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A shared variable x, initialized to zero, is operated on by four concurrent processes W, X, Y, Z as follows. Each of the processes W and X reads x from memory, increments by one, stores it to memory, and then terminates. Each of the processes Y and Z reads x from memory, decrements by two, stores it to memory, and then terminates. Each process before reading x invokes the P operation (i.e., wait) on a counting semaphore S and invokes the V operation (i.e., signal) on the semaphore S after storing x to memory. Semaphore S is initialized to two. What is the maximum possible value of x after all processes complete execution?

A shared variable x, initialized to zero, is operated on by four concurrent processes W, X, Y, Z as follows. Each of the processes W and X reads x from memory, increments by one, stores it to memory, and then terminates. Each of the processes Y and Z reads x from memory, decrements by two, stores it to memory, and then terminates. Each process before reading x invokes the P operation (i.e., wait) on a counting semaphore S and invokes the V operation (i.e., signal) on the semaphore S after storing x to memory. Semaphore S is initialized to two. What is the maximum possible value of x after all processes complete execution? Correct Answer 2

The correct answer is option 4.

Key Points

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X, W reads X and increment X by 1.

Y, W reads X and decrement by 2.

Start with X,  will perform P(S) then S=1, read X=0, X=X+1=1

Then Y will perform P(S) then S=0, read X=0, X=X-2=-2, then store X. V(S), S=1

Then Z will perform P(S) then S=0, read X=-2, X=X-2= -4, then store X, V(S), S=1 

Then X will  store X, V(S), S=2, X=1

Then W will perform  P(S), S=1, read X=1, X=X+1=2, store X, V(S), S=2, X=2
Hence the correct answer is 2.

Related Questions

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