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Gate2006_61

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;
}

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

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