microprocessors using HOL
Fox, Anthony
University of Cambridge, Computer Laboratory
2001-03
en
Text
UCAM-CL-TR-512
ISSN 1476-2986
This report describes an algebraic approach to the specification and
verification of microprocessor designs. Key results are expressed
and verified using the HOL proof tool. Particular attention is paid
to the models of time and temporal abstraction, culminating in a
number of one-step theorems. This work is then explained with a
small but complete case study, which verifies the correctness of a
datapath with microprogram control.
Generic summaries for indexing in information retrieval –
Detailed test results
Sakai, Tetsuya
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
2001-05
en
Text
UCAM-CL-TR-513
ISSN 1476-2986
This paper examines the use of generic summaries for indexing in
information retrieval. Our main observations are that:
– With or without pseudo-relevance feedback, a summary index may be
as effective as the corresponding fulltext index for
precision-oriented search of highly relevant documents. But a
reasonably sophisticated summarizer, using a compression ratio of
10–30%, is desirable for this purpose.
– In pseudo-relevance feedback, using a summary index at initial
search and a fulltext index at final search is possibly effective
for precision-oriented search, regardless of relevance levels. This
strategy is significantly more effective than the one using the
summary index only and probably more effective than using summaries
as mere term selection filters. For this strategy, the summary
quality is probably not a critical factor, and a compression ratio
of 5–10% appears best.
Nomadic π-calculi: expressing and verifying communication
infrastructure for mobile computation
Unyapoth, Asis
University of Cambridge, Computer Laboratory
2001-06
en
Text
UCAM-CL-TR-514
ISSN 1476-2986
This thesis addresses the problem of verifying distributed
infrastructure for mobile computation. In particular, we study
language primitives for communication between mobile agents. They
can be classified into two groups. At a low level there are
“location dependent” primitives that require a programmer to know
the current site of a mobile agent in order to communicate with it.
At a high level there are “location independent” primitives that
allow communication with a mobile agent irrespective of any
migrations. Implementation of the high level requires delicate
distributed infrastructure algorithms. In earlier work of Sewell,
Wojciechowski and Pierce, the two levels were made precise as
process calculi, allowing such algorithms to be expressed as
encodings of the high level into the low level; a distributed
programming language “Nomadic Pict” has been built for experimenting
with such encodings.
This thesis turns to semantics, giving a definition of the core
language (with a type system) and proving correctness of an example
infrastructure. This involves extending the standard semantics and
proof techniques of process calculi to deal with the new notions of
sites and agents. The techniques adopted include labelled transition
semantics, operational equivalences and preorders (e.g., expansion
and coupled simulation), “up to” equivalences, and uniform
receptiveness. We also develop two novel proof techniques for
capturing the design intuitions regarding mobile agents: we consider
“translocating” versions of operational equivalences that take
migration into account, allowing compositional reasoning; and
“temporary immobility”, which captures the intuition that while an
agent is waiting for a lock somewhere in the system, it will not
migrate.
The correctness proof of an example infrastructure is non-trivial.
It involves analysing the possible reachable states of the encoding
applied to an arbitrary high-level source program. We introduce an
intermediate language for factoring out as many ‘house-keeping’
reduction steps as possible, and focusing on the partially-committed
steps.
The UDP calculus: rigorous semantics for real
networking
Serjantov, Andrei
Sewell, Peter
Wansbrough, Keith
University of Cambridge, Computer Laboratory
2001-07
en
Text
UCAM-CL-TR-515
ISSN 1476-2986
Network programming is notoriously hard to understand: one has to
deal with a variety of protocols (IP, ICMP, UDP, TCP, etc.),
concurrency, packet loss, host failure, timeouts, the complex
sockets interface to the protocols, and subtle protability issues.
Moreover, the behavioural properties of operating systems and the
network are not well documented.
A few of these issues have been addressed in the process calculus
and distributed algorithm communities, but there remains a wide gulf
between what has been captured in semantic models and what is
required for a precise understanding of the behaviour of practical
distributed programs that use these protocols.
In this paper we demonstrate (in a preliminary way) that the gulf
can be bridged. We give an operational model for socket programming
with a substantial fraction of UDP and ICMP, including loss and
failure. The model has been validated by experiment against actual
systems. It is not tied to a particular programming language, but
can be used with any language equipped with an operational semantics
for system calls – here we give such a language binding for an OCaml
fragment. We illustrate the model with a few small network programs.
Dynamic provisioning of resource-assured and programmable
virtual private networks
Isaacs, Rebecca
University of Cambridge, Computer Laboratory
2001-09
en
Text
UCAM-CL-TR-516
ISSN 1476-2986
Virtual Private Networks (VPNs) provide dedicated connectivity to a
closed group of users on a shared network. VPNs have traditionally
been deployed for reasons of economy of scale, but have either been
statically defined, requiring manual configuration, or else unable
to offer any quality of service (QoS) guarantees.
This dissertation describes VServ, a service offering dynamic and
resource-assured VPNs that can be acquired and modified on demand.
In VServ, a VPN is both a subset of physical resources, such as
bandwidth and label space, together with the means to perform
fine-grained management of those resources. This network
programmability, combined with QoS guarantees, enables the
multiservice network – a single universal network that can support
all types of service and thus be efficient, cost-effective and
flexible.
VServ is deployed over a network control framework known as Tempest.
The Tempest explicitly distinguishes between inter- and intra-VPN
resource management mechanisms. This makes the dynamic resource
reallocation capabilities of VServ viable, whilst handling highly
dynamic VPNs or a large number of VPNs. Extensions to the original
implementation of the Tempest to support dynamically reconfigurable
QoS are detailed.
A key part of a dynamic and responsive VPN service is fully
automated VPN provisioning. A notation for VPN specification is
described, together with mechanisms for incorporating policies of
the service provider and the current resource availability in the
network into the design process. The search for a suitable VPN
topology can be expressed as a optimisation problem that is not
computationally tractable except for very small networks. This
dissertation describes how the search is made practical by tailoring
it according to the characteristics of the desired VPN.
Availability of VServ is addressed with a proposal for distributed
VPN creation. A resource revocation protocol exploits the dynamic
resource management capabilities of VServ to allow adaptation in the
control plane on a per-VPN basis. Managed resource revocation
supports highly flexible resource allocation and reallocation
policies, allowing VServ to efficiently provision for short-lived or
highly dynamic VPNs.
The Cambridge Multimedia Document Retrieval Project: summary
of experiments
Spärck Jones, Karen
Jourlin, P.
Johnson, S.E.
Woodland, P.C.
University of Cambridge, Computer Laboratory
2001-07
en
Text
UCAM-CL-TR-517
ISSN 1476-2986
This report summarises the experimental work done under the
Multimedia Document Retrieval (MDR) project at Cambridge from
1997-2000, with selected illustrations. The focus is primarily on
retrieval studies, and on speech tests directly related to
retrieval, not on speech recognition itself. The report draws on the
many and varied tests done during the project, but also presents a
new series of results designed to compare strategies across as many
different data sets as possible by using consistent system parameter
settings.
The project tests demonstrate that retrieval from files of audio
news material transcribed using a state of the art speech
recognition system can match the reference level defined by human
transcriptions; and that expansion techniques, especially when
applied to queries, can be very effective means for improving basic
search performance.
An attack on a traitor tracing scheme
Yan, Jeff Jianxin
Wu, Yongdong
University of Cambridge, Computer Laboratory
2001-07
en
Text
UCAM-CL-TR-518
ISSN 1476-2986
In Crypto’99, Boneh and Franklin proposed a public key traitor
tracing scheme, which was believed to be able to catch all traitors
while not accusing any innocent users (i.e., full-tracing and
error-free). Assuming that Decision Diffie-Hellman problem is
unsolvable in Gq, Boneh and Franklin proved that a decoder cannot
distinguish valid ciphertexts from invalid ones that are used for
tracing. However, our novel pirate decoder P3 manages to make some
invalid ciphertexts distinguishable without violating their
assumption, and it can also frame innocent user coalitions to fool
the tracer. Neither the single-key nor arbitrary pirate tracing
algorithm presented in [1] can identify all keys used by P3 as
claimed. Instead, it is possible for both algorithms to catch none
of the traitors. We believe that the construction of our novel
pirate also demonstrates a simple way to defeat some other black-box
traitor tracing schemes in general.
Local evidence in document retrieval
Choquette, Martin
University of Cambridge, Computer Laboratory
2001-08
en
Text
UCAM-CL-TR-519
ISSN 1476-2986
Ternary and three-point univariate subdivision
schemes
Hassan, Mohamed
Dodgson, Neil A.
University of Cambridge, Computer Laboratory
2001-09
en
Text
UCAM-CL-TR-520
ISSN 1476-2986
The generating function formalism is used to analyze the continuity
properties of univariate ternary subdivision schemes. These are
compared with their binary counterparts.
Operational congruences for reactive systems
Leifer, James
University of Cambridge, Computer Laboratory
2001-09
en
Text
UCAM-CL-TR-521
ISSN 1476-2986
The dynamics of process calculi, eg. CCS, have often been defined
using a labelled transaction system (LTS). More recently it has
become common when defining dynamics to use reaction rules –ie.
unlabelled transition rules– together with a structural congruence.
This form, which I call a reactive system, is highly expressive but
is limited in an important way: LTSs lead more naturally to
operational equivalences and preorders.
So one would like to derive from reaction rules a suitable LTS. This
dissertation shows how to derive an LTS for a wide range of reactive
systems. A label for an agent (process), a, is defined to be any
context, F, which intuitively is just large enough so that the agent
Fa (“a in context F”) is able to perform a reaction. The key
contribution of my work is the precise definition of “just large
enough”, in terms of the categorical notation of relative pushout
(RPO), which ensures that several operational equivalences and
preorders (strong bisimulation, weak bisimulation, the traces
preorder, and the failures preorder) are congruences when sufficient
RPOs exist.
I present a substantial example of a family of reactive systems
based on closed, shallow action calculi (those with no free names
and no nesting). I prove that RPOs exist for a category of such
contexts. The proof is carried out indirectly in terms of a category
of action graphs and embeddings and gives precise (necessary and
sufficient) conditions for the existance of RPOs. I conclude by
arguing that these conditions are satisfied for a wide class of
reaction rules. The thrust of this dissertation is, therefore,
towards easing the burden of exploring new models of computation by
providing a general method for achieving useful operational
congruences.
Practical behavioural animation based on vision and
attention
Gillies, Mark F.P.
University of Cambridge, Computer Laboratory
2001-09
en
Text
UCAM-CL-TR-522
ISSN 1476-2986
The animation of human like characters is a vital aspect of computer
animation. Most animations rely heavily on characters of some sort
or other. This means that one important aspect of computer animation
research is to improve the animation of these characters both by
making it easier to produce animations and by improving the quality
of animation produced. One approach to animating characters is to
produce a simulation of the behaviour of the characters which will
automatically animate the character.
The dissertation investigates the simulation of behaviour in
practical applications. In particular it focuses on models of visual
perception for use in simulating human behaviour. A simulation of
perception is vital for any character that interacts with its
surroundings. Two main aspects of the simulation of perception are
investigated:
– The use of psychology for designing visual algorithms.
– The simulation of attention in order to produce both behaviour and
gaze patterns.
Psychological theories are a useful starting point for designing
algorithms for simulating visual perception. The dissertation
investigates their use and presents some algorithms based on
psychological theories.
Attention is the focusing of a person’s perception on a particular
object. The dissertation presents a simulation of what a character
is attending to (looking at). This is used to simulate behaviour and
for animating eye movements.
