Process Concept

Process Concept

One hindrance to the discussion of operating systems is the question of what to call all the CPU activities. A batch system executes jobs, whereas a time-shared system has user programs, or tasks. Even on a single-user system, such as MS-DOS and Macintosh OS, a user may be able to run several programs at one time: one interactive and several batch programs. Even if the user can execute only one program at a time, the operating system may need to support its own internal programmed activities, such as spooling. In many respects, all of these activities are similar, so we call all of them processes.

The terms job and process are used almost interchangeably in this text. Although we personally prefer the term process, much of operating-system theory and terminology was developed during a time when the major activity of operating systems was job processing. It would be misleading to avoid the use of commonly accepted terms that include the word job (such as job scheduling) simply because the term process has superseded it.

The Process

Informally, a process is a program in execution. The execution of a process must progress in a sequential fashion. That is, at any time, at most one instruction is executed on behalf of the process.

A process is more than the program code (sometimes known as the text section). It also includes the current activity, as represented by the value of the program counter and the contents of the processor's registers. A process generally also includes the process stack, containing temporary data (such as subroutine parameters, return addresses, and temporary variables), and a data section containing global variables.

We emphasize that a program by itself is not a process; a program is a passive entity, such as the contents of a file stored on disk, whereas a process is an active entity, with a program counter specifying the next instruction to execute and a set of associated resources.

Although two processes may be associated with the same program, they are nevertheless considered two separate execution sequences. For instance, several users may be running copies of the mail program, or the same user may invoke many copies of the editor program. Each of these is a separate process, and, although the text sections are equivalent, the data sections will vary. It is also common to have a process that spawns many processes as it runs.