At a particular time of computation, the value of a counting semaphore is 10. Then 12 P operations and “x” V operations were performed on this semaphore. If the final value of semaphore is 7, x will be:

Concept:

A semaphore S is an integer variable that apart from initialization is accessed only through two atomic operations: wait and signal. These are termed as wait (P) and signal (V).

Semaphore operation

Wait(S): S.value = S. value – 1;

 If S. value < 0,

then add this process to S.L and block;

End;

Signal (S): S. value = S. value + 1;

If S. value = 0, then begin

remove process from S.L;

Wakeup(P);

End;

Explanation:

counting semaphore: 10

12 P operations are performed, value will be decreased by 1 with each P operation.

x V operations are performed, value will be increased by 1 with each V operation.

Final value of semaphore = 7

So, 10 – 12 + x = 7

-2 + x = 7

x = 7 + 2 = 9