Method and apparatus for locking and unlocking a semaphore

摘要

A data processing system that automatically changes a semaphore in response to a test and set or clear and invalidate instruction. When a device desires to either test and set or clear and invalidate a semaphore, it transfers an instruction having a test and set or clear and invalidate operation code and the address of the semaphore over the bus. The device responsible for managing the semaphore receives the instruction and automatically changes the semaphore. Therefore, a device is only required to transfer the instruction to test and set or clear and invalidate the semaphore. Moreover, because the test and set operation requires only a single instruction transfer, special techniques are not necessary to insure exclusive access to the semaphore during the operation.