Semaphores

• Spinlocks let a process continue to use the processor (wasting CPU) when waiting for a critical region or resource to become available
• Semaphores release the processor from the currently executing process, save the state of this process, and call the dispatcher to scheduler another process on the processor
• A semaphore can be used to protect a critical region of code

Skeleton Code for Binary Semaphore

• wait(sem): if sem = 1 then sem:= 0; else place process in sem.queue;
• signal(sem): if sem.queue not empty

then remove a process from the queue and place in ready list

else sem :=1;

• A binary semaphore allows just one process in the critical region at a time (mutual exclusion)

Skeleton Code for Counting Semaphore

• Counting semaphore is initialized to "n"
• It then allows n processes into critical region or allocation of n resources
• wait(sem): s:=s-1; if s<0 place process on sem.queue
• signal(sem): s:=s+1; if s <= n then move process from sem.queue to ready list