Semaphores in computers?

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Semaphores in computers?triumph

http://en.wikipedia.org/wiki/Semaphore_(...

Semaphores in computers?worms

A hardware or software flag. In multitasking systems, a semaphore is a variable with a value that indicates the status of a common resource. It's used to lock the resource that is being used. A process needing the resource checks the semaphore to determine the resource's status and then decides how to proceed.

Semaphore

A signal that is used to communicate the status of a shared resource. A semaphore is commonly a positive integer from 0 and higher, each value is a different status of that resource.

Apparatus and method for semaphore initialization in a multiprocessing computer system for process synchronization

Document Type and Number: United States Patent 4316245

Link to this Page: http://www.freepatentsonline.com/4316245...

Abstract: Apparatus in a data processing system to initialize a semaphore held in a memory field of the data processing system or, alternatively, to restore the semaphore to a previous predetermined state. A count field, or tally field, provided in the semaphore is initialized by a particular instruction. The semaphore can be either a non-message semaphore or a message semaphore. The instruction initializes the semaphore count field of a non message semaphore to zero or a preloaded positive value. For a message semaphore, this instruction initializes the count field to zero. If the message semaphore previously had a positive count, the messages tied to the semaphore are released and the message links holding the messages are transferred to a free message link queue tied to the free link semaphore in the same semaphore descriptor segment. The apparatus includes a retrieving member for reading the content of the semaphore in the memory of the data processing system, a testing member for testing the count field of the semaphore for generating a first signal when the count field represents the number of processes in queue and a second signal when the count field represents the number of times a particular kind of event has occurred, a changing unit responsive to the second signal and controlled by the particular instruction for changing the number inside the count field to a predetermined number and a control member responsive to the first signal and controlled by the particular instruction for controlling the data processing system to cease execution of the particular instruction and to begin to execute a different instruction.