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One of the essential parts of any computer system is a mechanism for allocating the processors of the system among the various competitors for their services. These allocations must be performed in even the simplest system, for example, by the action of an operator at the console of the machine. In larger systems more automatic techniques are usually adopted; batching of jobs, interrupts and interval timers are the most common ones. As the use of such techniques becomes more frequent, it becomes increasingly difficult to maintain the conventional view of a computer as a system which does only one job at a time; even though it may at any given instant be executing a particular sequence of instructions, its attention is switched from one such sequence to another with such rapidity that it appears desirable to describe the system in a manner which accommodates this multiplexing more naturally. It is worthwhile to observe that these remarks apply to any large modern computer system and not just to one which attempts to service a number of on-line users simultaneously.
Butler Lampson (Sun,) studied this question.