The concept of “process ” has come to play a central role in many efforts to master the complexity of large computer systems. The purpose of this paper is to discuss useful methods of structuring complex processes, and to relate these to the problems of improving the quality of large computer systems. Two distinct ways of structuring systems are presented, namely, process combination, and process abstraction; these are then used to discuss such topics as concurrency, synchronization, multiprogramming, interpreters, and programmable processors. This discussion is based on a set of precise definitions for such concepts as “process,” “processor, ” “computation, ” “combination, ” and “abstraction. ” The paper relates these definitions to both current research and practical applications, with particular concern for the problems of the performance, reliability, and modifiability of computer systems. Key words and phrases: sequential process, cooperating processes, asynchronous processes, parallelism, complexity, program, processor, interpreter, hierarchical structures, abstraction, refinement.

Abstract. A technique for proving the equivalence of implementations of multi-tasking programming languages is developed and applied to proving the equivalence of the contour model and a multi-tasking version of the copy rule. In multi-tasking programming languages, e.g. Algol 68 [vWN 69] and PL/I [Wlk 69], the inherent nondeterminism associated with tasking accentuates the difficulty of precisely describing computations

In block structumed languages, upon entry to a block or procedure, storage is allocated for the identifiers declared in the block or procedure. There are two choices as to'when to deallocate this

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