Process Description

A process in UNIX is a somewhat complex set of data structures that provides the operating system with all of the information necessary to dispatch and manage processes. The elements of the process image are categorized into two parts. They are as system context and user level context.

The user level context encloses the fundamental elements of the user's program and can be generated directly from a compiled object file. The user's program is divided into text and data areas. The text area is read-only as well as it is intended to hold the program's instructions. While the process is implementing the processor uses the user stack area for procedure calls and returns and parameter passing. The shared memory area is a data area specifically shared with other processes. There is merely one physical copy of a shared memory area other than, by the utilize of virtual memory it appears to every sharing process memory region is in its address space. When a process isn't running then the processor status information is stored in the register context area.

The system level context encloses the remaining information that the operating system needs to manage the process.

It consists of a static part which is fixed in size and stays with a process throughout its lifetime and a dynamic part which vary through the life of the process. The one element of the static part is the process table entry. This is in fact part of the process table maintained by the operating system with one entry per process. The process table entry table process a control information i.e. accessible to the kernel at all times therefore in a virtual memory system all process table entries are maintained in main memory.