Q. 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
Answer: (A)
Explanation:
