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 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 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 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 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 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 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 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 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 imporve 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 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 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 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 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 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 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 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 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 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 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 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 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 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 Modular reasoning in Isabelle Kammüller, Florian University of Cambridge, Computer Laboratory 1999-08 en Text UCAM-CL-TR-470 ISSN 1476-2986 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 Computer algebra and theorem proving Ballarin, Clemens University of Cambridge, Computer Laboratory 1999-10 en Text UCAM-CL-TR-473 ISSN 1476-2986 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 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 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 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 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 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 gesture to each other. I report the results of tests with individuals and with pairs collaborating remotely. Collaborating participants found an audio channel and the shared desk surface much more useful than a video channel showing their faces. The Escritoire is constructed from commodity components, and unlike multi-projector display walls its cost is feasible for an individual user and it fits into a normal office setting. It demonstrates a hardware configuration, calibration algorithm, graphics warping process, set of interface features, and distributed architecture that can make personal projected displays a reality. Role-based access control policy administration Belokosztolszki, András University of Cambridge, Computer Laboratory 2004-03 en Text UCAM-CL-TR-586 ISSN 1476-2986 The wide proliferation of the Internet has set new requirements for access control policy specification. Due to the demand for ad-hoc cooperation between organisations, applications are no longer isolated from each other; consequently, access control policies face a large, heterogeneous, and dynamic environment. Policies, while maintaining their main functionality, go through many minor adaptations, evolving as the environment changes. In this thesis we investigate the long-term administration of role-based access control (RBAC) – in particular OASIS RBAC – policies. With the aim of encapsulating persistent goals of policies we introduce extensions in the form of meta-policies. These meta-policies, whose expected lifetime is longer than the lifetime of individual policies, contain extra information and restrictions about policies. It is expected that successive policy versions are checked at policy specification time to ensure that they comply with the requirements and guidelines set by meta-policies. In the first of the three classes of meta-policies we group together policy components by annotating them with context labels. Based on this grouping and an information flow relation on context labels, we limit the way in which policy components may be connected to other component groups. We use this to partition conceptually disparate portions of policies, and reference these coherent portions to specify policy restrictions and policy enforcement behaviour. In our second class of meta-policies – compliance policies – we specify requirements on an abstract policy model. We then use this for static policy checking. As compliance tests are performed at policy specification time, compliance policies may include restrictions that either cannot be included in policies, or whose inclusion would result in degraded policy enforcement performance. We also indicate how to use compliance policies to provide information about organisational policies without disclosing sensitive information. The final class of our meta-policies, called interface policies, is used to help set up and maintain cooperation among organisations by enabling them to use components from each other’s policies. Being based on compliance policies, they use an abstract policy component model, and can also specify requirements for both component exporters and importers. Using such interface policies we can reconcile compatibility issues between cooperating parties automatically. Finally, building on our meta-policies, we consider policy evolution and self-administration, according to which we treat RBAC policies as distributed resources to which access is specified with the help of RBAC itself. This enables environments where policies are maintained by many administrators who have varying levels of competence, trust, and jurisdiction. We have tested all of these concepts in Desert, our proof of concept implementation. Verification of asynchronous circuits Cunningham, Paul Alexander University of Cambridge, Computer Laboratory 2004-04 en Text UCAM-CL-TR-587 ISSN 1476-2986 The purpose of this thesis is to introduce proposition-oriented behaviours and apply them to the verification of asynchronous circuits. The major contribution of proposition-oriented behaviours is their ability to extend existing formal notations to permit the explicit use of both signal levels and