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The atomic fetch-and-set x, y instruction unconditionally sets the memory location x to 1 and fetches the old value of x n y without allowing any intervening access to the memory location x. consider the following implementation of P and V functions on a binary semaphore S.
void P (binary_semaphore *s)
{
        unsigned y;
        unsigned *x = &(s->value);
        do
        {
                fetch-and-set x, y;
        }
        while (y);
}
void V (binary_semaphore *s)
{
        S->value = 0;
}
 
Q. Which one of the following is true?
  • a)
    The implementation may not work if context switching is disabled in P
  • b)
    Instead of using fetch-and –set, a pair of normal load/store can be used
  • c)
    The implementation of V is wrong
  • d)
    The code does not implement a binary semaphore
Correct answer is option 'A'. Can you explain this answer?
Verified Answer
The atomic fetch-and-set x, y instruction unconditionally sets the mem...
Let us talk about the operation P(). It stores the value of s in x, then it fetches the old value of x, stores it in y and sets x as 1. The while loop of a process will continue forever if some other process doesn’t execute V() and sets the value of s as 0. If context switching is disabled in P, the while loop will run forever as no other process will be able to execute V().
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Most Upvoted Answer
The atomic fetch-and-set x, y instruction unconditionally sets the mem...


Explanation:

Context Switching:
- Context switching involves saving the state of a process or thread so that it can be paused and resumed later.
- In the given implementation of the P function, the fetch-and-set operation is crucial for the correct functioning of the binary semaphore.
- If context switching is disabled, there is a possibility that another process may interrupt the execution of the P function before it completes the fetch-and-set operation.
- This interruption can lead to incorrect behavior of the semaphore, making the implementation unreliable.

Use of Fetch-and-Set:
- The fetch-and-set operation ensures atomicity in updating the semaphore value, which is essential for proper synchronization.
- Using a pair of normal load and store operations instead of fetch-and-set can lead to race conditions and data inconsistency.
- Therefore, the fetch-and-set operation is necessary for the correct functioning of the binary semaphore.

Implementation of V:
- The implementation of the V function sets the semaphore value to 0, indicating the release of the semaphore.
- This is a valid operation for the V function in the context of binary semaphores.
- Therefore, the implementation of the V function is correct.

Conclusion:
- The correct statement among the given options is that the implementation may not work if context switching is disabled in the P function.
- It is essential to ensure atomicity in semaphore operations to avoid race conditions and data inconsistency, which can be achieved through the fetch-and-set operation.
- Disabling context switching can potentially disrupt the atomicity of the semaphore operations, making the implementation unreliable.
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The atomic fetch-and-set x, y instruction unconditionally sets the memory location x to 1 and fetches the old value of x n y without allowing any intervening access to the memory location x. consider the following implementation of P and V functions on a binary semaphore S.void P (binary_semaphore *s){ unsigned y; unsigned *x = (s-value); do { fetch-and-set x, y; } while (y);}void V (binary_semaphore *s){ S-value = 0;}Q.Which one of the following is true?a)The implementation may not work if context switching is disabled in Pb)Instead of using fetch-and set, a pair of normal load/store can be usedc)The implementation of V is wrongd)The code does not implement a binary semaphoreCorrect answer is option 'A'. Can you explain this answer?
Question Description
The atomic fetch-and-set x, y instruction unconditionally sets the memory location x to 1 and fetches the old value of x n y without allowing any intervening access to the memory location x. consider the following implementation of P and V functions on a binary semaphore S.void P (binary_semaphore *s){ unsigned y; unsigned *x = (s-value); do { fetch-and-set x, y; } while (y);}void V (binary_semaphore *s){ S-value = 0;}Q.Which one of the following is true?a)The implementation may not work if context switching is disabled in Pb)Instead of using fetch-and set, a pair of normal load/store can be usedc)The implementation of V is wrongd)The code does not implement a binary semaphoreCorrect answer is option 'A'. Can you explain this answer? for Computer Science Engineering (CSE) 2024 is part of Computer Science Engineering (CSE) preparation. The Question and answers have been prepared according to the Computer Science Engineering (CSE) exam syllabus. Information about The atomic fetch-and-set x, y instruction unconditionally sets the memory location x to 1 and fetches the old value of x n y without allowing any intervening access to the memory location x. consider the following implementation of P and V functions on a binary semaphore S.void P (binary_semaphore *s){ unsigned y; unsigned *x = (s-value); do { fetch-and-set x, y; } while (y);}void V (binary_semaphore *s){ S-value = 0;}Q.Which one of the following is true?a)The implementation may not work if context switching is disabled in Pb)Instead of using fetch-and set, a pair of normal load/store can be usedc)The implementation of V is wrongd)The code does not implement a binary semaphoreCorrect answer is option 'A'. Can you explain this answer? covers all topics & solutions for Computer Science Engineering (CSE) 2024 Exam. Find important definitions, questions, meanings, examples, exercises and tests below for The atomic fetch-and-set x, y instruction unconditionally sets the memory location x to 1 and fetches the old value of x n y without allowing any intervening access to the memory location x. consider the following implementation of P and V functions on a binary semaphore S.void P (binary_semaphore *s){ unsigned y; unsigned *x = (s-value); do { fetch-and-set x, y; } while (y);}void V (binary_semaphore *s){ S-value = 0;}Q.Which one of the following is true?a)The implementation may not work if context switching is disabled in Pb)Instead of using fetch-and set, a pair of normal load/store can be usedc)The implementation of V is wrongd)The code does not implement a binary semaphoreCorrect answer is option 'A'. Can you explain this answer?.
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