Process

A process is unit of work in operating system. A process is a program in execution. The process is different from the program as program is a passive entity which is a file containing the list of instructions stored on the disk whereas process is an active entity with a program counter specifying the next instruction to execute and a set of associated resources. A process includes:

  • Program Counter: It specifies next instruction to be executed.
  • Stack: it contains temporary data such as return addresses.
  • Data Section: It contains global variables.

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Process State

As process executes, it changes its state. By its current activity, the process can be in one of the following state:

  • New: The process is being created.
  • Running: The instructions are being carried out.
  • Waiting: The process is waiting for some event to occur.
  • Ready: The process is waiting to be assigned to processor.
  • Terminated: The process has finished.

It must be noted that only one process can be running on any processor at any instant, other processes may be in ready or waiting state.

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Process Control Block

Each process is represented in operating system by Process Control Block (PCB). It contains process specific information:

  • Process State: As discussed above, the state can be new, ready, running, waiting, terminated.
  • Program Counter: The counter indicates the address of the next instruction to be executed for this process.
  • CPU Registers: They include accumulators, index registers, stack pointers, and general-purpose registers, plus any condition-code information.
  • CPU Scheduling information: This information includes a process priority, pointers to scheduling queues, and any other scheduling parameters.
  • Memory-management information
  • Accounting information: This information includes the amount of CPU and real time used, time limits, account numbers, job or process numbers, and so on.
  • I/O status information: This information includes the list of I/O devices allocated to the process, a list of open files, and so on.

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