transitions. This thesis begins with the formalisation of proposition-oriented behaviours in the context of gate networks, and with the set-theoretic extension of both regular-expressions and trace-expressions to reason over proposition-oriented behaviours. A new trace-expression construct, referred to as biased composition, is also introduced. Algorithmic realisation of these set-theoretic extensions is documented using a special form of finite automata called proposition automata. A verification procedure for conformance of gate networks to a set of proposition automata is described in which each proposition automaton may be viewed either as a constraint or a specification. The implementation of this procedure as an automated verification program called Veraci is summarised, and a number of example Veraci programs are used to demonstrate contributions of proposition-oriented behaviour to asynchronous circuit design. These contributions include level-event unification, event abstraction, and relative timing assumptions using biased composition. The performance of Veraci is also compared to an existing event-oriented verification program called Versify, the result of this comparison being a consistent performance gain using Veraci over Versify. This thesis concludes with the design and implementation of a 2048 bit dual-rail asynchronous Montgomery exponentiator, MOD_EXP, in a 0.18µm standard-cell process. The application of Veraci to the design of MOD_EXP is summarised, and the practical benefits of proposition-oriented verification are discussed. MulTEP: A MultiThreaded Embedded Processor Watcharawitch, Panit University of Cambridge, Computer Laboratory 2004-05 en Text UCAM-CL-TR-588 ISSN 1476-2986 Conventional embedded microprocessors have traditionally followed the footsteps of high-end processor design to achieve high performance. Their underlying architectures prioritise tasks by time-critical interrupts and rely on software to perform scheduling tasks. Single threaded execution relies on instruction-based probabilistic techniques, such as speculative execution and branch prediction, which are unsuitable for embedded systems when real-time performance guarantees need to be met. Multithreading appears to be a feasible solution for embedded processors. Thread-level parallelism has a potential to overcome the limitations of insufficient instruction-level parallelism to hide the increasing memory latencies. MulTEP is designed to provide high performance thread-level parallelism, real-time characteristics, a flexible number of threads and low incremental cost per thread for the embedded system. In its architecture, a matching-store synchronisation mechanism allows a thread to wait for multiple data items. A tagged up/down dynamic-priority hardware scheduler is provided for real-time scheduling. Pre-loading, pre-fetching and colour-tagging techniques are implemented to allow context switches without any overhead. The architecture provides four additional multithreading instructions for programmers and advance compilers to create programs with low-overhead multithreaded operations. Experimental results demonstrate that multithreading can be effectively used to improve performance and system utilisation. Latency operations that would otherwise stall the pipeline are hidden by the execution of the other threads. The hardware scheduler provides priority scheduling, which is suitable for real-time embedded applications. new-HOPLA — a higher-order process language with name generation Winskel, Glynn Nardelli, Francesco Zappa University of Cambridge, Computer Laboratory 2004-05 en Text UCAM-CL-TR-589 ISSN 1476-2986 This paper introduces new-HOPLA, a concise but powerful language for higher-order nondeterministic processes with name generation. Its origins as a metalanguage for domain theory are sketched but for the most part the paper concentrates on its operational semantics. The language is typed, the type of a process describing the shape of the computation paths it can perform. Its transition semantics, bisimulation, congruence properties and expressive power are explored. Encodings of π-calculus and HOπ are presented. Hermes: A scalable event-based middleware Pietzuch, Peter R. University of Cambridge, Computer Laboratory 2004-06 en Text UCAM-CL-TR-590 ISSN 1476-2986 Large-scale distributed systems require new middleware paradigms that do not suffer from the limitations of traditional request/reply middleware. These limitations include tight coupling between components, a lack of information filtering capabilities, and support for one-to-one communication semantics only. We argue that event-based middleware is a scalable and powerful new type of middleware for building large-scale distributed systems. However, it is important that an event-based middleware platform includes all the standard functionality that an application programmer expects from middleware. In this thesis we describe the design and implementation of Hermes, a distributed, event-based middleware platform. The power and flexibility of Hermes is illustrated throughout for two application domains: Internet-wide news distribution and a sensor-rich, active building. Hermes follows a type- and attribute-based publish/subscribe model that places particular emphasis on programming language integration by supporting type-checking of event data and event type inheritance. To handle dynamic, large-scale environments, Hermes uses peer-to-peer techniques for autonomic management of its overlay network of event brokers and for scalable event dissemination. Its routing algorithms, implemented on top of a distributed hash table, use rendezvous nodes to reduce routing state in the system, and include fault-tolerance features for repairing event dissemination trees. All this is achieved without compromising scalability and efficiency, as is shown by a simulational evaluation of Hermes routing. The core functionality of an event-based middleware is extended with three higher-level middleware services that address different requirements in