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Context switching steps

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Introduction ContextSwitching

Context switching is the process ofstoring the context of one processand switches to another process sothat the execution of the rst processcan be resumed later.

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Steps involved in contextswitching

The entire process of contextswitching takes seven steps tocomplete.

Let us assume processes task !and task ". The latter has higherpriorit#.

Task " has alread# executedpreviousl# and is now is read# state.

Task ! is currentl# executing

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Step $

Task ! is executing.

%C points to thenext instruction tobe executed.

S% points to thestack of task !.

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Step

&T'S tick occurs as theprocessor is about toexecute L(I &)* )

The tick is a Softwaregenerated interrupt thatoccurs after thecompletion of a time slice.

"efore execution of tickIS& routine* the %C valuegets pushed into stack

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Step + Tick IS& &outine

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Step + contd, %ort-S!/-C'0T/1T

234 saves theexecution context oftask ! in the stackof task !

Stack pointer of !will point to the topof context of !.

This functioncompletes b#storing a cop# ofthe stack pointer.

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Step 5

vTaskIncrementTick234 increments thetick value b# $ and as this occurs thetask " is in read# state.

vTaskSwitchContext234 chooses thenext task that gets to use theprocessor resources.

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Step 6

port-&/ST'&/-C'0T/1T234 executes.

The rst operation isdoes is it restores the

S% value of task ".

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Step 7

0ext the contexts of

task " are retrievedfrom the stack.

The %C value stillremains in the stack.

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Step 8

&/TI marks the endof the interrupt andso the value of %C isrestored from stackwhich is the value of%C of task ".

Thus* the processorhas switched fromexecuting task ! totask ".

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