Chapter 3

Chapter 3 The hardware interface, I/O and communications

Objectives

To study the mechanisms for interaction between a software system and the outside world. To set up real instances of the occurrence of events and the need for synchronisation.

Points to emphasise

Process-hardware synchronisation at the device interface level.
Process-process synchronisation between client requests (top down) and the device-handling level (driven bottom up by hardware events).
The need for buffers for DMA interfaces and between client and device driving software.
Similarities and differences between dedicated, shared and communications device interfaces (speed, throughput, unit of transfer) and the higher levels of software associated with handling them.

Possible difficulties

There is a lot of material here.

Communications handling belongs here because the device interfaces can easily be taught with those for dedicated devices and shared devices. The danger is that too much time may be spent on re-teaching hardware.

Communications software, again, could be taught in great detail and take up a great deal of time. The difficulty is to convey the essential concepts fairly briefly but without being superficial.

Teaching hints

Device interfacing and interrupt handling will have been taught elsewhere: AR6. It is probably worth revising polling and interrupt handling. The machines (and network) with which the students are familiar could be used rather than the M68000 and R2000 examples in the book. It is not essential to cover both CISC and RISC. It is useful to cover communications interfaces at the same time.
Facts and figures comparing, for example, disk and network interfaces could be given. How often does the software have to respond to hardware events? How much data can be dumped in your memory from across the network? What happens if you run out of buffer space? Convey the idea of concurrent hardware events to which the software must respond and more concurrent events caused by clients’ requests.
One should not attempt to cover in detail the implementation of the layers of communications software, for example the use of buffering between layers. The concepts of a service interface and a protocol between peers at a given level can be explained. Examples of applications protocols rather than layer protocols could be used. Examples such as a mail service or an FTP service could be used.