a distributed computing environment. We introduce a novel congestion control service that avoids congestion in the overlay broker network during normal operation and recovery after failure, and therefore enables a resource-efficient deployment of the middleware. The expressiveness of subscriptions in the event-based middleware is enhanced with a composite event service that performs the distributed detection of complex event patterns, thus taking the burden away from clients. Finally, a security service adds access control to Hermes according to a secure publish/subscribe model. This model supports fine-grained access control decisions so that separate trust domains can share the same overlay broker network. Conversion of notations Brown, Silas S. University of Cambridge, Computer Laboratory 2004-06 en Text UCAM-CL-TR-591 ISSN 1476-2986 Music, engineering, mathematics, and many other disciplines have established notations for writing their documents. The effectiveness of each of these notations can be hampered by the circumstances in which it is being used, or by a user’s disability or cultural background. Adjusting the notation can help, but the requirements of different cases often conflict, meaning that each new document will have to be transformed between many versions. Tools that support the programming of such transformations can also assist by allowing the creation of new notations on demand, which is an under-explored option in the relief of educational difficulties. This thesis reviews some programming tools that can be used to manipulate the tree-like structure of a notation in order to transform it into another. It then describes a system “4DML” that allows the programmer to create a “model” of the desired result, from which the transformation is derived. This is achieved by representing the structure in a geometric space with many dimensions, where the model acts as an alternative frame of reference. Example applications of 4DML include the transcription of songs and musical scores into various notations, the production of specially-customised notations to assist a sight-impaired person in learning Chinese, an unusual way of re-organising personal notes, a “website scraping” system for extracting data from on-line services that provide only one presentation, and an aid to making mathematics and diagrams accessible to people with severe print disabilities. The benefits and drawbacks of the 4DML approach are evaluated, and possible directions for future work are explored. Unwrapping the Chrysalis Bond, Mike Cvrček, Daniel Murdoch, Steven J. University of Cambridge, Computer Laboratory 2004-06 en Text UCAM-CL-TR-592 ISSN 1476-2986 We describe our experiences reverse engineering the Chrysalis-ITS Luna CA³ – a PKCS#11 compliant cryptographic token. Emissions analysis and security API attacks are viewed by many to be simpler and more efficient than a direct attack on an HSM. But how difficult is it to actually “go in the front door”? We describe how we unpicked the CA³ internal architecture and abused its low-level API to impersonate a CA³ token in its cloning protocol – and extract PKCS#11 private keys in the clear. We quantify the effort involved in developing and applying the skills necessary for such a reverse-engineering attack. In the process, we discover that the Luna CA³ has far more undocumented code and functionality than is revealed to the end-user. Paxos at war Zieliński, Piotr University of Cambridge, Computer Laboratory 2004-06 en Text UCAM-CL-TR-593 ISSN 1476-2986 The optimistic latency of Byzantine Paxos can be reduced from three communication steps to two, without using public-key cryptography. This is done by making a decision when more than (n+3f)/2 acceptors report to have received the same proposal from the leader, with n being the total number of acceptors and f the number of the faulty ones. No further improvement in latency is possible, because every Consensus algorithm must take at least two steps even in benign settings. Moreover, if the leader is correct, our protocol achieves the latency of at most three steps, even if some other processes fail. These two properties make this the fastest Byzantine agreement protocol proposed so far. By running many instances of this algorithm in parallel, we can implement Vector Consensus and Byzantine Atomic Broadcast in two and three steps, respectively, which is two steps faster than any other known algorithm. Designing and attacking anonymous communication systems Danezis, George University of Cambridge, Computer Laboratory 2004-07 en Text UCAM-CL-TR-594 ISSN 1476-2986 This report contributes to the field of anonymous communications over widely deployed communication networks. It describes novel schemes to protect anonymity; it also presents powerful new attacks and new ways of analysing and understanding anonymity properties. We present Mixminion, a new generation anonymous remailer, and examine its security against all known passive and active cryptographic attacks. We use the secure anonymous replies it provides, to describe a pseudonym server, as an example of the anonymous protocols that mixminion can support. The security of mix systems is then assessed against a compulsion threat model, in which an adversary can request the decryption of material from honest nodes. A new construction, the fs-mix, is presented that makes tracing messages by such an adversary extremely expensive. Moving beyond the static security of anonymous communication protocols, we define a metric based on information theory that can be used to measure anonymity. The analysis of the pool mix serves as an example of its use. We then create a framework within which we compare the traffic analysis resistance provided by different mix network topologies. A new topology, based on expander graphs, proves to be efficient and secure. The rgb-mix is also presented; this implements a strategy to detect flooding attacks against honest mix nodes and neutralise them by the use of cover traffic. Finally a set of generic attacks are studied. Statistical