A scheduler is a key component of kernel. It has algorithms that determine which task executes when. Scheduler uses following entities/ policies for task scheduling.

i) Scheduling Entities:

• Any task that can be scheduled for execution is a scheduling entity.
• Example: ISR, a function or any process.
• When a scheduling entity enters ready state, it is scheduled to be executed by the processor.

ii) Multitasking:

• The kernel schedules multiple tasks such that many tasks appear to be executing concurrently.
• The scheduler must ensure that appropriate task is executed without any latency.

iii) Context switching:

• Whenever a task is in execution, the register holds the contents of that task.
• Whenever the OS decides to switch the task, it needs to store this context (contents of ongoing task).
• For this OS uses special memory block called Task Control Block (TCB).
• In TCB, OS stores all the information required by itself and the task for execution.

iv) Dispatcher:

• It is responsible for dispatching the tasks in running state.
• It controls task switching and execution flow.

v) Scheduling algorithms:

• It implements scheduling algorithm to determine which task will run.
• There are two types of scheduling algorithms –

a) Preemptive priority based scheduling: If higher priority task enters ready state, then it preempts or suspends the lower priority task and higher priority task enters execution state.

b) Round Robin scheduling: Each task is given fixed time slot.