An abstract entity (not provided by hardware)
Named by a unique semaphore id.
Consists of a tuple (count, queue), where count is an integer and queue is a list of processes. The following invariant is true for semaphores: a nonnegative count means that the queue is empty; a semaphore count of negative n means that the queue contains n waiting processes.
Associated with four operations, which are described below.
The four operations are:
Wait: decrements count, and enqueues process if count is negative.
Signal: increments count and make a process ready if waiting.
Create: generates a new semaphore.
Delete: destroys an existing semaphore.
The last two operations are non-standard and allow semaphores to be created and