disclosure attacks model the whole anonymous system as a black box, and are able to uncover the relationships between long-term correspondents. Stream attacks trace streams of data travelling through anonymizing networks, and uncover the communicating parties very quickly. They both use statistical methods to drastically reduce the anonymity of users. Other minor attacks are described against peer discovery and route reconstruction in anonymous networks, as well as the naïve use of anonymous replies. Representations of quantum operations, with applications to quantum cryptography Arrighi, Pablo J. University of Cambridge, Computer Laboratory 2004-07 en Text UCAM-CL-TR-595 ISSN 1476-2986 Representations of quantum operations – We start by introducing a geometrical representation (real vector space) of quantum states and quantum operations. To do so we exploit an isomorphism from positive matrices to a subcone of the Minkowski future light-cone. Pure states map onto certain light-like vectors, whilst the axis of revolution encodes the overall probability of occurrence for the state. This extension of the Generalized Bloch Sphere enables us to cater for non-trace-preserving quantum operations, and in particular to view the per-outcome effects of generalized measurements. We show that these consist of the product of an orthogonal transform about the axis of the cone of revolution and a positive real symmetric linear transform. In the case of a qubit the representation becomes all the more interesting since it elegantly associates, to each measurement element of a generalized measurement, a Lorentz transformation in Minkowski space. We formalize explicitly this correspondence between ‘observation of a quantum system’ and ‘special relativistic change of inertial frame’. To end this part we review the state-operator correspondence, which was successfully exploited by Choi to derive the operator-sum representation of quantum operations. We go further and show that all of the important theorems concerning quantum operations can in fact be derived as simple corollaries of those concerning quantum states. Using this methodology we derive novel composition laws upon quantum states and quantum operations, Schmidt-type decompositions for bipartite pure states and some powerful formulae relating to the correspondence. Quantum cryptography – The key principle of quantum cryptography could be summarized as follows. Honest parties communicate using quantum states. To the eavesdropper these states are random and non-orthogonal. In order to gather information she must measure them, but this may cause irreversible damage. Honest parties seek to detect her mischief by checking whether certain quantum states are left intact. Thus tradeoff between the eavesdropper’s information gain, and the disturbance she necessarily induces, can be viewed as the power engine behind quantum cryptographic protocols. We begin by quantifying this tradeoff in the case of a measure distinguishing two non-orthogonal equiprobable pure states. A formula for this tradeoff was first obtained by Fuchs and Peres, but we provide a shorter, geometrical derivation (within the framework of the above mentioned conal representation). Next we proceed to analyze the Information gain versus disturbance tradeoff in a scenario where Alice and Bob interleave, at random, pairwise superpositions of two message words within their otherwise classical communications. This work constitutes one of the few results currently available regarding d-level systems quantum cryptography, and seems to provide a good general primitive for building such protocols. The proof crucially relies on the state-operator correspondence formulae derived in the first part, together with some methods by Banaszek. Finally we make use of this analysis to prove the security of a ‘blind quantum computation’ protocol, whereby Alice gets Bob to perform some quantum algorithm for her, but prevents him from learning her input to this quantum algorithm. Reconstructing I/O Fraser, Keir Hand, Steven Neugebauer, Rolf Pratt, Ian Warfield, Andrew Williamson, Mark University of Cambridge, Computer Laboratory 2004-08 en Text UCAM-CL-TR-596 ISSN 1476-2986 We present a next-generation architecture that addresses problems of dependability, maintainability, and manageability of I/O devices and their software drivers on the PC platform. Our architecture resolves both hardware and software issues, exploiting emerging hardware features to improve device safety. Our high-performance implementation, based on the Xen virtual machine monitor, provides an immediate transition opportunity for today’s systems. Syntactic simplification and text cohesion Siddharthan, Advaith University of Cambridge, Computer Laboratory 2004-08 en Text UCAM-CL-TR-597 ISSN 1476-2986 Syntactic simplification is the process of reducing the grammatical complexity of a text, while retaining its information content and meaning. The aim of syntactic simplification is to make text easier to comprehend for human readers, or process by programs. In this thesis, I describe how syntactic simplification can be achieved using shallow robust analysis, a small set of hand-crafted simplification rules and a detailed analysis of the discourse-level aspects of syntactically rewriting text. I offer a treatment of relative clauses, apposition, coordination and subordination. I present novel techniques for relative clause and appositive attachment. I argue that these attachment decisions are not purely syntactic. My approaches rely on a shallow discourse model and on animacy information obtained from a lexical knowledge base. I also show how clause and appositive boundaries can be determined reliably using a decision procedure based on local context, represented by part-of-speech tags and noun chunks. I then formalise the interactions that take place between syntax and discourse during the simplification process. This is important because the usefulness of syntactic simplification in making a text accessible to a wider audience can be