Dispatcher in CPU Scheduling

By | September 14, 2021

Dispatcher

  • ┬áIt is a component of operating system involved in CPU scheduling.
  • Dispatcher is a program responsible for assigning the CPU to the process, which has been selected by the short-term scheduler.

Dispatcher in CPU Scheduling

  • Dispatching a process involves three functions:
  1. Context switching
  2. Switching to user mode from monitor mode
  3. Jumping to the proper location in the user program to restart that program.

1. Context Switching:

It means switching of CPU from one process to another.

  • This involves saving the state of old process (one that was running previously) and loading the saved state, if any, for this ready process (scheduled to be run now).
  • There will be no saved state if this ready process has been newly created and is being allocated to CPU for the first time.
  • If this ready process was previously in the blocked state (because of I/O), then the process registers and memory must be loaded with all the previously saved data.

2. Switching to user mode from monitor mode

A user process must be run in user mode. For this it is necessary to change its mode from monitor to user.

3. Restart the execution of process

  • The execution of process should be restarted by jumping to the instruction that was supposed to be executed when this process was last interrupted.
  • In case, if the process is being executed for the first time after creation, jump should be made to the first instruction.

 

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