Sema Kudu

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 or hundreds of milliseconds. While, strictly speaking, at any instant of time, the CPU is
running only one program, in the course of 1 second, it may work on several programs, thus
giving the users the illusion of parallelism. Sometimes people speak of pseudoparallelism in this
context, to contrast it with the true hardware parallelism of multiprocessor systems (which have
two or more CPUs 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 conceptual model (sequential processes) that makes parallelism.