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.
Definition
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).
Process State
The process state consist of everything necessary to resume the process execution if it is somehow put aside temporarily. The process state consists of at least following:
• Code for the program.
• Program's static data.
• Program's dynamic data.
• Program's procedure call stack.
• Contents of general purpose registers.
• Contents of program counter (PC)
• Contents of program status word (PSW).
• Operating Systems resource in use.
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.
Process State
The process state consist of everything necessary to resume the process execution if it is somehow put aside temporarily. The process state consists of at least following:
• Code for the program.
• Program's static data.
• Program's dynamic data.
• Program's procedure call stack.
• Contents of general purpose registers.
• Contents of program counter (PC)
• Contents of program status word (PSW).
• Operating Systems resource in use.
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.
Process Control Block
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.
The PCB is a certain store that allows the operating systems to locate key information about a process. Thus, the PCB is the data structure that defines a process to the operating systems.
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