Operating Systems : Introduction To Processes

All modern computers can do several things at the same time. While running a user program, a computer can also be reading from a disk and outputting text to a screen or printer. In a multiprogramming system, the CPU also switches from program to program, running each for tens of hundreds of milliseconds. While, strictly speaking, at any instant of time, the CPU is running only one program, in the course of i second, it may work on several program, thus giving the users the illusion of parallelism.

Sometimes people speak of pseudoparallelism to mean this rapid switching back and forth of the CPU between programs, to contrast it with the true harware parallelism of multiprocessor systems (which have two or more CPU's sharing the same physical memory). Keeping track of multiple, parallel activities is hard for people to do. Therefore, operating system designers over the years have evolved a model (sequential processes) that makes parallelism easier to deal with.