Cache coherency protocol with tagged state and historical information

摘要

A cache coherency protocol uses a "Tagged" coherency state to track responsibility for writing a modified value back to system memory, allowing intervention of the value without immediately writing it back to system memory, thus increasing memory bandwidth. The Tagged state can migrate across the caches (horizontally) when assigned to a cache line that has most recently loaded the modified value. Historical states relating to the Tagged state may further be used. The invention may also be applied to a multi-processor computer system having clustered processing units, such that the Tagged state can be applied to one of the cache lines in each group of caches that support separate processing unit clusters. Priorities are assigned to different cache states, including the Tagged state, for responding to a request to access a corresponding memory block. Any tagged intervention response can be forwarded only to selected caches that could be affected by the intervention response, using cross-bars. The Tagged protocol can be combined with existing and new cache coherency protocols. The invention further contemplates independent optimization of cache operations using the Tagged state. <IMAGE>