Concurrent internal and input/output activities

In certain sequential-state machines, the input of data precedes the internal processing operations which are then followed by the output of data. Each separate process (input, operation, output) is executed in turn, in violation, and there is no concurrency. Such a sequence may be represented as a structured program for the total information-processing task. Performance may require the apparent concurrency of the different processes by time-sharing one processing element amongst them. This technique has been used for some time in large mainframe computers that use complex operating systems to manage the allocation of the main processor amongst several concurrent tasks.