Abstract: 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.

Abstract: A cache coherency method and system are provided for ensuring coherency of accessed data for each bus master of a plurality of bus masters in a processing system, wherein at least some bus masters have a cache means connected to a system bus, which provides communication to a main memory for access of data stored therein. Each of these at least some bus masters also includes snoop monitor logic, e.g., residing within a bus interface unit (BIU), for monitoring the presence of a coherent memory transaction on the system bus and for broadcasting in response thereto a unidirectional snoop response signal with reference to the bus master's caching means whenever the coherent memory transaction is initiated by other than that bus master. The snoop monitors are electrically interconnected, with each snoop monitor receiving at a separate signal input the unidirectional snoop response signal broadcast by each other snoop monitor of the plurality of snoop monitors.