The algorithms for the simulation of vision and attention are
applied to two tasks in the simulation of behaviour. The first is a
method for designing generic behaviour patterns from simple pieces
of motion. The second is a behaviour pattern for navigating a
cluttered environment. The simulation of vision and attention gives
advantages over existing work on both problems. The approaches to
the simulation of perception will be evaluated in the context of
these examples.
Bigraphical reactive systems: basic theory
Milner, Robin
University of Cambridge, Computer Laboratory
2001-09
en
Text
UCAM-CL-TR-523
ISSN 1476-2986
A notion of bigraph is proposed as the basis for a model of mobile
interaction. A bigraph consists of two independent structures: a
topograph representing locality and a monograph representing
connectivity. Bigraphs are equipped with reaction rules to form
bigraphical reactive systems (BRSs), which include versions of the
π-calculus and the ambient calculus. Bigraphs are shown to be a
special case of a more abstract notion, wide reactive systems
(WRSs), not assuming any particular graphical or other structure but
equipped with a notion of width, which expresses that agents,
contexts and reactions may all be widely distributed entities.
A behavioural theory is established for WRSs using the categorical
notion of relative pushout; it allows labelled transition systems to
be derived uniformly, in such a way that familiar behavioural
preorders and equivalences, in particular bisimilarity, are
congruential under certain conditions. Then the theory of bigraphs
is developed, and they are shown to meet these conditions. It is
shown that, using certain functors, other WRSs which meet the
conditions may also be derived; these may, for example, be forms of
BRS with additional structure.
Simple examples of bigraphical systems are discussed; the theory is
developed in a number of ways in preparation for deeper application
studies.
Verifying the SET purchase protocols
Bella, Giampaolo
Massacci, Fabio
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
2001-11
en
Text
UCAM-CL-TR-524
ISSN 1476-2986
The Secure Electronic Transaction (SET) protocol has been proposed
by a consortium of credit card companies and software corporations
to guarantee the authenticity of e-commerce transactions and the
confidentiality of data. When the customer makes a purchase, the SET
dual signature keeps his account details secret from the merchant
and his choice of goods secret from the bank. This paper reports
verification results for the purchase step of SET, using the
inductive method. The credit card details do remain confidential.
The customer, merchant and bank can confirm most details of a
transaction even when some of those details are kept from them. The
usage of dual signatures requires repetition in protocol messages,
making proofs more difficult but still feasible. The formal analysis
has revealed a significant defect. The dual signature lacks
explicitness, giving rise to potential vulnerabilities.
Extensible virtual machines
Harris, Timothy L.
University of Cambridge, Computer Laboratory
2001-12
en
Text
UCAM-CL-TR-525
ISSN 1476-2986
Virtual machines (VMs) have enjoyed a resurgence as a way of
allowing the same application program to be used across a range of
computer systems. This flexibility comes from the abstraction that
the provides over the native interface of a particular computer.
However, this also means that the application is prevented from
taking the features of particular physical machines into account in
its implementation.
This dissertation addresses the question of why, where and how it is
useful, possible and practicable to provide an application with
access to lower-level interfaces. It argues that many aspects of
implementation can be devolved safely to untrusted applications and
demonstrates this through a prototype which allows control over
run-time compilation, object placement within the heap and thread
scheduling. The proposed architecture separates these
application-specific policy implementations from the application
itself. This allows one application to be used with different
policies on different systems and also allows naïve or premature
optimizations to be removed.
Extending lossless image compression
Penrose, Andrew J.
University of Cambridge, Computer Laboratory
2001-12
en
Text
UCAM-CL-TR-526
ISSN 1476-2986
“It is my thesis that worthwhile improvements can be made to
lossless image compression schemes, by considering the correlations
between the spectral, temporal and interview aspects of image data,
in extension to the spatial correlations that are traditionally
exploited.”
Images are an important part of today’s digital world. However, due
to the large quantity of data needed to represent modern imagery the
storage of such data can be expensive. Thus, work on efficient image
storage (image compression) has the potential to reduce storage
costs and enable new applications.
Many image compression schemes are lossy; that is they sacrifice
image informationto achieve very compact storage. Although this is
acceptable for many applications, some environments require that
compression not alter the image data. This lossless image
compression has uses in medical, scientific and professional video
processing applications.
Most of the work on lossless image compression has focused on
monochrome images and has made use of the spatial smoothness of
image data. Only recently have researchers begun to look
specifically at the lossless compression of colour images and video.
By extending compression schemes for colour images and video, the
storage requirements for these important classes of image data can
be further reduced.
Much of the previous research into lossless colour image and video
compression has been exploratory. This dissertation studies the
problem in a structured way. Spatial, spectral and temporal
correlations are all considered to facilitate improved compression.
This has lead to a greater data reduction than many existing schemes
for lossless colour image and colour video compression.
Furthermore, this work has considered the application of extended
lossless image coding to more recent image types, such as multiview
imagery. Thus, systems that use multiple views of the same scene to
provide 3D viewing, have beenprovided with a completely novel
solution for the compression of multiview colour video.
Architectures for ubiquitous systems
Saif, Umar
University of Cambridge, Computer Laboratory
2002-01
en
Text
UCAM-CL-TR-527
ISSN 1476-2986
Advances in digital electronics over the last decade have made
computers faster, cheaper and smaller. This coupled with the
revolution in communication technology has led to the development of
sophisticated networked appliances and handheld devices. “Computers”
are no longer boxes sitting on a desk, they are all around us,
embedded in every nook and corner of our environment. This
increasing complexity in our environment leads to the desire to
design a system that could allow this pervasive functionality to
disappear in the infrastructure, automatically carrying out everyday
tasks of the users.
Such a system would enable devices embedded in the environment to
cooperate with one another to make a wide range of new and useful
applications possible, not originally conceived by the manufacturer,
to achieve greater functionality, flexibility and utility.
The compelling question then becomes “what software needs to be
embedded in these devices to enable them to participate in such a
ubiquitous system”? This is the question addressed by the
dissertation.
Based on the experience with home automation systems, as part of the
AutoHAN project, the dissertation presents two compatible but
different architectures; one to enable dumb devices to be controlled
by the system and the other to enable intelligent devices to
control, extend and program the system.
Control commands for dumb devices are managed using an HTTP-based
publish/subscribe/notify architecture; devices publish their control
commands to the system as XML-typed discrete messages, applications
discover and subscribe interest in these events to send and receive
control commands from these devices, as typed messages, to control
their behavior. The architecture handles mobility and failure of
devices by using soft-state, redundent subscriptions and “care-of”
nodes. The system is programmed with event scripts that encode
automation rules as condition-action bindings. Finally, the use of
XML and HTTP allows devices to be controlled by a simple Internet
browser.
While the publish/subscribe/notify defines a simple architecture to
enable interoperability of limited capability devices, intelligent
devices can afford more complexity that can be utilized to support
user applications and services to control, manage and program the
system. However, the operating system embedded in these devices
needs to address the heterogeneity, longevity, mobility and dynamism
of the system.
The dissertation presents the architecture of an embedded
distributed operating system that lends itself to safe
context-driven adaptation. The operating system is instrumented with
four artifacts to address the challenges posed by a ubiquitous
system. 1) An XML-based directory service captures and notifies the
applications and services about changes in the device context, as
resources move, fail, leave or join the system, to allow
context-driven adaptation. 2) A Java-based mobile agent system
allows new software to be injected in the system and moved and
replicated with the changing characteristics of the system to define
a self-organizing system. 3) A subscribe/notify interface allows
context-specific extensions to be dynamically added to the operating
system to enable it to efficiently interoperate in its current
context according to application requirements. 4) Finally, a
Dispatcher module serves as the context-aware system call interface
for the operating system; when requested to invoke a service, the
Dispatcher invokes the resource that best satisfies the requirements
given the characteristics of the system.
Definition alone is not sufficient to prove the validity of an
architecture. The dissertation therefore describes a prototype
implementation of the operating system and presents both a
quantitative comparison of its performance with related systems and
its qualitative merit by describing new applications made possible
by its novel architecture.
Measurement-based management of network resources
Moore, Andrew William
University of Cambridge, Computer Laboratory
2002-04
en
Text
UCAM-CL-TR-528
ISSN 1476-2986
Measurement-Based Estimators are able to characterise data flows,
enabling improvements to existing management techniques and access
to previously impossible management techniques. It is the thesis of
this dissertation that in addition to making practical adaptive
management schemes, measurement-based estimators can be practical
within current limitations of resource.
Examples of network management include the characterisation of
current utilisation for explicit admission control and the
configuration of a scheduler to divide link-capacity among competing
traffic classes. Without measurements, these management techniques
have relied upon the accurate characterisation of traffic – without
accurate traffic characterisation, network resources may be under or
over utilised.
Embracing Measurement-Based Estimation in admission control,
Measurement-Based Admission Control (MBAC) algorithms have allowed
characterisation of new traffic flows while adapting to changing
flow requirements. However, there have been many MBAC algorithms
proposed, often with no clear differentiation between them. This has
motivated the need for a realistic, implementation-based comparison
in order to identify an ideal MBAC algorithm.
This dissertation reports on an implementation-based comparison of
MBAC algorithms conducted using a purpose built test environment.
The use of an implementation-based comparison has allowed the MBAC
algorithms to be tested under realistic conditions of traffic load
and realistic limitations on memory, computational resources and
measurements. Alongside this comparison is a decomposition of a
group of MBAC algorithms, illustrating the relationship among MBAC
algorithm components, as well as highlighting common elements among
different MBAC algorithms.
The MBAC algorithm comparison reveals that, while no single
algorithm is ideal, the specific resource demands, such as
computation overheads, can dramatically impact on the MBAC
algorithm’s performance. Further, due to the multiple timescales
present in both traffic and management, the estimator of a robust
MBAC algorithm must base its estimate on measurements made over a
wide range of timescales. Finally, a reliable estimator must account
for the error resulting from random properties of measurements.
Further identifying that the estimator components used in MBAC
algorithms need not be tied to the admission control problem, one of
the estimators (originally constructed as part of an MBAC algorithm)
is used to continuously characterise resource requirements for a
number of classes of traffic. Continuous characterisation of
traffic, whether requiring similar or orthogonal resources, leads to
the construction and demonstration of a network switch that is able
to provide differentiated service while being adaptive to the
demands of each traffic class. The dynamic allocation of resources
is an approach unique to a measurement-based technique that would
not be possible if resources were based upon static declarations of
requirement.
The triVM intermediate language reference manual
Johnson, Neil
University of Cambridge, Computer Laboratory
2002-02
en
Text
UCAM-CL-TR-529
ISSN 1476-2986
The triVM intermediate language has been developed as part of a
research programme concentrating on code space optimization. The
primary aim in developing triVM is to provide a language that
removes the complexity of high-level languages, such as C or ML,
while maintaining sufficient detail, at as simple a level as
possible, to support reseach and experimentation into code size
optimization. The basic structure of triVM is a notional Static
Single Assignment-based three-address machine. A secondary aim is to
develop an intermediate language that supports graph-based
translation, using graph rewrite rules, in a textual, human-readable
format. Experience has shown that text-format intermediate files are
much easier to use for experimentation, while the penalty in
translating this human-readable form to the internal data structures
used by the software is negligible. Another aim is to provide a
flexible language in which features and innovations can be
evaluated; for example, this is one of the first intermediate
languages directly based on the Static Single Assignment technique,
and which explicitly exposes the condition codes as a result of
arithmetic operations. While this paper is concerned solely with the
description of triVM, we present a brief summary of other
research-orientated intermediate languages.
