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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)
    -2
  • b)
    -1
  • c)
    1
  • d)
    2
Correct answer is option 'D'. Can you explain this answer?
Verified Answer
A shared variable x, initialized to zero, is operated on by four concu...
Processes can run in many ways, below is one of the cases in which x attains max value
Semaphore S is initialized to 2
Process W executes S=1, x=1 but it doesn't update the x variable.
Then process Y executes S=0, it decrements x, now x= -2 and signal semaphore S=1
Now process Z executes s=0, x=-4, signal semaphore S=1
Now process W updates x=1, S=2 Then process X executes X=2 
So correct option is D
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Most Upvoted Answer
A shared variable x, initialized to zero, is operated on by four concu...
Explanation:
Let's analyze the given scenario step by step to determine the maximum possible value of x after all processes complete execution.

Step 1: Initialization
- The shared variable x is initialized to zero.
- Semaphore S is initialized to two.

Step 2: Processes W and X
- Both process W and process X read the value of x from memory. At this point, x is still zero.
- Each process increments the value of x by one.
- Both processes store the updated value of x to memory.
- Before storing x to memory, both processes invoke the P operation on semaphore S, which decreases the value of S by one. Since S was initialized to two, its value becomes one after both processes complete the P operation.
- After storing x to memory, both processes invoke the V operation on semaphore S, which increases the value of S by one. As a result, the value of S becomes two again.

Step 3: Processes Y and Z
- Both process Y and process Z read the value of x from memory. At this point, x is incremented by two (due to the operations performed by process W and process X).
- Each process decrements the value of x by two.
- Both processes store the updated value of x to memory.
- Before storing x to memory, both processes invoke the P operation on semaphore S, which decreases the value of S by one. Since S was already two, its value becomes one after both processes complete the P operation.
- After storing x to memory, both processes invoke the V operation on semaphore S, which increases the value of S by one. As a result, the value of S becomes two again.

Step 4: Final value of x
- After all processes have completed execution, the final value of x can be calculated as follows:
- The initial value of x is zero.
- Processes W and X increment x by one each, resulting in an increase of two.
- Processes Y and Z decrement x by two each, resulting in a decrease of four.
- Therefore, the maximum possible value of x after all processes complete execution is 2 (initial value of x + increase of two - decrease of four).

Hence, the correct answer is option 'D' - 2.
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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)-2b)-1c)1d)2Correct answer is option 'D'. Can you explain this answer?
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