undermined if the rewritten text lacks cohesion. I describe how various generation issues like sentence ordering, cue-word selection, referring-expression generation, determiner choice and pronominal use can be resolved so as to preserve conjunctive and anaphoric cohesive-relations during syntactic simplification. In order to perform syntactic simplification, I have had to address various natural language processing problems, including clause and appositive identification and attachment, pronoun resolution and referring-expression generation. I evaluate my approaches to solving each problem individually, and also present a holistic evaluation of my syntactic simplification system. Transition systems, link graphs and Petri nets Leifer, James J. Milner, Robin University of Cambridge, Computer Laboratory 2004-08 en Text UCAM-CL-TR-598 ISSN 1476-2986 A framework is defined within which reactive systems can be studied formally. The framework is based upon s-categories, a new variety of categories, within which reactive systems can be set up in such a way that labelled transition systems can be uniformly extracted. These lead in turn to behavioural preorders and equivalences, such as the failures preorder (treated elsewhere) and bisimilarity, which are guaranteed to be congruential. The theory rests upon the notion of relative pushout previously introduced by the authors. The framework is applied to a particular graphical model known as link graphs, which encompasses a variety of calculi for mobile distributed processes. The specific theory of link graphs is developed. It is then applied to an established calculus, namely condition-event Petri nets. In particular, a labelled transition system is derived for condition-event nets, corresponding to a natural notion of observable actions in Petri net theory. The transition system yields a congruential bisimilarity coinciding with one derived directly from the observable actions. This yields a calibration of the general theory of reactive systems and link graphs against known specific theories. Further analysis of ternary and 3-point univariate subdivision schemes Hassan, Mohamed F. University of Cambridge, Computer Laboratory 2004-08 en Text UCAM-CL-TR-599 ISSN 1476-2986 The precision set, approximation order and Hölder exponent are derived for each of the univariate subdivision schemes described in Technical Report UCAM-CL-TR-520. Trust for resource control: Self-enforcing automatic rational contracts between computers Shand, Brian Ninham University of Cambridge, Computer Laboratory 2004-08 en Text UCAM-CL-TR-600 ISSN 1476-2986 Computer systems need to control access to their resources, in order to give precedence to urgent or important tasks. This is increasingly important in networked applications, which need to interact with other machines but may be subject to abuse unless protected from attack. To do this effectively, they need an explicit resource model, and a way to assess others’ actions in terms of it. This dissertation shows how the actions can be represented using resource-based computational contracts, together with a rich trust model which monitors and enforces contract compliance. Related research in the area has focused on individual aspects of this problem, such as resource pricing and auctions, trust modelling and reputation systems, or resource-constrained computing and resource-aware middleware. These need to be integrated into a single model, in order to provide a general framework for computing by contract. This work explores automatic computerized contracts for negotiating and controlling resource usage in a distributed system. Contracts express the terms under which client and server promise to exchange resources, such as processor time in exchange for money, using a constrained language which can be automatically interpreted. A novel, distributed trust model is used to enforce these promises, and this also supports trust delegation through cryptographic certificates. The model is formally proved to have appropriate properties of safety and liveness, which ensure that cheats cannot systematically gain resources by deceit, and that mutually profitable contracts continue to be supported. The contract framework has many applications, in automating distributed services and in limiting the disruptiveness of users’ programs. Applications such as resource-constrained sandboxes, operating system multimedia support and automatic distribution of personal address book entries can all treat the user’s time as a scarce resource, to trade off computational costs against user distraction. Similarly, commercial Grid services can prioritise computations with contracts, while a cooperative service such as distributed composite event detection can use contracts for detector placement and load balancing. Thus the contract framework provides a general purpose tool for managing distributed computation, allowing participants to take calculated risks and rationally choose which contracts to perform. Combining model checking and theorem proving Amjad, Hasan University of Cambridge, Computer Laboratory 2004-09 en Text UCAM-CL-TR-601 ISSN 1476-2986 We implement a model checker for the modal mu-calculus as a derived rule in a fully expansive mechanical theorem prover, without causing an unacceptable performance penalty. We use a restricted form of a higher order logic representation calculus for binary decision diagrams (BDDs) to interface the model checker to a high-performance BDD engine. This is used with a formalised theory of the modal mu-calculus (which we also develop) for model checking in which all steps of the algorithm are justified by fully expansive proof. This provides a fine-grained integration of model checking and theorem proving using a mathematically rigourous interface. The generality of our theories allows us to perform much of the proof offline, in contrast with earlier work. This substantially reduces the inevitable performance penalty of doing model checking by proof. To demonstrate the feasibility of our approach, optimisations