Subcategorization acquisition
Korhonen, Anna
University of Cambridge, Computer Laboratory
2002-02
en
Text
UCAM-CL-TR-530
ISSN 1476-2986
Manual development of large subcategorised lexicons has proved
difficult because predicates change behaviour between sublanguages,
domains and over time. Yet access to a comprehensive
subcategorization lexicon is vital for successful parsing capable of
recovering predicate-argument relations, and probabilistic parsers
would greatly benefit from accurate information concerning the
relative likelihood of different subcategorisation frames SCFs of a
given predicate. Acquisition of subcategorization lexicons from
textual corpora has recently become increasingly popular. Although
this work has met with some success, resulting lexicons indicate a
need for greater accuracy. One significant source of error lies in
the statistical filtering used for hypothesis selection, i.e. for
removing noise from automatically acquired SCFs.
This thesis builds on earlier work in verbal subcategorization
acquisition, taking as a starting point the problem with statistical
filtering. Our investigation shows that statistical filters tend to
work poorly because not only is the underlying distribution zipfian,
but there is also very little correlation between conditional
distribution of SCFs specific to a verb and unconditional
distribution regardless of the verb. More accurate back-off
estimates are needed for SCF acquisition than those provided by
unconditional distribution.
We explore whether more accurate estimates could be obtained by
basing them on linguistic verb classes. Experiments are reported
which show that in terms of SCF distributions, individual verbs
correlate more closely with syntactically similar verbs and even
more closely with semantically similar verbs, than with all verbs in
general. On the basis of this result, we suggest classifying verbs
according to their semantic classes and obtaining back-off estimates
specific to these classes.
We propose a method for obtaining such semantically based back-off
estimates, and a novel approach to hypothesis selection which makes
use of these estimates. This approach involves automatically
identifying the semantic class of a predicate, using
subcategorization acquisition machinery to hypothesise conditional
SCF distribution for the predicate, smoothing the conditional
distribution with the back-off estimates of the respective semantic
verb class, and employing a simple method for filtering, which uses
a threshold on the estimates from smoothing. Adopting Briscoe and
Carroll’s (1997) system as a framework, we demonstrate that this
semantically-driven approach to hypothesis selection can
significantly improve the accuracy of large-scale subcategorization
acquisition.
Verifying the SET registration protocols
Bella, Giampaolo
Massacci, Fabio
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
2002-03
en
Text
UCAM-CL-TR-531
ISSN 1476-2986
SET (Secure Electronic Transaction) is an immense e-commerce
protocol designed to improve the security of credit card purchases.
In this paper we focus on the initial bootstrapping phases of SET,
whose objective is the registration of customers and merchants with
a SET certification authority. The aim of registration is twofold:
getting the approval of the cardholder’s or merchant’s bank, and
replacing traditional credit card numbers with electronic
credentials that customers can present to the merchant, so that
their privacy is protected. These registration sub-protocols present
a number of challenges to current formal verification methods.
First, they do not assume that each agent knows the public keys of
the other agents. Key distribution is one of the protocols’ tasks.
Second, SET uses complex encryption primitives (digital envelopes)
which introduce dependency chains: the loss of one secret key can
lead to potentially unlimited losses. Building upon our previous
work, we have been able to model and formally verify SET’s
registration with the inductive method in Isabelle/HOL solving its
challenges with very general techniques.
Internet traffic engineering
Mortier, Richard
University of Cambridge, Computer Laboratory
2002-04
en
Text
UCAM-CL-TR-532
ISSN 1476-2986
Due to the dramatically increasing popularity of the services
provided over the public Internet, problems with current mechanisms
for control and management of the Internet are becoming apparent. In
particular, it is increasingly clear that the Internet and other
networks built on the Internet protocol suite do not provide
sufficient support for the efficient control and management of
traffic, i.e. for Traffic Engineering.
This dissertation addresses the problem of traffic engineering in
the Internet. It argues that traffic management techniques should be
applied at multiple timescales, and not just at data timescales as
is currently the case. It presents and evaluates mechanisms for
traffic engineering in the Internet at two further timescales: flow
admission control and control of per-flow packet marking, enabling
control timescale traffic engineering; and support for load based
inter-domain routeing in the Internet, enabling management timescale
traffic engineering.
This dissertation also discusses suitable policies for the
application of the proposed mechanisms. It argues that the proposed
mechanisms are able to support a wide range of policies useful to
both users and operators. Finally, in a network of the size of the
Internet consideration must also be given to the deployment of
proposed solutions. Consequently, arguments for and against the
deployment of these mechanisms are presented and the conclusion
drawn that there are a number of feasible paths toward deployment.
The work presented argues the following: firstly, it is possible to
implement mechanisms within the Internet framework that enable
traffic engineering to be carried out by operators; secondly, that
applying these mechanisms with suitable policies can ease the
management problems faced by operators and at the same time improve
the efficiency with which the network can be run; thirdly, that
these improvements can correspond to increased network performance
as viewed by the user; and finally, that not only the resulting
deployment but also the deployment process itself are feasible.
The acquisition of a unification-based generalised
categorial grammar
Villavicencio, Aline
University of Cambridge, Computer Laboratory
2002-04
en
Text
UCAM-CL-TR-533
ISSN 1476-2986
The purpose of this work is to investigate the process of
grammatical acquisition from data. In order to do that, a
computational learning system is used, composed of a Universal
Grammar with associated parameters, and a learning algorithm,
following the Principles and Parameters Theory. The Universal
Grammar is implemented as a Unification-Based Generalised Categorial
Grammar, embedded in a default inheritance network of lexical types.
The learning algorithm receives input from a corpus of spontaneous
child-directed transcribed speech annotated with logical forms and
sets the parameters based on this input. This framework is used as a
basis to investigate several aspects of language acquisition. In
this thesis I concentrate on the acquisition of subcategorisation
frames and word order information, from data. The data to which the
learner is exposed can be noisy and ambiguous, and I investigate how
these factors affect the learning process. The results obtained show
a robust learner converging towards the target grammar given the
input data available. They also show how the amount of noise present
in the input data affects the speed of convergence of the learner
towards the target grammar. Future work is suggested for
investigating the developmental stages of language acquisition as
predicted by the learning model, with a thorough comparison with the
developmental stages of a child. This is primarily a cognitive
computational model of language learning that can be used to
investigate and gain a better understanding of human language
acquisition, and can potentially be relevant to the development of
more adaptive NLP technology.
Resource control in network elements
Donnelly, Austin
University of Cambridge, Computer Laboratory
2002-04
en
Text
UCAM-CL-TR-534
ISSN 1476-2986
Increasingly, substantial data path processing is happening on
devices within the network. At or near the edges of the network,
data rates are low enough that commodity workstations may be used to
process packet flows. However, the operating systems such machines
use are not suited to the needs of data-driven processing. This
dissertation shows why this is a problem, how current work fails to
address it, and proposes a new approach.
The principal problem is that crosstalk occurs in the processing of
different data flows when they contend for a shared resource and
their accesses to this resource are not scheduled appropriately;
typically the shared resource is located in a server process.
Previous work on vertically structured operating systems reduces the
need for such shared servers by making applications responsible for
performing as much of their own processing as possible, protecting
and multiplexing devices at the lowest level consistent with
allowing untrusted user access.
However, shared servers remain on the data path in two
circumstances: firstly, dumb network adaptors need non-trivial
processing to allow safe access by untrusted user applications.
Secondly, shared servers are needed wherever trusted code must be
executed for security reasons.
This dissertation presents the design and implementation of Expert,
an operating system which avoids crosstalk by removing the need for
such servers.
This dissertation describes how Expert handles dumb network adaptors
to enable applications to access them via a low-level interface
which is cheap to implement in the kernel, and retains application
responsibility for the work involved in running a network stack.
Expert further reduces the need for application-level shared servers
by introducing paths which can trap into protected modules of code
to perform actions which would otherwise have to be implemented
within a server.
Expert allows traditional compute-bound tasks to be freely mixed
with these I/O-driven paths in a single system, and schedules them
in a unified manner. This allows the processing performed in a
network element to be resource controlled, both for background
processing tasks such as statistics gathering, and for data path
processing such as encryption.
Designs, disputes and strategies
Faggian, Claudia
Hyland, Martin
University of Cambridge, Computer Laboratory
2002-05
en
Text
UCAM-CL-TR-535
ISSN 1476-2986
Important progresses in logic are leading to interactive and
dynamical models. Geometry of Interaction and Games Semantics are
two major examples. Ludics, initiated by Girard, is a further step
in this direction.
The objects of Ludics which correspond to proofs are designs. A
design can be described as the skeleton of a sequent calculus
derivation, where we do not manipulate formulas, but their location
(the address where the formula is stored). To study the traces of
the interactions between designs as primitive leads to an
alternative presentation, which is to describe a design as the set
of its possible interactions, called disputes. This presentation has
the advantage to make precise the correspondence between the basic
notions of Ludics (designs, disputes and chronicles) and the basic
notions of Games semantics (strategies, plays and views).
Low temperature data remanence in static RAM
Skorobogatov, Sergei
University of Cambridge, Computer Laboratory
2002-06
en
Text
UCAM-CL-TR-536
ISSN 1476-2986
Security processors typically store secret key material in static
RAM, from which power is removed if the device is tampered with. It
is commonly believed that, at temperatures below −20 °C, the
contents of SRAM can be ‘frozen’; therefore, many devices treat
temperatures below this threshold as tampering events. We have done
some experiments to establish the temperature dependency of data
retention time in modern SRAM devices. Our experiments show that the
conventional wisdom no longer holds.
Parallel systems in symbolic and algebraic
computation
Matooane, Mantsika
University of Cambridge, Computer Laboratory
2002-06
en
Text
UCAM-CL-TR-537
ISSN 1476-2986
This report describes techniques to exploit distributed memory
massively parallel supercomputers to satisfy the peak memory demands
of some very large computer algebra problems (over 10 GB). The
memory balancing is based on a randomized hashing algorithm for
dynamic data distribution. Fine grained partitioning is used to
provide flexibility in the memory allocation, at the cost of higher
communication cost. The main problem areas are multivariate
polynomial algebra, and linear algebra with polynomial matrices. The
system was implemented and tested on a Hitachi SR2201 supercomputer.
The Escritoire: A personal projected display for interacting
with documents
Ashdown, Mark
Robinson, Peter
University of Cambridge, Computer Laboratory
2002-06
en
Text
UCAM-CL-TR-538
ISSN 1476-2986
The Escritoire is a horizontal desk interface that uses two
projectors to create a foveal display. Items such as images,
documents, and the interactive displays of other conventional
computers, can be manipulated on the desk using pens in both hands.
The periphery covers the desk, providing ample space for laying out
the objects relevant to a task, allowing them to be identified at a
glance and exploiting human spatial memory for rapid retrieval. The
fovea is a high resolution focal area that can be used to view any
item in detail. The projected images are continuously warped with
commodity graphics hardware before display, to reverse the effects
of misaligned projectors and ensure registration between fovea and
periphery. The software is divided into a hardware-specific client
driving the display, and a platform-independent server imposing
control.
Towards a ternary interpolating subdivision scheme for the
triangular mesh
Dodgson, N.A.
Sabin, M.A.
Barthe, L.
Hassan, M.F.
University of Cambridge, Computer Laboratory
2002-07
en
Text
UCAM-CL-TR-539
ISSN 1476-2986
We derive a ternary interpolating subdivision scheme which works on
the regular triangular mesh. It has quadratic precision and fulfils
the standard necessary conditions for C2 continuity. Further
analysis is required to determine its actual continuity class and to
define its behaviour around extraordinary points.
