Concurrency

Up until now, underlying everything we have said is the basic assumption that when we run a program, only one statement is executed at any given time. This presents no problem if the model of the computer which we intend to use is a uniprocessor of the von Neumann variety. But there are now many reasons for considering this view of computing to be out of date. Similarly when we discussed procedures and coroutines, in both cases only one procedure (or coroutine) could be active at a given time. The language concept of the procedure (and the coroutine) is simply the next higher level of abstraction one gets by starting with a machine which permits only sequential execution. New hardware configurations will mean that new language features will be devised for exploiting the capabilities of these new machines.