to the model checking algorithm are added. We add naive caching and also perform advanced caching for nested non-alternating fixed-point computations. Finally, the usefulness of the work is demonstrated. We leverage our theory by proving translations to simpler logics that are in more widespread use. We then implement an executable theory for counterexample-guided abstraction refinement that also uses a SAT solver. We verify properties of a bus architecture in use in industry as well as a pedagogical arithmetic and logic unit. The benchmarks show an acceptable performance penalty, and the results are correct by construction. Model checking the AMBA protocol in HOL Amjad, Hasan University of Cambridge, Computer Laboratory 2004-09 en Text UCAM-CL-TR-602 ISSN 1476-2986 The Advanced Microcontroller Bus Architecture (AMBA) is an open System-on-Chip bus protocol for high-performance buses on low-power devices. In this report we implement a simple model of AMBA and use model checking and theorem proving to verify latency, arbitration, coherence and deadlock freedom properties of the implementation. Bigraphs whose names have multiple locality Milner, Robin University of Cambridge, Computer Laboratory 2004-09 en Text UCAM-CL-TR-603 ISSN 1476-2986 The previous definition of binding bigraphs is generalised so that local names may be located in more than one region, allowing more succinct and flexible presentation of bigraphical reactive systems. This report defines the generalisation, verifies that it retains relative pushouts, and introduces a new notion of bigraph extension; this admits a wider class of parametric reaction rules. Extension is shown to be well-behaved algebraically; one consequence is that, as in the original definition of bigraphs, discrete parameters are sufficient to generate all reactions. On the anonymity of anonymity systems Serjantov, Andrei University of Cambridge, Computer Laboratory 2004-10 en Text UCAM-CL-TR-604 ISSN 1476-2986 Anonymity on the Internet is a property commonly identified with privacy of electronic communications. A number of different systems exist which claim to provide anonymous email and web browsing, but their effectiveness has hardly been evaluated in practice. In this thesis we focus on the anonymity properties of such systems. First, we show how the anonymity of anonymity systems can be quantified, pointing out flaws with existing metrics and proposing our own. In the process we distinguish the anonymity of a message and that of an anonymity system. Secondly, we focus on the properties of building blocks of mix-based (email) anonymity systems, evaluating their resistance to powerful blending attacks, their delay, their anonymity under normal conditions and other properties. This leads us to methods of computing anonymity for a particular class of mixes – timed mixes – and a new binomial mix. Next, we look at the anonymity of a message going through an entire anonymity system based on a mix network architecture. We construct a semantics of a network with threshold mixes, define the information observable by an attacker, and give a principled definition of the anonymity of a message going through such a network. We then consider low latency connection-based anonymity systems, giving concrete attacks and describing methods of protection against them. In particular, we show that Peer-to-Peer anonymity systems provide less anonymity against the global passive adversary than ones based on a “classic” architecture. Finally, we give an account of how anonymity can be used in censorship resistant systems. These are designed to provide availability of documents, while facing threats from a powerful adversary. We show how anonymity can be used to hide the identity of the servers where each of the documents are stored, thus making them harder to remove from the system. Acute: High-level programming language design for distributed computation : Design rationale and language definition Sewell, Peter Leifer, James J. Wansbrough, Keith Allen-Williams, Mair Nardelli, Francesco Zappa Habouzit, Pierre Vafeiadis, Viktor University of Cambridge, Computer Laboratory 2004-10 en Text UCAM-CL-TR-605 ISSN 1476-2986 This paper studies key issues for distributed programming in high-level languages. We discuss the design space and describe an experimental language, Acute, which we have defined and implemented. Acute extends an OCaml core to support distributed development, deployment, and execution, allowing type-safe interaction between separately-built programs. It is expressive enough to enable a wide variety of distributed infrastructure layers to be written as simple library code above the byte-string network and persistent store APIs, disentangling the language runtime from communication. This requires a synthesis of novel and existing features: (1) type-safe marshalling of values between programs; (2) dynamic loading and controlled rebinding to local resources; (3) modules and abstract types with abstraction boundaries that are respected by interaction; (4) global names, generated either freshly or based on module hashes: at the type level, as runtime names for abstract types; and at the term level, as channel names and other interaction handles; (5) versions and version constraints, integrated with type identity; (6) local concurrency and thread thunkification; and (7) second-order polymorphism with a namecase construct. We deal with the interplay among these features and the core, and develop a semantic definition that tracks abstraction boundaries, global names, and hashes throughout compilation and execution, but which still admits an efficient implementation strategy. Dynamic binary analysis and instrumentation Nethercote, Nicholas University of Cambridge, Computer Laboratory 2004-11 en Text UCAM-CL-TR-606 ISSN 1476-2986 Dynamic binary analysis (DBA) tools such as profilers and checkers help programmers create better software. Dynamic binary instrumentation (DBI) frameworks make it easy to build new DBA tools. This dissertation advances the theory