ISRO CSE 2017 | Question: 71

Question

At a particular time the value of counting semaphore is 10. It will become 7 after:

• A. 3 V operations
• B. 3 P operations
• C. 5 V operations and 2 P operations
• D. 2 V operations and 5 P operations

Comments

Were we supposed to circle both B and D ??

Answer 1

We know that there are two operations are used in semaphores

1) wait : It is denoted by P, It is used when some process enter into the critical section by decreasing semaphore value by 1.

2) signal :It is denoted by V, It is used when some process exit into the critical section by increasing semaphore value by 1.

So, in the question they are asking about value is decreasing from 10 to 7 so, there would be 3 P operations used..

And other possibility may be firstly 2 V happen, because of this value increases from 10 to 12 and then 5 P happen, because of this value decreases from 12 to 7..

so both B and D will be correct...

Comments

Why not D?

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yes shiva u are correct, have they given any other information about number of processes????

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No extra information. I think both B and D can be the answer.

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Ya ,I agree with U

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both answers are correct

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were we supposed to circle both B) and D) ????

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@akash @shiva

What i think is there B option is only correct , and they show that in official answer key too ..

now why option B , that is because of  P, It is used when some process enter into the critical section

so when counting semaphore is 10, after enter into it's CS , the counting semaphore value become 7 after 3 P operations.

But , it can not be START with V, as V   is only used when some process exit into the critical section 

so a process have to enter into it's critical section first , not to exit first .

Now, you can think if the process is middle of it's critical section then it exit first means perform V , then again enter into it's CS means perform P . But this is not the general scenario .

Hence only B is the  correct option :) .

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No, I think answer is B because ISRO made the key. We can do V any time we want. This should be debated -- and hopefully they will agree.

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@arjun Sir,

I agree with u , that we can do V operations any time. i have seen this in lot of codes too..

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this question is taken from some made easy psu book ..but they did not opy whole question

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they forgot to copy whole question

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ISRO is unbelievable :)

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I hope all the PSUs stop their exams and only recruit via GATE. And also hope GATE is conducted twice a year soon.

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@sonam vyas

And Made easy took it from Timothy j Williams book  , see question number 21 .

options given there both B and C .

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it is mentioned that, "at particular time", not initialized with. So, i think both B and D are correct.

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It is my opinion we should mark the answer D since we also know the number of V operations in D.
 

Answer 2

Option B,D both will give same answer