What do you mean by process? Draw and explain process state diagram in Unix.

Subject: Operating System

Topic: Process Concept and Scheduling

Difficulty: Medium

1 Answer
  • A process is an instance of a program in execution
  • A process is the basic unit of execution in an operating system. Each process has a process identifier associated with it. Known as the process_id.

The process state diagram for UNIX system is given below:

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  • UNIX uses two categories of processes: System processes and user processes.
  • System processes operate in kernel mode and execute operating system code to perform administrative and housekeeping functions.
  • User processes operate in user mode to execute user programs and utilities  

The various states over here are:

  • Created: Process is newly created and is not ready to run.
  • User Running: The process is currently executing in user mode.
  • Kernel Running: The process is currently running in kernel mode.
  • Ready to run, in memory: The process is ready to run and is waiting for the Kernel to schedule it.
  • Ready to run, swapped: The process is ready to run, but is currently no in the main memory and is waiting for the swapper to swap it to the main memory.
  • Sleep, Swapped: A blocked state wherein the process is awaiting an event and has been swapped to secondary storage.
  • Asleep in memory: A blocked state wherein the process is waiting for an event to occur and is currently in the main memory.
  • Pre-empted: Process is running from kernel to user mode. But the kernel pre-empts it and does a process switch and schedules another process.
  • Zombie: Process no longer exists. It still leaves behind a record of it for its parent process to access.


  • A process after creation is not immediately put into Ready state.
  • Instead it is put to Created state. This is done so that we create a process without enough memory available to run.
  • Then it can move to Ready to run in memory or Ready to run swapped state based on the available memory.
  • Now, when it arrives in memory, based on the scheduling we move to kernel mode or User mode as per the requirements. Interrupts in between will send the system into a Sleep state (Blocked).
  • When the process finishes execution, it invokes an Exit system call and goes into Zombie state.
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