The use of computer graphics rendering software in the
analysis of a novel autostereoscopic display design
Dodgson, N.A.
Moore, J.R.
University of Cambridge, Computer Laboratory
2002-08
en
Text
UCAM-CL-TR-540
ISSN 1476-2986
Computer graphics ‘ray tracing’ software has been used in the design
and evaluation of a new autostereoscopic 3D display. This software
complements the conventional optical design software and provides a
cost-effective method of simulating what is actually seen by a
viewer of the display. It may prove a useful tool in similar design
problems.
Different applications of two-dimensional potential fields
for volume modeling
Barthe, L.
Dodgson, N.A.
Sabin, M.A.
Wyvill, B.
Gaildrat, V.
University of Cambridge, Computer Laboratory
2002-08
en
Text
UCAM-CL-TR-541
ISSN 1476-2986
Current methods for building models using implicit volume techniques
present problems defining accurate and controllable blend shapes
between implicit primitives. We present new methods to extend the
freedom and controllability of implicit volume modeling. The main
idea is to use a free-form curve to define the profile of the blend
region between implicit primitives.
The use of a free-form implicit curve, controlled point-by-point in
the Euclidean user space, allows us to group boolean composition
operators with sharp transitions or smooth free-form transitions in
a single modeling metaphor. This idea is generalized for the
creation, sculpting and manipulation of volume objects, while
providing the user with simplicity, controllability and freedom in
volume modeling.
Bounded volume objects, known as “Soft objects” or “Metaballs”, have
specific properties. We also present binary Boolean composition
operators that gives more control on the form of the transition when
these objects are blended.
To finish, we show how our free-form implicit curves can be used to
build implicit sweep objects.
A generative classification of mesh refinement rules with
lattice transformations
Ivrissimtzis, I.P.
Dodgson, N.A.
Sabin, M.A.
University of Cambridge, Computer Laboratory
2002-09
en
Text
UCAM-CL-TR-542
ISSN 1476-2986
We give a classification of the subdivision refinement rules using
sequences of similar lattices. Our work expands and unifies recent
results in the classification of primal triangular subdivision
[Alexa, 2001], and results on the refinement of quadrilateral
lattices [Sloan, 1994, 1989]. In the examples we concentrate on the
cases with low ratio of similarity and find new univariate and
bivariate refinement rules with the lowest possible such ratio,
showing that this very low ratio usually comes at the expense of
symmetry.
Evaluating similarity-based visualisations as interfaces for
image browsing
Rodden, Kerry
University of Cambridge, Computer Laboratory
2002-09
en
Text
UCAM-CL-TR-543
ISSN 1476-2986
Large collections of digital images are becoming more and more
common, and the users of these collections need computer-based
systems to help them find the images they require. Digital images
are easy to shrink to thumbnail size, allowing a large number of
them to be presented to the user simultaneously. Generally, current
image browsing interfaces display thumbnails in a two-dimensional
grid, in some default order, and there has been little exploration
of possible alternatives to this model.
With textual document collections, information visualisation
techniques have been used to produce representations where the
documents appear to be clustered according to their mutual
similarity, which is based on the words they have in common. The
same techniques can be applied to images, to arrange a set of
thumbnails according to a defined measure of similarity. In many
collections, the images are manually annotated with descriptive
text, allowing their similarity to be measured in an analogous way
to textual documents. Alternatively, research in content-based image
retrieval has made it possible to measure similarity based on
low-level visual features, such as colour.
The primary goal of this research was to investigate the usefulness
of such similarity-based visualisations as interfaces for image
browsing. We concentrated on visual similarity, because it is
applicable to any image collection, regardless of the availability
of annotations. Initially, we used conventional information
retrieval evaluation methods to compare the relative performance of
a number of different visual similarity measures, both for retrieval
and for creating visualisations.
Thereafter, our approach to evaluation was influenced more by
human-computer interaction: we carried out a series of user
experiments where arrangements based on visual similarity were
compared to random arrangements, for different image browsing tasks.
These included finding a given target image, finding a group of
images matching a generic requirement, and choosing subjectively
suitable images for a particular purpose (from a shortlisted set).
As expected, we found that similarity-based arrangements are
generally more helpful than random arrangements, especially when the
user already has some idea of the type of image she is looking for.
Images are used in many different application domains; the ones we
chose to study were stock photography and personal photography. We
investigated the organisation and browsing of personal photographs
in some depth, because of the inevitable future growth in usage of
digital cameras, and a lack of previous research in this area.
On the support of recursive subdivision
Ivrissimtzis, I.P.
Sabin, M.A.
Dodgson, N.A.
University of Cambridge, Computer Laboratory
2002-09
en
Text
UCAM-CL-TR-544
ISSN 1476-2986
We study the support of subdivision schemes, that is, the area of
the subdivision surface that will be affected by the displacement of
a single control point. Our main results cover the regular case,
where the mesh induces a regular Euclidean tessellation of the
parameter space. If n is the ratio of similarity between the
tessellation at step k and step k−1 of the subdivision, we show that
this number determines if the support is polygonal or fractal. In
particular if n=2, as it is in the most schemes, the support is a
polygon whose vertices can be easily determined. If n is not equal
to two as, for example, in the square root of three scheme, the
support is usually fractal and on its boundary we can identify sets
like the classic ternary Cantor set.
A HOL specification of the ARM instruction set
architecture
Fox, Anthony C.J.
University of Cambridge, Computer Laboratory
2001-06
en
Text
UCAM-CL-TR-545
ISSN 1476-2986
This report gives details of a HOL specification of the ARM
instruction set architecture. It is shown that the HOL proof tool
provides a suitable environment in which to model the architecture.
The specification is used to execute fragments of ARM code generated
by an assembler. The specification is based primarily around the
third version of the ARM architecture, and the intent is to provide
a target semantics for future microprocessor verifications.
Depth perception in computer graphics
Pfautz, Jonathan David
University of Cambridge, Computer Laboratory
2002-09
en
Text
UCAM-CL-TR-546
ISSN 1476-2986
With advances in computing and visual display technology, the
interface between man and machine has become increasingly complex.
The usability of a modern interactive system depends on the design
of the visual display. This dissertation aims to improve the design
process by examining the relationship between human perception of
depth and three-dimensional computer-generated imagery (3D CGI).
Depth is perceived when the human visual system combines various
different sources of information about a scene. In Computer
Graphics, linear perspective is a common depth cue, and systems
utilising binocular disparity cues are of increasing interest. When
these cues are inaccurately and inconsistently presented, the
effectiveness of a display will be limited. Images generated with
computers are sampled, meaning they are discrete in both time and
space. This thesis describes the sampling artefacts that occur in 3D
CGI and their effects on the perception of depth. Traditionally,
sampling artefacts are treated as a Signal Processing problem. The
approach here is to evaluate artefacts using Human Factors and
Ergonomics methodology; sampling artefacts are assessed via
performance on relevant visual tasks.
A series of formal and informal experiments were performed on human
subjects to evaluate the effects of spatial and temporal sampling on
the presentation of depth in CGI. In static images with perspective
information, the relative size of an object can be inconsistently
presented across depth. This inconsistency prevented subjects from
making accurate relative depth judgements. In moving images, these
distortions were most visible when the object was moving slowly,
pixel size was large, the object was located close to the line of
sight and/or the object was located a large virtual distance from
the viewer. When stereo images are presented with perspective cues,
the sampling artefacts found in each cue interact. Inconsistencies
in both size and disparity can occur as the result of spatial and
temporal sampling. As a result, disparity can vary inconsistently
across an object. Subjects judged relative depth less accurately
when these inconsistencies were present. An experiment demonstrated
that stereo cues dominated in conflict situations for static images.
In moving imagery, the number of samples in stereo cues is limited.
Perspective information dominated the perception of depth for
unambiguous (i.e., constant in direction and velocity) movement.
Based on the experimental results, a novel method was developed that
ensures the size, shape and disparity of an object are consistent as
it moves in depth. This algorithm manipulates the edges of an object
(at the expense of positional accuracy) to enforce consistent size,
shape and disparity. In a time-to-contact task using only stereo and
perspective depth cues, velocity was judged more accurately using
this method. A second method manipulated the location and
orientation of the viewpoint to maximise the number of samples of
perspective and stereo depth in a scene. This algorithm was tested
in a simulated air traffic control task. The experiment demonstrated
that knowledge about where the viewpoint is located dominates any
benefit gained in reducing sampling artefacts.
This dissertation provides valuable information for the visual
display designer in the form of task-specific experimental results
and computationally inexpensive methods for reducing the effects of
sampling.
Semantic optimization of OQL queries
Trigoni, Agathoniki
University of Cambridge, Computer Laboratory
2002-10
en
Text
UCAM-CL-TR-547
ISSN 1476-2986
This work explores all the phases of developing a query processor
for OQL, the Object Query Language proposed by the Object Data
Management Group (ODMG 3.0). There has been a lot of research on the
execution of relational queries and their optimization using
syntactic or semantic transformations. However, there is no context
that has integrated and tested all the phases of processing an
object query language, including the use of semantic optimization
heuristics. This research is motivated by the need for query
execution tools that combine two valuable properties: i) the
expressive power to encompass all the features of the
object-oriented paradigm and ii) the flexibility to benefit from the
experience gained with relational systems, such as the use of
semantic knowledge to speed up query execution.
The contribution of this work is twofold. First, it establishes a
rigorous basis for OQL by defining a type inference model for OQL
queries and proposing a complete framework for their translation
into calculus and algebraic representations. Second, in order to
enhance query execution it provides algorithms for applying two
semantic optimization heuristics: constraint introduction and
constraint elimination techniques. By taking into consideration a
set of association rules with exceptions, it is possible to add or
remove predicates from an OQL query, thus transforming it to a more
efficient form.
We have implemented this framework, which enables us to measure the
benefits and the cost of exploiting semantic knowledge during query
execution. The experiments showed significant benefits, especially
in the application of the constraint introduction technique. In
contexts where queries are optimized once and are then executed
repeatedly, we can ignore the cost of optimization, and it is always
worth carrying out the proposed transformation. In the context of
adhoc queries the cost of the optimization becomes an important
consideration. We have developed heuristics to estimate the cost as
well as the benefits of optimization. The optimizer will carry out a
semantic transformation only when the overhead is less than the
expected benefit. Thus transformations are performed safely even
with adhoc queries. The framework can often speed up the execution
of an OQL query to a considerable extent.
Formal verification of the ARM6
micro-architecture
Fox, Anthony
University of Cambridge, Computer Laboratory
2002-11
en
Text
UCAM-CL-TR-548
ISSN 1476-2986
This report describes the formal verification of the ARM6
micro-architecture using the HOL theorem prover. The correctness of
the microprocessor design compares the micro-architecture with an
abstract, target instruction set semantics. Data and temporal
abstraction maps are used to formally relate the state spaces and to
capture the timing behaviour of the processor. The verification is
carried out in HOL and one-step theorems are used to provide the
framework for the proof of correctness. This report also describes
the formal specification of the ARM6’s three stage pipelined
micro-architecture.
Two remarks on public key cryptology
Anderson, Ross
University of Cambridge, Computer Laboratory
2002-12
en
Text
UCAM-CL-TR-549
ISSN 1476-2986
In some talks I gave in 1997-98, I put forward two observations on
public-key cryptology, concerning forward-secure signatures and
compatible weak keys. I did not publish a paper on either of them as
they appeared to be rather minor footnotes to public key cryptology.
