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Definition of Process

The notion of process is central to the understanding of operating systems. There are quite a few definitions presented in the literature, but no "perfect" definition has yet appeared.


The term "process" was first used by the designers of the MULTICS in 1960's. Since then, the term process, used somewhat interchangeably with 'task' or 'job'. The process has been given many definitions for instance

  • A program in Execution.
  • An asynchronous activity.
  • The 'animated sprit' of a procedure in execution.
  • The entity to which processors are assigned.
  • The 'dispatchable' unit.

and many more definitions have given. As we can see from above that there is no universally agreed upon definition, but the definition "Program in Execution" seem to be most frequently used. And this is a concept are will use in the present study of operating systems.

Now that we agreed upon the definition of process, the question is what is the relation between process and program. It is same beast with different name or when this beast is sleeping (not executing) it is called program and when it is executing becomes process. Well, to be very precise. Process is not the same as program. In the following discussion we point out some of the difference between process and program. As we have mentioned earlier.

Process is not the same as program. A process is more than a program code. A process is an 'active' entity as oppose to program which consider to be a 'passive' entity. As we all know that a program is an algorithm expressed in some suitable notation, (e.g., programming language). Being a passive, a program is only a part of process. Process, on the other hand, includes:

  • Current value of Program Counter (PC)
  • Contents of the processors registers
  • Value of the variables
  • The process stack (SP) which typically contains temporary data such as subroutine parameter, return address, and temporary variables.
  • A data section that contains global variables.

A process is the unit of work in a system.

In Process model, all software on the computer is organized into a number of sequential processes. A process includes PC, registers, and variables. Conceptually, each process has its own virtual CPU. In reality, the CPU switches back and forth among processes. (The rapid switching back and forth is called multiprogramming).