UGC NET CSE | December 2019 | Part 2 | Question: 24

Question

A counting semaphore is initialized to $8$. $3$ wait ( ) operations and $4$ signal ( )  operations are applied. Find the current value of semaphore variable.

• A. $9$
• B. $5$
• C. $1$
• D. $4$

Answer 1

wait() operation decrements the value of a semaphore.

signal() operation increments the value of a semaphore.

3 wait() implies -3 to value of semaphore.

4 signal() implies +4 to value of semaphore.

So 8-3+4=9 is the value of semaphore after  3 wait() and 4 signal() operations.

Answer 2

In terms of counting semaphore

• wait/down/$P()$ operation decrement $(\downarrow)$ the value by $-1$ 
• signal/up/$V()$ operation increment $(\uparrow)$ the value by $+1$

Now initial value of CS= $8$

final value of CS after $3P$ and $4V$ operation = $(8-3P+4V)=(8-3+4)=9$

So current values of CS is $9.$

Option $(A)$ is correct.

Ref:  Gate 1992