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Process States

Definition

The life cycle of a process can be divided into a set of states, which characterize the entire process. The process state reflects the life state of the process.

Five states of a process

Generally speaking, a process has five states: Running state, ready state, blocked (waiting) state, created state and terminated state. The first three states are the basic states of the process.

  • Created state: When a process is created, it needs to apply for a blank control block (PCB) and fill in the information of controlling and managing the process to complete resource allocation.

    • Running state: The process occupies processor resources and runs on the processor.
    • Ready state: The process has the conditions to run, but cannot run because it is not allocated to a processor.
    • Blocked state: The process cannot run temporarily because it is waiting for an event to occur (such as I/O operation).

  • Terminated state: The process ends normally, or enters the terminated state due to an error or being terminated by the system.

Process state transition

Process state transition

The three basic states of a process (ready, running, blocked) can be divided into six transition situations:

  • Ready → Running:

When the scheduler selects the process from the ready queue, the process state changes from ready to running.

  • Running → Ready:

When the time slice allocated to the process by the system is used up, the process changes from the running state to the ready state.

In a system with a preemptive priority scheduling algorithm, when a higher priority process is to be run, the current process is forced to give up the processor and enter the ready state.

  • Running → Blocked:

If the running process cannot continue to execute because it is waiting for an event (such as an I/O request), the process state changes from running to blocked.

  • Blocked → Ready: When the event the process is waiting for is completed, the process enters the ready queue and the state changes from blocked to ready.

Note: The following two state transitions are impossible:

  • Blocked → Running
  • Ready → Blocked