المحاضرة التاسعة-نظم تشغيل 1

Semaphores
A semaphore S is an integer variable that, apart from initialization, is accessed only through two standard atomic operations: wait( ) and signal( ).
The wait() operation was originally termed P (from the Dutch proberen, “to test”); signal( ) was originally called V (from verhogen, “to increment”). The definition of wait( ) is as follows:

wait(S) {
while S <= 0 ; // no-op
S--;
}
The definition of signal ( ) is as follows:

signal(S) {
S++;
}
All modifications to the integer value of the semaphore in the wait( ) and signal( ) operations must be executed indivisibly. That is, when one process modifies the semaphore value, no other process can simultaneously modify that same semaphore value. In addition, in the case of wait(S), the testing of the integer value of S (S ? 0), as well as its possible modification (S--), must be executed without interruption.
Semaphores
A semaphore S is an integer variable that, apart from initialization, is accessed only through two standard atomic operations: wait( ) and signal( ).
The wait() operation was originally termed P (from the Dutch proberen, “to test”); signal( ) was originally called V (from verhogen, “to increment”). The definition of wait( ) is as follows:

wait(S) {
while S <= 0 ; // no-op
S--;
}
The definition of signal ( ) is as follows:

signal(S) {
S++;
}
All modifications to the integer value of the semaphore in the wait( ) and signal( ) operations must be executed indivisibly. That is, when one process modifies the semaphore value, no other process can simultaneously modify that same semaphore value. In addition, in the case of wait(S), the testing of the integer value of S (S ? 0), as well as its possible modification (S--), must be executed without interruption.