ISRO-DEC2017-70

Question

A counting semaphore was initialized t o $7$. Then $\text{20 P (wait)}$ operations and$\text{x V (signal)}$ operations were completed on this semaphore. If the final value of semaphore is $5$, then the value $x$ will be

• A. $0$
• B. $13$
• C. $18$
• D. $5$

Comments

option c

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answer key they have given as b..

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In revised key answer is changed to C

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Shiva

Give me link of revised answer key??

Answer 1

Counting  semaphore is a protected variable whose value can be changed only by predefined atomic operations

• Wait :- P(Sem s) Decrements value of S by 1. If S<=0, the thread must go for sleep and get inserted in wait_queue
• Signal: V(Sem s) Increments value of S by 1 and wakes up one of the threads in waiting queue. 

In given problem

• Initially value =  7
• After 20 P (wait) operations, value = 7- 20 = -13
• After x V (signal) operations, value = -13+x =5
• x = 5+13 =18

Answer is C

Answer 2

it should be 18

P refers to down operation and V refers to up i.e 7-20(P)+x(V)=5

=> x = 18

Comments

explain a bit.

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P refers to down operation and V refers to up i.e 7-20(P)+x(V)=5

=> x = 18

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in answer put it

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Smart representation. I'm noting it down as a formula