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Wednesday, August 26, 2009

Overview Of Process

A process is an activity which takes place over time and which has a precise aim regarding the result to be achieved. The concept of a process is hierarchical which means that a process may consist of a partially ordered set of sub-processes. A process is rather abstract. It describes the essentials of the purpose, structure, rationale, roles and timing, leaving plenty of implementation freedom. The power of a process is its abstraction, which enables its application in a wide range of applications, by tailoring its implementation to the specific application. A process can be tailored and elaborated in one or more procedures, which describe cookbook-like what need to be done when and by whom.
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.
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.
A process goes through a series of discrete process states :
* New State: The process being created.
* Running State: A process is said to be running if it has the CPU, that is, process actually using the CPU at that particular instant.
* Blocked (or waiting) State: A process is said to be blocked if it is waiting for some event to happen such that as an I/O completion before it can proceed. Note that a process is unable to run until some external event happens.
* Ready State: A process is said to be ready if it use a CPU if one were available. A ready state process is runable but temporarily stopped running to let another process run.
* Terminated state: The process has finished execution.

A process in an operating system is represented by a data structure known as a process control block (PCB) or process descriptor. The PCB contains important information about the specific process including :
* The current state of the process i.e., whether it is ready, running, waiting, or whatever.
* Unique identification of the process in order to track "which is which" information. * A pointer to parent process.
* Similarly, a pointer to child process (if it exists).
* The priority of process (a part of CPU scheduling information).
* Pointers to locate memory of processes.
* A register save area.
* The processor it is running on.

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