MULTIPROCESSOR SYSTEM BUS PROTOCOL INCLUDING READ REQUEST HAVING FLAG INDICATING DEALLOCATION UPON SOURCING A REQUESTED MODIFIED VALUE

A novel cache coherency protocol provides a modified-unsolicited (MU) cache state to indicate that a value held in a cache line has been modified (i.e., is not currently consistent with system memory), but was modified by another processing unit, not by the processing unit associated with the cache that currently contains the value in the MU state, and that the value is held exclusive of any other horizontally adjacent caches. Because the value is exclusively held, it may be modified in that cache without the necessity of issuing a bus transaction to other horizontal caches in the memory hierarchy. The MU state may be applied as a result of a snoop response to a read request. The read request can include a flag to indicate that the requesting cache is capable of utilizing the MU state. Alternatively, a flag may be provided with intervention data to indicate that the requesting cache should utilize the modified-unsolicited state.