History
See also: History of operating systemsBy the early 1960s computer control software had evolved from Monitor control software, e.g., IBSYS, to Executive control software. Computers got "faster" and computer time was still neither "cheap" nor fully used. It made multiprogramming possible and necessary.
Multiprogramming means that several programs run "at the same time" (concurrently, including parallel and non-parallel). At first they ran on a single processor (i.e., uniprocessor) and shared scarce resources. Multiprogramming is also basic form of multiprocessing, a much broader term.
Programs consist of sequences of instructions for processors. A single processor can run only one instruction at a time: it is impossible to run more programs at the same time. A program might need some resource (input ...) which has a large delay, or a program might start some slow operation (output to printer ...). This would lead to processor being "idle" (unused). To use processor at all times, the execution of such a program is halted. At that point, a second (or nth) program is started or restarted. To the user, it will appear that the programs run at the same time (hence the term, concurrent).
Shortly thereafter, the notion of a 'program' was expanded to the notion of an 'executing program and its context'. The concept of a process was born.
This became necessary with the invention of re-entrant code.
Threads came somewhat later. However, with the advent of time-sharing; computer networks; multiple-CPU, shared memory computers; etc., the old "multiprogramming" gave way to true multitasking, multiprocessing and, later, multithreading.
Read more about this topic: Process (computing)
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