But the work has occasionally been cited, and I’ve been asked to
write a permanent record.
Computer security – a layperson’s guide, from the bottom
up
Spärck Jones, Karen
University of Cambridge, Computer Laboratory
2002-06
en
Text
UCAM-CL-TR-550
ISSN 1476-2986
Computer security as a technical matter is complex, and opaque for
those who are not themselves computer professionals but who
encounter, or are ultimately responsible for, computer systems. This
paper presents the essentials of computer security in non-technical
terms, with the aim of helping people affected by computer systems
to understand what security is about and to withstand the blinding
with science mantras that too often obscure the real issues.
The relative consistency of the axiom of choice — mechanized
using Isabelle/ZF
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
2002-12
en
Text
UCAM-CL-TR-551
ISSN 1476-2986
The proof of the relative consistency of the axiom of choice has
been mechanized using Isabelle/ZF. The proof builds upon a previous
mechanization of the reflection theorem. The heavy reliance on
metatheory in the original proof makes the formalization unusually
long, and not entirely satisfactory: two parts of the proof do not
fit together. It seems impossible to solve these problems without
formalizing the metatheory. However, the present development follows
a standard textbook, Kunen’s “Set Theory”, and could support the
formalization of further material from that book. It also serves as
an example of what to expect when deep mathematics is formalized.
The Xenoserver computing infrastructure
Fraser, Keir A.
Hand, Steven M.
Harris, Timothy L.
Leslie, Ian M.
Pratt, Ian A.
University of Cambridge, Computer Laboratory
2003-01
en
Text
UCAM-CL-TR-552
ISSN 1476-2986
The XenoServer project will build a public infrastructure for
wide-area distributed computing. We envisage a world in which
XenoServer execution platforms will be scattered across the globe
and available for any member of the public to submit code for
execution. Crucially, the code’s sponsor will be billed for all the
resources used or reserved during its execution. This will encourage
load balancing, limit congestion, and make the platform
self-financing.
Such a global infrastructure is essential to address the fundamental
problem of communication latency. By enabling principals to run
programs at points throughout the network they can ensure that their
code executes close to the entities with which it interacts. As well
as reducing latency this can be used to avoid network bottlenecks,
to reduce long-haul network charges and to provide a network
presence for transiently-connected mobile devices.
This project will build and deploy a global XenoServer test-bed and
make it available to authenticated external users; initially members
of the scientific community and ultimately of the general public. In
this environment accurate resource accounting and pricing is
critical – whether in an actual currency or one that is fictitious.
As with our existing work on OS resource management, pricing
provides the feedback necessary for applications that can adapt, and
prevents over-use by applications that cannot.
Xen 2002
Barham, Paul R.
Dragovic, Boris
Fraser, Keir A.
Hand, Steven M.
Harris, Timothy L.
Ho, Alex C.
Kotsovinos, Evangelos
Madhavapeddy, Anil V.S.
Neugebauer, Rolf
Pratt, Ian A.
Warfield, Andrew K.
University of Cambridge, Computer Laboratory
2003-01
en
Text
UCAM-CL-TR-553
ISSN 1476-2986
This report describes the design of Xen, the hypervisor developed as
part of the XenoServer wide-area computing project. Xen enables the
hardware resources of a machine to be virtualized and dynamically
partitioned such as to allow multiple different ‘guest’ operating
system images to be run simultaneously.
Virtualizing the machine in this manner provides flexibility,
allowing different users to choose their preferred operating system
(Windows, Linux, NetBSD), and also enables use of the platform as a
testbed for operating systems research. Furthermore, Xen provides
secure partitioning between these ‘domains’, and enables better
resource accounting and QoS isolation than can be achieved within a
conventional operating system. We show these benefits can be
achieved at negligible performance cost.
We outline the design of Xen’s main sub-systems, and the interface
exported to guest operating systems. Initial performance results are
presented for our most mature guest operating system port, Linux
2.4. This report covers the initial design of Xen, leading up to our
first public release which we plan to make available for download in
April 2003. Further reports will update the design as our work
progresses and present the implementation in more detail.
Towards a field theory for networks
Crowcroft, Jon
University of Cambridge, Computer Laboratory
2003-01
en
Text
UCAM-CL-TR-554
ISSN 1476-2986
It is often claimed that Internet Traffic patterns are interesting
because the Internet puts few constraints on sources. This leads to
innovation. It also makes the study of Internet traffic, what we
might cal the search for the Internet Erlang, very difficult. At the
same time, traffic control (congestion control) and engineering are
both hot topics.
What if “flash crowds” (a.k.a. slashdot), cascades, epidemics and so
on are the norm? What if the trend continues for network link
capacity to become flatter, with more equal capacity in the access
and core, or even more capacity in the access than the core (as in
the early 1980s with 10Mbps LANs versus Kbps links in the ARPANET)?
How could we cope?
This is a paper about the use of field equations (e.g.
gravitational, electrical, magnetic, strong and weak atomic and so
forth) as a future model for managing network traffic. We believe
that in the future, one could move from this model to a very general
prescriptive technique for designing network control on different
timescales, including traffic engineering and the set of admission
and congestion control laws. We also speculate about the use of the
same idea in wireless networks.
BOURSE – Broadband Organisation of Unregulated Radio Systems
through Economics
Crowcroft, Jon
Gibbens, Richard
Hailes, Stephen
University of Cambridge, Computer Laboratory
2003-01
en
Text
UCAM-CL-TR-555
ISSN 1476-2986
This is a technical report about an idea for research in the
intersection of active nets, cognitive radio and power laws of
network topologies.
Turing Switches – Turing machines for all-optical Internet
routing
Crowcroft, Jon
University of Cambridge, Computer Laboratory
2003-01
en
Text
UCAM-CL-TR-556
ISSN 1476-2986
This is technical report outlining an idea for basic long term
research into the architectures for programmable all-optical
Internet routers.
We are revisiting some of the fundamental tenets of computer science
to carry out this work, and so it is necessarily highly speculative.
Currently, the processing elements in all-electronic routers are
typically fairly conventional von-Neumann architecture computers
with processors that have large, complex instruction sets (even RISC
is relatively complex compared with the actual requirements for
packet processing) and Random Access Memory.
As the need for speed increases, first this architecture, and then
the classical computing hardware components, and finally,
electronics cease to be able to keep up.
At this time, optical device technology is making great strides, and
we see the availability of gates, as well as a plethora of invention
in providing buffering mechanisms.
However, a critical problem we foresee is the ability to re-program
devices for different packet processing functions such as
classification and scheduling. This proposal is aimed at researching
one direction for adding optical domain programmability.
Iota: A concurrent XML scripting language with applications
to Home Area Networking
Bierman, G.M.
Sewell, P.
University of Cambridge, Computer Laboratory
2003-01
en
Text
UCAM-CL-TR-557
ISSN 1476-2986
Iota is a small and simple concurrent language that provides native
support for functional XML computation and for typed channel-based
communication. It has been designed as a domain-specific language to
express device behaviour within the context of Home Area Networking.
In this paper we describe Iota, explaining its novel treatment of
XML and describing its type system and operational semantics. We
give a number of examples including Iota code to program Universal
Plug ’n’ Play (UPnP) devices.
A role and context based security model
Beresnevichiene, Yolanta
University of Cambridge, Computer Laboratory
2003-01
en
Text
UCAM-CL-TR-558
ISSN 1476-2986
Security requirements approached at the enterprise level initiate
the need for models that capture the organisational and distributed
aspects of information usage. Such models have to express
organisation-specific security policies and internal controls aiming
to protect information against unauthorised access and modification,
and against usage of information for unintended purposes. This
technical report describes a systematic approach to modelling the
security requirements from the perspective of job functions and
tasks performed in an organisation. It deals with the design,
analysis, and management of security abstractions and mechanisms in
a unified framework.
The basis of access control policy in this framework is formulated
around a semantic construct of a role. Roles are granted permissions
according to the job functions that exist in an organisation, and
then users are assigned to roles on basis of their specific job
responsibilities. In order to ensure that permissions included in
the roles are used by users only for purposes corresponding to the
organisation’s present business needs, a novel approach of “active”
context-based access control is proposed. The usage of role
permissions in this approach is controlled according to the emerging
context associated with progress of various tasks in the
organisation.
The work explores formally the security properties of the
established model, in particular, support for separation of duty and
least privilege principles that are important requirements in many
commercial systems. Results have implications for understanding
different variations of separation of duty policy that are currently
used in the role-based access control.
Finally, a design architecture of the defined security model is
presented detailing the components and processing phases required
for successful application of the model to distributed computer
environments. The model provides opportunities for the implementers,
based on application requirements, to choose between several
alternative design approaches.
Pronto: MobileGateway with publish-subscribe paradigm over
wireless network
Yoneki, Eiko
Bacon, Jean
University of Cambridge, Computer Laboratory
2003-02
en
Text
UCAM-CL-TR-559
ISSN 1476-2986
This paper presents the design, implementation, and evaluation of
Pronto, a middleware system for mobile applications with messaging
as a basis. It provides a solution for mobile application specific
problems such as resource constraints, network characteristics, and
data optimization. Pronto consists of three main functions: 1)
MobileJMS Client, a lightweight client of Message Oriented
Middleware (MOM) based on Java Message Service (JMS), 2) Gateway for
reliable and efficient transmission between mobile devices and a
server with pluggable components, and 3) Serverless JMS based on IP
multicast. The publish-subscribe paradigm is ideal for mobile
applications, as mobile devices are commonly used for data
collection under conditions of frequent disconnection and changing
numbers of recipients. This paradigm provides greater flexibility
due to the decoupling of publisher and subscriber. Adding a gateway
as a message hub to transmit information in real-time or with
store-and-forward messaging provides powerful optimization and data
transformation. Caching is an essential function of the gateway, and
SmartCaching is designed for generic caching in an N-tier
architecture. Serverless JMS aims at a decentralized messaging
model, which supports an ad-hoc network, as well as creating a
high-speed messaging BUS. Pronto is an intelligent MobileGateway,
providing a useful MOM intermediary between a server and mobile
devices over a wireless network.
Decimalisation table attacks for PIN cracking
Bond, Mike
Zieliński, Piotr
University of Cambridge, Computer Laboratory
2003-02
en
Text
UCAM-CL-TR-560
ISSN 1476-2986
We present an attack on hardware security modules used by retail
banks for the secure storage and verification of customer PINs in
ATM (cash machine) infrastructures. By using adaptive decimalisation
tables and guesses, the maximum amount of information is learnt
about the true PIN upon each guess. It takes an average of 15
guesses to determine a four digit PIN using this technique, instead
of the 5000 guesses intended. In a single 30 minute lunch-break, an
attacker can thus discover approximately 7000 PINs rather than 24
with the brute force method. With a £300 withdrawal limit per card,
the potential bounty is raised from £7200 to £2.1 million and a
single motivated attacker could withdraw £30–50 thousand of this
each day. This attack thus presents a serious threat to bank
security.
Resource control of untrusted code in an open network
environment
Menage, Paul B.
University of Cambridge, Computer Laboratory
2003-03
en
Text
UCAM-CL-TR-561
ISSN 1476-2986
Current research into Active Networks, Open Signalling and other
forms of mobile code have made use of the ability to execute
user-supplied code at locations within the network infrastructure,
in order to avoid the inherent latency associated with wide area
networks or to avoid sending excessive amounts of data across
bottleneck links or nodes. Existing research has addressed the
design and evaluation of programming environments, and testbeds have
been implemented on traditional operating systems. Such work has
deferred issues regarding resource control; this has been
reasonable, since this research has been conducted in a closed
environment.
