Скачать или смотреть Lecture 10 Semaphores (Binary and Counting)

Semaphore is a very powerful tool for process synchronization, solving critical section problem and for multi-instances resource sharing. It has two types; binary and counting. In binary semaphore the value of semaphore can either be 0 or 1 where 1 shows that a process can enter the critical section problem while 0 shows that the process is not allowed to enter the critical section problem. The value of the semaphore is incremented and decremented through two standard atomic operations. The wait (also P or down ) operation decrements the semaphore while Signal( also V or up ) operation increments the semaphore. By atomic operation we mean that once a processes is in either wait or signal function then no other processes can enter these functions because interrupts are disabled when a process is in the wait or signal function and thus, cannot be preempted. This goes true for uni-processor system but if there are two processors and one processor cannot block the interrupts of the other processors so the buses to this piece of code are disabled.