UP and DOWN Operations

  DOWN OPERATION (SEMAPHORE)

    if r > 0;
    then r = r - 1;
    else SLEEP(process);
 
 

Lets look at an example of semaphores. In figure A, there are three semaphores. S1 which has a value of zero. Process 1 is blocked on S1. S2 has a value of three, meaning three more processes may execute a down operation on S2 without being put to sleep. S3 has a value of zero, and has two sleeping processes, P4 and P5.

Low lets look at what happens when a process executes an UP on S1. Process P1 is activated by the UP operation. It executes a DOWN on S1 and enters its critical section. The state of the semaphores after these actions is shown in figure B.