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Question

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 .

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

Answer: (a).The implementation may not work if context switching is disabled in P

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Q. 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...

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