The meaning and implementation of PRI ALT in occamGeo

This paper describes what we believe to be an error in the implementation of PRI ALT on the inmos transputer, and exhibits anomalous behaviour of that implementation. A correct implementation of comparable complexity is described, and some of the properties of the construct are described. Finally, an attempt is made to describe circumstances in which the behaviour of the existing implementation is adequate for the correctness of a program which uses it. Concepts An alternation is a process which is simultaneously prepared to communicate with a number of other processes, and which selects exactly one of those that are prepared to communicate with it. In the occam languagess1, 2], the simplest form of alternation consists of a number of branches each beginning with an input process. The alternation executes exactly one of these inputs and the process which follows it, making its selection from amongst those inputs using channels on which another process is already prepared to output. The initial input of each branch of an alternation is called its guard, and a guard is said to be ready when another process has committed itself to performing an output on that channel. Two observations are worth making about readiness. Firstly, since output processes are unconditional, a guard which becomes ready remains ready until discharged by the corresponding input. Secondly, it is possible for several branches of an alternation to be guarded by inputs from the same channel; in this case they would all become ready simultaneously. For the most part we connne ourselves in this paper to considering only guards that consist of a simple input process; the treatment can be extended to accommodate SKIP guards, delay guards, and guards accompanied by Boolean conditions.