and practice of dynamic binary analysis and instrumentation, with an emphasis on the importance of the use and support of metadata. The dissertation has three main parts. The first part describes a DBI framework called Valgrind which provides novel features to support heavyweight DBA tools that maintain rich metadata, especially location metadata—the shadowing of every register and memory location with a metavalue. Location metadata is used in shadow computation, a kind of DBA where every normal operation is shadowed by an abstract operation. The second part describes three powerful DBA tools. The first tool performs detailed cache profiling. The second tool does an old kind of dynamic analysis—bounds-checking—in a new way. The third tool produces dynamic data flow graphs, a novel visualisation that cuts to the essence of a program’s execution. All three tools were built with Valgrind, and rely on Valgrind’s support for heavyweight DBA and rich metadata, and the latter two perform shadow computation. The third part describes a novel system of semi-formal descriptions of DBA tools. It gives many example descriptions, and also considers in detail exactly what dynamic analysis is. The dissertation makes six main contributions. First, the descriptions show that metadata is the key component of dynamic analysis; in particular, whereas static analysis predicts approximations of a program’s future, dynamic analysis remembers approximations of a program’s past, and these approximations are exactly what metadata is. Second, the example tools show that rich metadata and shadow computation make for powerful and novel DBA tools that do more than the traditional tracing and profiling. Third, Valgrind and the example tools show that a DBI framework can make it easy to build heavyweight DBA tools, by providing good support for rich metadata and shadow computation. Fourth, the descriptions are a precise and concise way of characterising tools, provide a directed way of thinking about tools that can lead to better implementations, and indicate the theoretical upper limit of the power of DBA tools in general. Fifth, the three example tools are interesting in their own right, and the latter two are novel. Finally, the entire dissertation provides many details, and represents a great deal of condensed experience, about implementing DBI frameworks and DBA tools. Code size optimization for embedded processors Johnson, Neil E. University of Cambridge, Computer Laboratory 2004-11 en Text UCAM-CL-TR-607 ISSN 1476-2986 This thesis studies the problem of reducing code size produced by an optimizing compiler. We develop the Value State Dependence Graph (VSDG) as a powerful intermediate form. Nodes represent computation, and edges represent value (data) and state (control) dependencies between nodes. The edges specify a partial ordering of the nodes—sufficient ordering to maintain the I/O semantics of the source program, while allowing optimizers greater freedom to move nodes within the program to achieve better (smaller) code. Optimizations, both classical and new, transform the graph through graph rewriting rules prior to code generation. Additional (semantically inessential) state edges are added to transform the VSDG into a Control Flow Graph, from which target code is generated. We show how procedural abstraction can be advantageously applied to the VSDG. Graph patterns are extracted from a program’s VSDG. We then select repeated patterns giving the greatest size reduction, generate new functions from these patterns, and replace all occurrences of the patterns in the original VSDG with calls to these abstracted functions. Several embedded processors have load- and store-multiple instructions, representing several loads (or stores) as one instruction. We present a method, benefiting from the VSDG form, for using these instructions to reduce code size by provisionally combining loads and stores before code generation. The final contribution of this thesis is a combined register allocation and code motion (RACM) algorithm. We show that our RACM algorithm formulates these two previously antagonistic phases as one combined pass over the VSDG, transforming the graph (moving or cloning nodes, or spilling edges) to fit within the physical resources of the target processor. We have implemented our ideas within a prototype C compiler and suite of VSDG optimizers, generating code for the Thumb 32-bit processor. Our results show improvements for each optimization and that we can achieve code sizes comparable to, and in some cases better than, that produced by commercial compilers with significant investments in optimization technology. Trust management for widely distributed systems Yao, Walt University of Cambridge, Computer Laboratory 2004-11 en Text UCAM-CL-TR-608 ISSN 1476-2986 In recent years, we have witnessed the evolutionary development of a new breed of distributed systems. Systems of this type share a number of characteristics – highly decentralized, of Internet-grade scalability, and autonomous within their administrative domains. Most importantly, they are expected to operate collaboratively across both known and unknown domains. Prime examples include peer-to-peer applications and open web services. Typically, authorization in distributed systems is identity-based, e.g. access control lists. However, approaches based on predefined identities are unsuitable for the new breed of distributed systems because of the need to deal with unknown users, i.e. strangers, and the need to manage a potentially large number of users and/or resources. Furthermore, effective administration and management of authorization in such systems requires: (1) natural mapping of organizational policies into security policies; (2) managing collaboration of independently administered domains/organizations; (3) decentralization of security policies and policy enforcement. This thesis describes Fidelis, a trust management framework designed to address the authorization needs for the next-generation distributed systems. A trust