In an open environment, which is required for widespread deployment
of such technologies, the code supplied to the network nodes may not
be from a trusted source. Thus, it cannot be assumed that such code
will behave non-maliciously, nor that it will avoid consuming more
than its fair share of the available system resources.
The computing resources consumed by end-users on programmable nodes
within a network are not free, and must ultimately be paid for in
some way. Programmable networks allow users substantially greater
complexity in the way that they may consume network resources. This
dissertation argues that, due to this complexity, it is essential to
be able control and account for the resources used by untrusted
user-supplied code if such technology is to be deployed effectively
in a wide-area open environment.
The Resource Controlled Active Node Environment (RCANE) is presented
to facilitate the control of untrusted code. RCANE supports the
allocation, scheduling and accounting of the resources available on
a node, including CPU and network I/O scheduling, memory allocation,
and garbage collection overhead.
Fast Marching farthest point sampling
Moenning, Carsten
Dodgson, Neil A.
University of Cambridge, Computer Laboratory
2003-04
en
Text
UCAM-CL-TR-562
ISSN 1476-2986
Using Fast Marching for the incremental computation of distance maps
across the sampling domain, we obtain an efficient farthest point
sampling technique (FastFPS). The method is based on that of Eldar
et al. (1992, 1997) but extends more naturally to the case of
non-uniform sampling and is more widely applicable. Furthermore, it
can be applied to both planar domains and curved manifolds and
allows for weighted domains in which different cost is associated
with different points on the surface. We conclude with considering
the extension of FastFPS to the sampling of point clouds without the
need for prior surface reconstruction.
MJ: An imperative core calculus for Java and Java with
effects
Bierman, G.M.
Parkinson, M.J.
Pitts, A.M.
University of Cambridge, Computer Laboratory
2003-04
en
Text
UCAM-CL-TR-563
ISSN 1476-2986
In order to study rigorously object-oriented languages such as Java
or C#, a common practice is to define lightweight fragments, or
calculi, which are sufficiently small to facilitate formal proofs of
key properties. However many of the current proposals for calculi
lack important language features. In this paper we propose
Middleweight Java, MJ, as a contender for a minimal imperative core
calculus for Java. Whilst compact, MJ models features such as object
identity, field assignment, constructor methods and block structure.
We define the syntax, type system and operational semantics of MJ,
and give a proof of type safety. In order to demonstrate the
usefulness of MJ to reason about operational features, we consider a
recent proposal of Greenhouse and Boyland to extend Java with an
effects system. This effects system is intended to delimit the scope
of computational effects within a Java program. We define an
extension of MJ with a similar effects system and instrument the
operational semantics. We then prove the correctness of the effects
system; a question left open by Greenhouse and Boyland. We also
consider the question of effect inference for our extended calculus,
detail an algorithm for inferring effects information and give a
proof of correctness.
Access policies for middleware
Lang, Ulrich
University of Cambridge, Computer Laboratory
2003-05
en
Text
UCAM-CL-TR-564
ISSN 1476-2986
This dissertation examines how the architectural layering of
middleware constrains the design of a middleware security
architecture, and analyses the complications that arise from that.
First, we define a precise notion of middleware that includes its
architecture and features. Our definition is based on the Common
Object Request Broker Architecture (CORBA), which is used throughout
this dissertation both as a reference technology and as a basis for
a proof of concept implementation. In several steps, we construct a
security model that fits to the described middleware architecture.
The model facilitates conceptual reasoning about security. The
results of our analysis indicate that the cryptographic identities
available on the lower layers of the security model are only of
limited use for expressing fine-grained security policies, because
they are separated from the application layer entities by the
middleware layer. To express individual application layer entities
in access policies, additional more fine-grained descriptors are
required. To solve this problem for the target side (i.e., the
receiving side of an invocation), we propose an improved middleware
security model that supports individual access policies on a
per-target basis. The model is based on so-called “resource
descriptors”, which are used in addition to cryptographic identities
to describe application layer entities in access policies. To be
useful, descriptors need to fulfil a number of properties, such as
local uniqueness and persistency. Next, we examine the information
available at the middleware layer for its usefulness as resource
descriptors, in particular the interface name and the instance
information inside the object reference. Unfortunately neither
fulfils all required properties. However, it is possible to obtain
resource descriptors on the target side through a mapping process
that links target instance information to an externally provided
descriptor. We describe both the mapping configuration when the
target is instantiated and the mapping process at invocation time. A
proof of concept implementation, which contains a number of
technical improvements over earlier attempts to solve this problem,
shows that this approach is useable in practice, even for complex
architectures, such as CORBA and CORBASec (the security services
specified for CORBA). Finally, we examine the security approaches of
several related middleware technologies that have emerged since the
specification of CORBA and CORBASec, and show the applicability of
the resource descriptor mapping.
Fast Marching farthest point sampling for point clouds and
implicit surfaces
Moenning, Carsten
Dodgson, Neil A.
University of Cambridge, Computer Laboratory
2003-05
en
Text
UCAM-CL-TR-565
ISSN 1476-2986
In a recent paper (Moenning and Dodgson, 2003), the Fast Marching
farthest point sampling strategy (FastFPS) for planar domains and
curved manifolds was introduced. The version of FastFPS for curved
manifolds discussed in the paper deals with surface domains in
triangulated form only. Due to a restriction of the underlying Fast
Marching method, the algorithm further requires the splitting of any
obtuse into acute triangles to ensure the consistency of the Fast
Marching approximation. In this paper, we overcome these
restrictions by using Memoli and Sapiro’s (Memoli and Sapiro, 2001
and 2002) extension of the Fast Marching method to the handling of
implicit surfaces and point clouds. We find that the extended
FastFPS algorithm can be applied to surfaces in implicit or point
cloud form without the loss of the original algorithm’s
computational optimality and without the need for any preprocessing.
Formal verification of probabilistic algorithms
Hurd, Joe
University of Cambridge, Computer Laboratory
2003-05
en
Text
UCAM-CL-TR-566
ISSN 1476-2986
This thesis shows how probabilistic algorithms can be formally
verified using a mechanical theorem prover.
We begin with an extensive foundational development of probability,
creating a higher-order logic formalization of mathematical measure
theory. This allows the definition of the probability space we use
to model a random bit generator, which informally is a stream of
coin-flips, or technically an infinite sequence of IID
Bernoulli(1/2) random variables.
Probabilistic programs are modelled using the state-transformer
monad familiar from functional programming, where the random bit
generator is passed around in the computation. Functions remove
random bits from the generator to perform their calculation, and
then pass back the changed random bit generator with the result.
Our probability space modelling the random bit generator allows us
to give precise probabilistic specifications of such programs, and
then verify them in the theorem prover.
We also develop technical support designed to expedite verification:
probabilistic quantifiers; a compositional property subsuming
measurability and independence; a probabilistic while loop together
with a formal concept of termination with probability 1. We also
introduce a technique for reducing properties of a probabilistic
while loop to properties of programs that are guaranteed to
terminate: these can then be established using induction and
standard methods of program correctness.
We demonstrate the formal framework with some example probabilistic
programs: sampling algorithms for four probability distributions;
some optimal procedures for generating dice rolls from coin flips;
the symmetric simple random walk. In addition, we verify the
Miller-Rabin primality test, a well-known and commercially used
probabilistic algorithm. Our fundamental perspective allows us to
define a version with strong properties, which we can execute in the
logic to prove compositeness of numbers.
Using inequalities as term ordering constraints
Hurd, Joe
University of Cambridge, Computer Laboratory
2003-06
en
Text
UCAM-CL-TR-567
ISSN 1476-2986
In this paper we show how linear inequalities can be used to
approximate Knuth-Bendix term ordering constraints, and how term
operations such as substitution can be carried out on systems of
inequalities. Using this representation allows an off-the-shelf
linear arithmetic decision procedure to check the satisfiability of
a set of ordering constraints. We present a formal description of a
resolution calculus where systems of inequalities are used to
constrain clauses, and implement this using the Omega test as a
satisfiability checker. We give the results of an experiment over
problems in the TPTP archive, comparing the practical performance of
the resolution calculus with and without inherited inequality
constraints.
Dynamic rebinding for marshalling and update, with
destruct-time λ
Bierman, Gavin
Hicks, Michael
Sewell, Peter
Stoyle, Gareth
Wansbrough, Keith
University of Cambridge, Computer Laboratory
2004-02
en
Text
UCAM-CL-TR-568
ISSN 1476-2986
Most programming languages adopt static binding, but for distributed
programming an exclusive reliance on static binding is too
restrictive: dynamic binding is required in various guises, for
example when a marshalled value is received from the network,
containing identifiers that must be rebound to local resources.
Typically it is provided only by ad-hoc mechanisms that lack clean
semantics.
In this paper we adopt a foundational approach, developing core
dynamic rebinding mechanisms as extensions to the simply-typed
call-by-value λ-calculus. To do so we must first explore refinements
of the call-by-value reduction strategy that delay instantiation, to
ensure computations make use of the most recent versions of rebound
definitions. We introduce redex-time and destruct-time strategies.
The latter forms the basis for a λ-marsh calculus that supports
dynamic rebinding of marshalled values, while remaining as far as
possible statically-typed. We sketch an extension of λ-marsh with
concurrency and communication, giving examples showing how wrappers
for encapsulating untrusted code can be expressed. Finally, we show
that a high-level semantics for dynamic updating can also be based
on the destruct-time strategy, defining a λ-update calculus with
simple primitives to provide type-safe updating of running code. We
thereby establish primitives and a common semantic foundation for a
variety of real-world dynamic rebinding requirements.
Global abstraction-safe marshalling with hash
types
Leifer, James J.
Peskine, Gilles
Sewell, Peter
Wansbrough, Keith
University of Cambridge, Computer Laboratory
2003-06
en
Text
UCAM-CL-TR-569
ISSN 1476-2986
Type abstraction is a key feature of ML-like languages for writing
large programs. Marshalling is necessary for writing distributed
programs, exchanging values via network byte-streams or persistent
stores. In this paper we combine the two, developing compile-time
and run-time semantics for marshalling, that guarantee
abstraction-safety between separately-built programs.
We obtain a namespace for abstract types that is global, ie
meaningful between programs, by hashing module declarations. We
examine the scenarios in which values of abstract types are
communicated from one program to another, and ensure, by
constructing hashes appropriately, that the dynamic and static
notions of type equality mirror each other. We use singleton kinds
to express abstraction in the static semantics; abstraction is
tracked in the dynamic semantics by coloured brackets. These allow
us to prove preservation, erasure, and coincidence results. We argue
that our proposal is a good basis for extensions to existing ML-like
languages, pragmatically straightforward for language users and for
implementors.
Bigraphs and mobile processes
Jensen, Ole Høgh
Milner, Robin
University of Cambridge, Computer Laboratory
2003-07
en
Text
UCAM-CL-TR-570
ISSN 1476-2986
A bigraphical reactive system (BRS) involves bigraphs, in which the
nesting of nodes represents locality, independently of the edges
connecting them; it also allows bigraphs to reconfigure themselves.
BRSs aim to provide a uniform way to model spatially distributed
systems that both compute and communicate. In this memorandum we
develop their static and dynamic theory.
In Part I we illustrate bigraphs in action, and show how they
correspond to to process calculi. We then develop the abstract
(non-graphical) notion of wide reactive system (WRS), of which BRSs
are an instance. Starting from reaction rules —often called
rewriting rules— we use the RPO theory of Leifer and Milner to
derive (labelled) transition systems for WRSs, in a way that leads
automatically to behavioural congruences.
