Q61

MCQ2 marks, −0.5

The atomic fetch-and-set $x, y$ instruction unconditionally sets the memory location $x$ to $1$ and fetches the old value of $x$ in $y$ 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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