management system is a term coined to refer to a unified framework for the specification of security policies, the representation of credentials, and the evaluation and enforcement of policy compliances. Based on the concept of trust conveyance and a generic abstraction for trusted information as trust statements, Fidelis provides a generic platform for building secure, trust-aware distributed applications. At the heart of the Fidelis framework is a language for the specification of security policies, the Fidelis Policy Language (FPL), and the inference model for evaluating policies expressed in FPL. With the policy language and its inference model, Fidelis is able to model recommendation-style policies and policies with arbitrarily complex chains of trust propagation. Web services have rapidly been gaining significance both in industry and research as a ubiquitous, next-generation middleware platform. The second half of the thesis describes the design and implementation of the Fidelis framework for the standard web service platform. The goal of this work is twofold: first, to demonstrate the practical feasibility of Fidelis, and second, to investigate the use of a policy-driven trust management framework for Internet-scale open systems. An important requirement in such systems is trust negotiation that allows unfamiliar principals to establish mutual trust and interact with confidence. Addressing this requirement, a trust negotiation framework built on top of Fidelis is developed. This thesis examines the application of Fidelis in three distinctive domains: implementing generic role-based access control, trust management in the World Wide Web, and an electronic marketplace comprising unfamiliar and untrusted but collaborative organizations. Using camera-phones to interact with context-aware mobile services Toye, Eleanor Madhavapeddy, Anil Sharp, Richard Scott, David Blackwell, Alan Upton, Eben University of Cambridge, Computer Laboratory 2004-12 en Text UCAM-CL-TR-609 ISSN 1476-2986 We describe an interaction technique for controlling site-specific mobile services using commercially available camera-phones, public information displays and visual tags. We report results from an experimental study validating this technique in terms of pointing speed and accuracy. Our results show that even novices can use camera-phones to “point-and-click” on visual tags quickly and accurately. We have built a publicly available client/server software framework for rapid development of applications that exploit our interaction technique. We describe two prototype applications that were implemented using this framework and present findings from user-experience studies based on these applications. Influence of syntax on prosodic boundary prediction Ingulfsen, Tommy University of Cambridge, Computer Laboratory 2004-12 en Text UCAM-CL-TR-610 ISSN 1476-2986 In this thesis we compare the effectiveness of different syntactic features and syntactic representations for prosodic boundary prediction, setting out to clarify which representations are most suitable for this task. The results of a series of experiments show that it is not possible to conclude that a single representation is superior to all others. Three representations give rise to similar experimental results. One of these representations is composed only of shallow features, which were originally thought to have less predictive power than deep features. Conversely, one of the deep representations that seemed to be best suited for our purposes (syntactic chunks) turns out not to be among the three best. An heuristic analysis of the classification of bivariate subdivision schemes Dodgson, Neil A. University of Cambridge, Computer Laboratory 2004-12 en Text UCAM-CL-TR-611 ISSN 1476-2986 Alexa [*] and Ivrissimtzis et al. [+] have proposed a classification mechanism for bivariate subdivision schemes. Alexa considers triangular primal schemes, Ivrissimtzis et al. generalise this both to quadrilateral and hexagonal meshes and to dual and mixed schemes. I summarise this classification and then proceed to analyse it in order to determine which classes of subdivision scheme are likely to contain useful members. My aim is to ascertain whether there are any potentially useful classes which have not yet been investigated or whether we can say, with reasonable confidence, that all of the useful classes have already been considered. I apply heuristics related to the mappings of element types (vertices, face centres, and mid-edges) to one another, to the preservation of symmetries, to the alignment of meshes at different subdivision levels, and to the size of the overall subdivision mask. My conclusion is that there are only a small number of useful classes and that most of these have already been investigated in terms of linear, stationary subdivision schemes. There is some space for further work, particularly in the investigation of whether there are useful ternary linear, stationary subdivision schemes, but it appears that future advances are more likely to lie elsewhere. [*] M. Alexa. Refinement operators for triangle meshes. Computer Aided Geometric Design, 19(3):169-172, 2002. [+] I. P. Ivrissimtzis, N. A. Dodgson, and M. A. Sabin. A generative classification of mesh refinement rules with lattice transformations. Computer Aided Geometric Design, 22(1):99-109, 2004. Location privacy in ubiquitous computing Beresford, Alastair R. University of Cambridge, Computer Laboratory 2005-01 en Text UCAM-CL-TR-612 ISSN 1476-2986 The field of ubiquitous computing envisages an era when the average consumer owns hundreds or thousands of mobile and embedded computing devices. These devices will perform actions based on the context of their users, and therefore ubiquitous systems will gather, collate and distribute much more personal information about individuals than computers do today. Much of this personal information will be considered private, and therefore mechanisms which allow users to control the dissemination of these data are vital. Location information is a parti