The atomic fetch-and-set x, y instruction unconditionally sets memory location…

GATE · 2006 · CS

The atomic fetch-and-set x, y instruction unconditionally sets memory location x to 1 and fetches the old value of x in y without allowing any intervening access to 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 operations can be used
C.
The implementation of V is wrong
D.
The code does not implement a binary semaphore

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Correct answer: A

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