Process Concept

• A process is an instance of a program in execution.

• Batch systems work in terms of "jobs". Many modern process concepts are still expressed in terms of jobs, (e.g. job scheduling), and the two terms are often used interchangeably.

The Process

Process memory is divided into four sections as shown in Figure below:

• The text section comprises the compiled program code, read in from non-volatile storage when the program is launched.
• The data section stores global and static variables, allocated and initialized prior to executing main.
• The heap is used for dynamic memory allocation, and is managed via calls to new, delete, malloc, free, etc.
• The stack is used for local variables. Space on the stack is reserved for local variables when they are declared (at function entrance or elsewhere, depending on the language), and the space is freed up when the variables go out of scope. Note that the stack is also used for function return values, and the exact mechanisms of stack management may be language specific.

Note that the stack and the heap start at opposite ends of the process's free space and grow towards each other. If they should ever meet, then either a stack overflow error will occur, or else a call to new or malloc will fail due to insufficient memory available.

When processes are swapped out of memory and later restored, additional information must also be stored and restored. Key among them are the program counter and the value of all program registers.