Question

The atomic fetch-and-set x, y instruction unconditionally sets the memory lcoation x to 1 and fetches the old value of x in y without allowing any intervening access to the memory location and x. Consider the follwoing implementation of P and V functions on a binary semaphore S.
void P(binary_semaphore *S) {
unsigned y;
unsinged *x=&(S$\to$ value);
do {
fetch-and-set x,y;
} while (y);
}
void V (binary_semphore *){
S$\to$ value =0;
}
Which one of the following is true?

• A. The code does not implement a binary semaphore
• B. Instead of using fetch-and-set, a pair normal load/store can be used
• C. The implementation of V is wrong
• D. The implementatioin may not work if context switching is disabled in P

Answer 1

The correct option is D The implementatioin may not work if context switching is disabled in P

The P and V function is as follows:

1. void P(binary-semaphore*S){
2. unsigned y;
3. unsigned *x=&(S$\to$ value);
4. do {
5. fetch-and-set x,y;
6. } while (y);
7. }
8. void V(binary-semaphore *S){
9. S$\to$=0;
10. }

fetch-and-set instruciton always sets the memory location x =1 and fetches the old value of x into y. The binary semphore * S takes only two values either 0 and 1. When we initiliaze S = 0 then in statement 3 his value will be start at location x and fetch-and-set instruction changes the value of x =0 to x =1 and y beocmes 0. If there are more thean two processes and context switching between processes is disabled in P then this implementation doesn't work properly and can't synchronize the processes.