In Part II we develop bigraphs and BRSs formally. The theory is
based directly on graphs, not on syntax. Key results in the static
theory are that sufficient RPOs exist (enabling the results of
Part I to be applied), that parallel combinators familiar from
process calculi may be defined, and that a complete algebraic theory
exists at least for pure bigraphs (those without binding). Key
aspects in the dynamic theory —the BRSs— are the definition of
parametric reaction rules that may replicate or discard parameters,
and the full application of the behavioural theory of Part I.
In Part III we introduce a special class: the simple BRSs. These
admit encodings of many process calculi, including the π-calculus
and the ambient calculus. A still narrower class, the basic BRSs,
admits an easy characterisation of our derived transition systems.
We exploit this in a case study for an asynchronous π-calculus. We
show that structural congruence of process terms corresponds to
equality of the representing bigraphs, and that classical strong
bisimilarity corresponds to bisimilarity of bigraphs. At the end, we
explore several directions for further work.
Multi-layer network monitoring and analysis
Hall, James
University of Cambridge, Computer Laboratory
2003-07
en
Text
UCAM-CL-TR-571
ISSN 1476-2986
Passive network monitoring offers the possibility of gathering a
wealth of data about the traffic traversing the network and the
communicating processes generating that traffic. Significant
advantages include the non-intrusive nature of data capture and the
range and diversity of the traffic and driving applications which
may be observed. Conversely there are also associated practical
difficulties which have restricted the usefulness of the technique:
increasing network bandwidths can challenge the capacity of monitors
to keep pace with passing traffic without data loss, and the bulk of
data recorded may become unmanageable.
Much research based upon passive monitoring has in consequence been
limited to that using a sub-set of the data potentially available,
typically TCP/IP packet headers gathered using Tcpdump or similar
monitoring tools. The bulk of data collected is thereby minimised,
and with the possible exception of packet filtering, the monitor’s
available processing power is available for the task of collection
and storage. As the data available for analysis is drawn from only a
small section of the network protocol stack, detailed study is
largely confined to the associated functionality and dynamics in
isolation from activity at other levels. Such lack of context
severely restricts examination of the interaction between protocols
which may in turn lead to inaccurate or erroneous conclusions.
The work described in this report attempts to address some of these
limitations. A new passive monitoring architecture — Nprobe — is
presented, based upon ‘off the shelf’ components and which, by using
clusters of probes, is scalable to keep pace with current high
bandwidth networks without data loss. Monitored packets are fully
captured, but are subject to the minimum processing in real time
needed to identify and associate data of interest across the target
set of protocols. Only this data is extracted and stored. The data
reduction ratio thus achieved allows examination of a wider range of
encapsulated protocols without straining the probe’s storage
capacity.
Full analysis of the data harvested from the network is performed
off-line. The activity of interest within each protocol is examined
and is integrated across the range of protocols, allowing their
interaction to be studied. The activity at higher levels informs
study of the lower levels, and that at lower levels infers detail of
the higher. A technique for dynamically modelling TCP connections is
presented, which, by using data from both the transport and higher
levels of the protocol stack, differentiates between the effects of
network and end-process activity.
The balance of the report presents a study of Web traffic using
Nprobe. Data collected from the IP, TCP, HTTP and HTML levels of the
stack is integrated to identify the patterns of network activity
involved in downloading whole Web pages: by using the links
contained in HTML documents observed by the monitor, together with
data extracted from the HTML headers of downloaded contained
objects, the set of TCP connections used, and the way in which
browsers use them, are studied as a whole. An analysis of the degree
and distribution of delay is presented and contributes to the
understanding of performance as perceived by the user. The effects
of packet loss on whole page download times are examined,
particularly those losses occurring early in the lifetime of
connections before reliable estimations of round trip times are
established. The implications of such early packet losses for pages
downloads using persistent connections are also examined by
simulations using the detailed data available.
Design choices for language-based transactions
Harris, Tim
University of Cambridge, Computer Laboratory
2003-08
en
Text
UCAM-CL-TR-572
ISSN 1476-2986
This report discusses two design choices which arose in our recent
work on introducing a new ‘atomic’ keyword as an extension to the
Java programming language. We discuss the extent to which programs
using atomic blocks should be provided with an explicit ‘abort’
operation to roll-back the effects of the current block. We also
discuss mechanisms for supporting blocks that perform I/O operations
or external database transactions.
Mechanizing compositional reasoning for concurrent systems:
some lessons
Paulson, Lawrence C.
University of Cambridge, Computer Laboratory
2003-08
en
Text
UCAM-CL-TR-573
ISSN 1476-2986
The paper reports on experiences of mechanizing various proposals
for compositional reasoning in concurrent systems. The work uses the
UNITY formalism and the Isabelle proof tool. The proposals
investigated include existential/universal properties, guarantees
properties and progress sets. The paper mentions some alternative
proposals that are also worth of investigation. The conclusions are
that many of these methods work and are suitable candidates for
further development.
Sketchpad: A man-machine graphical communication
system
Sutherland, Ivan Edward
University of Cambridge, Computer Laboratory
Blackwell, Alan
Rodden, Kerry
2003-09
en
Text
UCAM-CL-TR-574
ISSN 1476-2986
The Sketchpad system uses drawing as a novel communication medium
for a computer. The system contains input, output, and computation
programs which enable it to interpret information drawn directly on
a computer display. It has been used to draw electrical, mechanical,
scientific, mathematical, and animated drawings; it is a general
purpose system. Sketchpad has shown the most usefulness as an aid to
the understanding of processes, such as the notion of linkages,
which can be described with pictures. Sketchpad also makes it easy
to draw highly repetitive or highly accurate drawings and to change
drawings previously drawn with it. The many drawings in this thesis
were all made with Sketchpad.
A Sketchpad user sketches directly on a computer display with a
“light pen.” The light pen is used both to position parts of the
drawing on the display and to point to them to change them. A set of
push buttons controls the changes to be made such as “erase,” or
“move.” Except for legends, no written language is used.
Information sketched can include straight line segments and circle
arcs. Arbitrary symbols may be defined from any collection of line
segments, circle arcs, and previously defined symbols. A user may
define and use as many symbols as he wishes. Any change in the
definition of a symbol is at once seen wherever that symbol appears.
Sketchpad stores explicit information about the topology of a
drawing. If the user moves one vertex of a polygon, both adjacent
sides will be moved. If the user moves a symbol, all lines attached
to that symbol will automatically move to stay attached to it. The
topological connections of the drawing are automatically indicated
by the user as he sketches. Since Sketchpad is able to accept
topological information from a human being in a picture language
perfectly natural to the human, it can be used as an input program
for computation programs which require topological data, e.g.,
circuit simulators.
Sketchpad itself is able to move parts of the drawing around to meet
new conditions which the user may apply to them. The user indicates
conditions with the light pen and push buttons. For example, to make
two lines parallel, he successively points to the lines with the
light pen and presses a button. The conditions themselves are
displayed on the drawing so that they may be erased or changed with
the light pen language. Any combination of conditions can be defined
as a composite condition and applied in one step.
It is easy to add entirely new types of conditions to Sketchpad’s
vocabulary. Since the conditions can involve anything computable,
Sketchpad can be used for a very wide range of problems. For
example, Sketchpad has been used to find the distribution of forces
in the members of truss bridges drawn with it.
Sketchpad drawings are stored in the computer in a specially
designed “ring” structure. The ring structure features rapid
processing of topological information with no searching at all. The
basic operations used in Sketchpad for manipulating the ring
structure are described.
Reconfigurable wavelength-switched optical networks for the
Internet core
Granger, Tim
University of Cambridge, Computer Laboratory
2003-11
en
Text
UCAM-CL-TR-575
ISSN 1476-2986
With the quantity of data traffic carried on the Internet doubling
each year, there is no let up in the demand for ever increasing
network capacity. Optical fibres have a theoretical capacity of many
tens of terabits per second. Currently six terabits per second has
been achieved using Dense Wavelength Division Multiplexing: multiple
signals at different wavelengths carried on the same fibre.
This large available bandwidth moves the performance bottlenecks to
the processing required at each network node to receive, buffer,
route, and transmit each individual packet. For the last 10 years
the speed of the electronic routers has been, in relative terms,
increasing slower than optical capacity. The space required and
power consumed by these routers is also becoming a significant
limitation.
One solution examined in this dissertation is to create a virtual
topology in the optical layer by using all-optical switches to
create lightpaths across the network. In this way nodes that are not
directly connected can appear to be a single virtual hop away, and
no per-packet processing is required at the intermediate nodes. With
advances in optical switches it is now possible for the network to
reconfigure lightpaths dynamically. This allows the network to share
the resources available between the different traffic streams
flowing across the network, and track changes in traffic volumes by
allocating bandwidth on demand.
This solution is inherently a circuit-switched approach, but taken
into account are characteristics of optical switching, in particular
waveband switching (where we switch a contiguous range of
wavelengths as a single unit) and latency required to achieve non
disruptive switching.
This dissertation quantifies the potential gain from such a system
and how that gain is related to the frequency of reconfiguration. It
outlines possible network architectures which allow reconfiguration
and, through simulation, measures the performance of these
architectures. It then discusses the possible interactions between a
reconfiguring optical layer and higher-level network layers.
This dissertation argues that the optical layer should be distinct
from higher network layers, maintaining stable full-mesh
connectivity, and dynamically reconfiguring the sizes and physical
routes of the virtual paths to take advantage of changing traffic
levels.
An implementation of a coordinate based location
system
Spence, David R.
University of Cambridge, Computer Laboratory
2003-11
en
Text
UCAM-CL-TR-576
ISSN 1476-2986
This paper explains the co-ordinate based location system built for
XenoSearch, a resource discovery system in the XenoServer Open
Platform. The system is builds on the work of GNP, Lighthouse and
many more recent schemes. We also present results from various
combinations of algorithms to perform the actual co-ordinate
calculation based on GNP, Lighthouse and spring based systems and
show our implementations of the various algorithms give similar
prediction errors.
Compromising emanations: eavesdropping risks of computer
displays
Kuhn, Markus G.
University of Cambridge, Computer Laboratory
2003-12
en
Text
UCAM-CL-TR-577
ISSN 1476-2986
Electronic equipment can emit unintentional signals from which
eavesdroppers may reconstruct processed data at some distance. This
has been a concern for military hardware for over half a century.
The civilian computer-security community became aware of the risk
through the work of van Eck in 1985. Military “Tempest” shielding
test standards remain secret and no civilian equivalents are
available at present. The topic is still largely neglected in
security textbooks due to a lack of published experimental data.
This report documents eavesdropping experiments on contemporary
computer displays. It discusses the nature and properties of
compromising emanations for both cathode-ray tube and liquid-crystal
monitors. The detection equipment used matches the capabilities to
be expected from well-funded professional eavesdroppers. All
experiments were carried out in a normal unshielded office
environment. They therefore focus on emanations from display refresh
signals, where periodic averaging can be used to obtain reproducible
results in spite of varying environmental noise.
Additional experiments described in this report demonstrate how to
make information emitted via the video signal more easily
receivable, how to recover plaintext from emanations via
radio-character recognition, how to estimate remotely precise
video-timing parameters, and how to protect displayed text from
radio-frequency eavesdroppers by using specialized screen drivers
with a carefully selected video card. Furthermore, a proposal for a
civilian radio-frequency emission-security standard is outlined,
based on path-loss estimates and published data about radio noise
levels.
Finally, a new optical eavesdropping technique is demonstrated that
reads CRT displays at a distance. It observes high-frequency
variations of the light emitted, even after diffuse reflection.
Experiments with a typical monitor show that enough video signal
remains in the light to permit the reconstruction of readable text
from signals detected with a fast photosensor. Shot-noise
calculations provide an upper bound for this risk.
Linear types for packet processing (extended
version)
Ennals, Robert
Sharp, Richard
Mycroft, Alan
University of Cambridge, Computer Laboratory
2004-01
en
Text
UCAM-CL-TR-578
ISSN 1476-2986
We present PacLang: an imperative, concurrent, linearly-typed
language designed for expressing packet processing applications.
PacLang’s linear type system ensures that no packet is referenced by
more than one thread, but allows multiple references to a packet
within a thread. We argue (i) that this property greatly simplifies
compilation of high-level programs to the distributed memory
architectures of modern Network Processors; and (ii) that PacLang’s
type system captures that style in which imperative packet
processing programs are already written. Claim (ii) is justified by
means of a case-study: we describe a PacLang implementation of the
IPv4 unicast packet forwarding algorithm.
PacLang is formalised by means of an operational semantics and a
Unique Ownership theorem formalises its correctness with respect to
the type system.
Practical lock-freedom
Fraser, Keir
University of Cambridge, Computer Laboratory
2004-02
en
Text
UCAM-CL-TR-579
ISSN 1476-2986
Mutual-exclusion locks are currently the most popular mechanism for
interprocess synchronisation, largely due to their apparent
simplicity and ease of implementation. In the parallel-computing
environments that are increasingly commonplace in high-performance
applications, this simplicity is deceptive: mutual exclusion does
not scale well with large numbers of locks and many concurrent
threads of execution. Highly-concurrent access to shared data
demands a sophisticated ‘fine-grained’ locking strategy to avoid
serialising non-conflicting operations. Such strategies are hard to
design correctly and with good performance because they can harbour
problems such as deadlock, priority inversion and convoying. Lock
manipulations may also degrade the performance of cache-coherent
multiprocessor systems by causing coherency conflicts and increased
interconnect traffic, even when the lock protects read-only data.
In looking for solutions to these problems, interest has developed
in lock-free data structures. By eschewing mutual exclusion it is
hoped that more efficient and robust systems can be built.
Unfortunately the current reality is that most lock-free algorithms
are complex, slow and impractical. In this dissertation I address
these concerns by introducing and evaluating practical abstractions
and data structures that facilitate the development of large-scale
lock-free systems.
Firstly, I present an implementation of two useful abstractions that
make it easier to develop arbitrary lock-free data structures.
Although these abstractions have been described in previous work, my
designs are the first that can be practically implemented on current
multiprocessor systems.
Secondly, I present a suite of novel lock-free search structures.
This is interesting not only because of the fundamental importance
of searching in computer science and its wide use in real systems,
but also because it demonstrates the implementation issues that
arise when using the practical abstractions I have developed.
Finally, I evaluate each of my designs and compare them with
existing lock-based and lock-free alternatives. To ensure the
strongest possible competition, several of the lock-based
alternatives are significant improvements on the best-known
solutions in the literature. These results demonstrate that it is
possible to build useful data structures with all the perceived
benefits of lock-freedom and with performance better than
sophisticated lock-based designs. Furthermore, and contrary to
popular belief, this work shows that existing hardware primitives
are sufficient to build practical lock-free implementations of
complex data structures.
Bigraphs and mobile processes (revised)
Jensen, Ole Høgh
Milner, Robin
University of Cambridge, Computer Laboratory
2004-02
en
Text
UCAM-CL-TR-580
ISSN 1476-2986
A bigraphical reactive system (BRS) involves bigraphs, in which the
nesting of nodes represents locality, independently of the edges
connecting them; it also allows bigraphs to reconfigure themselves.
BRSs aim to provide a uniform way to model spatially distributed
systems that both compute and communicate. In this memorandum we
develop their static and dynamic theory.
In Part I we illustrate bigraphs in action, and show how they
correspond to to process calculi. We then develop the abstract
(non-graphical) notion of wide reactive system (WRS), of which BRSs
are an instance. Starting from reaction rules —often called
rewriting rules— we use the RPO theory of Leifer and Milner to
derive (labelled) transition systems for WRSs, in a way that leads
automatically to behavioural congruences.
In Part II we develop bigraphs and BRSs formally. The theory is
based directly on graphs, not on syntax. Key results in the static
theory are that sufficient RPOs exist (enabling the results of
Part I to be applied), that parallel combinators familiar from
process calculi may be defined, and that a complete algebraic theory
exists at least for pure bigraphs (those without binding). Key
aspects in the dynamic theory —the BRSs— are the definition of
parametric reaction rules that may replicate or discard parameters,
and the full application of the behavioural theory of Part I.
In Part III we introduce a special class: the simple BRSs. These
admit encodings of many process calculi, including the π-calculus
and the ambient calculus. A still narrower class, the basic BRSs,
admits an easy characterisation of our derived transition systems.
We exploit this in a case study for an asynchronous π-calculus. We
show that structural congruence of process terms corresponds to
equality of the representing bigraphs, and that classical strong
bisimilarity corresponds to bisimilarity of bigraphs. At the end, we
explore several directions for further work.
Axioms for bigraphical structure
Milner, Robin
University of Cambridge, Computer Laboratory
2004-02
en
Text
UCAM-CL-TR-581
ISSN 1476-2986
This paper axiomatises the structure of bigraphs, and proves that
the resulting theory is complete. Bigraphs are graphs with double
structure, representing locality and connectivity. They have been
shown to represent dynamic theories for the π-calculus, mobile
ambients and Petri nets, in a way that is faithful to each of those
models of discrete behaviour. While the main purpose of bigraphs is
to understand mobile systems, a prerequisite for this understanding
is a well-behaved theory of the structure of states in such systems.
The algebra of bigraph structure is surprisingly simple, as the
paper demonstrates; this is because bigraphs treat locality and
connectivity orthogonally.
Latency-optimal Uniform Atomic Broadcast
algorithm
Zieliński, Piotr
University of Cambridge, Computer Laboratory
2004-02
en
Text
UCAM-CL-TR-582
ISSN 1476-2986
We present a new asynchronous Uniform Atomic Broadcast algorithm
with a delivery latency of two communication steps in optimistic
settings, which is faster than any other known algorithm and has
been shown to be the lower bound. It also has the weakest possible
liveness requirements (the Ω failure detector and a majority of
correct processes) and achieves three new lower bounds presented in
this paper. Finally, we introduce a new notation and several new
abstractions, which are used to construct and present the algorithm
in a clear and modular way.
Subdivision as a sequence of sampled Cp surfaces and
conditions for tuning schemes
Gérot, Cédric
Barthe, Loïc
Dodgson, Neil A.
Sabin, Malcolm A.
University of Cambridge, Computer Laboratory
2004-03
en
Text
UCAM-CL-TR-583
ISSN 1476-2986
We deal with practical conditions for tuning a subdivision scheme in
order to control its artifacts in the vicinity of a mark point. To
do so, we look for good behaviour of the limit vertices rather than
good mathematical properties of the limit surface. The good
behaviour of the limit vertices is characterised with the definition
of C2-convergence of a scheme. We propose necessary explicit
conditions for C2-convergence of a scheme in the vicinity of any
mark point being a vertex of valency n or the centre of an n-sided
face with n greater or equal to three. These necessary conditions
concern the eigenvalues and eigenvectors of subdivision matrices in
the frequency domain. The components of these matrices may be
complex. If we could guarantee that they were real, this would
simplify numerical analysis of the eigenstructure of the matrices,
especially in the context of scheme tuning where we manipulate
symbolic terms. In this paper we show that an appropriate choice of
the parameter space combined with a substitution of vertices lets us
transform these matrices into pure real ones. The substitution
consists in replacing some vertices by linear combinations of
themselves. Finally, we explain how to derive conditions on the
eigenelements of the real matrices which are necessary for the
C2-convergence of the scheme.
Concise texture editing
Brooks, Stephen
University of Cambridge, Computer Laboratory
2004-03
en
Text
UCAM-CL-TR-584
ISSN 1476-2986
Many computer graphics applications remain in the domain of the
specialist. They are typically characterized by complex
user-directed tasks, often requiring proficiency in design, colour
spaces, computer interaction and file management. Furthermore, the
demands of this skill set are often exacerbated by an equally
complex collection of image or object manipulation commands embedded
in a variety of interface components. The complexity of these
graphic editing tools often requires that the user possess a
correspondingly high level of expertise.
Concise Texture Editing is aimed at addressing the over-complexity
of modern graphics tools and is based on the intuitive notion that
the human user is skilled at high level decision making while the
computer is proficient at rapid computation. This thesis has focused
on the development of interactive editing tools for 2D texture
images and has led to the development of a novel texture
manipulation system that allows:
– the concise painting of a texture;
– the concise cloning of textures;
– the concise alteration of texture element size.
The system allows complex operations to be performed on images with
minimal user interaction. When applied to the domain of image
editing, this implies that the user can instruct the system to
perform complex changes to digital images without having to specify
copious amounts of detail. In order to reduce the user’s workload,
the inherent self-similarity of textures is exploited to
interactively replicate editing operations globally over an image.
This unique image system thereby reduces the user’s workload through
semi-automation, resulting in an acutely concise user interface.
Personal projected displays
Ashdown, Mark S. D.
University of Cambridge, Computer Laboratory
2004-03
en
Text
UCAM-CL-TR-585
ISSN 1476-2986
Since the inception of the personal computer, the interface
presented to users has been defined by the monitor screen, keyboard,
and mouse, and by the framework of the desktop metaphor. It is very
different from a physical desktop which has a large horizontal
surface, allows paper documents to be arranged, browsed, and
annotated, and is controlled via continuous movements with both
hands. The desktop metaphor will not scale to such a large display;
the continuing profusion of paper, which is used as much as ever,
attests to its unsurpassed affordances as a medium for manipulating
documents; and despite its proven manual and cognitive benefits,
two-handed input is still not used in computer interfaces.
I present a system called the Escritoire that uses a novel
configuration of overlapping projectors to create a large desk
display that fills the area of a conventional desk and also has a
high resolution region in front of the user for precise work. The
projectors need not be positioned exactly—the projected imagery is
warped using standard 3D video hardware to compensate for rough
projector positioning and oblique projection. Calibration involves
computing planar homographies between the 2D co-ordinate spaces of
the warped textures, projector framebuffers, desk, and input
devices.
The video hardware can easily perform the necessary warping and
achieves 30 frames per second for the dual-projector display.
Oblique projection has proved to be a solution to the problem of
occlusion common to front-projection systems. The combination of an
electromagnetic digitizer and an ultrasonic pen allows simultaneous
input with two hands. The pen for the non-dominant hand is simpler
and coarser than that for the dominant hand, reflecting the
differing roles of the hands in bimanual manipulation. I give a new
algorithm for calibrating a pen, that uses piecewise linear
interpolation between control points. I also give an algorithm to
calibrate a wall display at distance using a device whose position
and orientation are tracked in three dimensions.
The Escritoire software is divided into a client that exploits the
video hardware and handles the input devices, and a server that
processes events and stores all of the system state. Multiple
clients can connect to a single server to support collaboration.
Sheets of virtual paper on the Escritoire can be put in piles which
can be browsed and reordered. As with physical paper this allows
items to be arranged quickly and informally, avoiding the premature
work required to add an item to a hierarchical file system. Another
interface feature is pen traces, which allow remote users to gest