Concurrent processes and coroutines

In this chapter we introduce concepts for multiprogramming, i.e. the programming of several, concurrently executed computations. These facilities are deliberately restricted to the formulation of so-called loosely coupled processes. We exclude the area of tightly coupled arrays of processes, considering their field of application as fairly narrow and specialized. Instead, we restrict the discussion to programs that describe several processes that interact relatively infrequently, and are therefore said to be loosely coupled. By infrequently is meant that interaction occurs at a few, well-defined, explicit points in the program, and by a process we understand a sequence of actions, i.e. an inherently sequential process. Programming as encountered so far can therefore be considered as a special case involving one single process only. Inversely, we can inherit for multiprogramming all facilities and techniques learned so far, and we need to add merely a few facilities for the designation of concurrent processes and for their interaction. In this regard we follow the tradition of earlier languages for multiprogramming such as Modula-1 and Brinch Hansen’s Concurrent Pascal.