Reducing bandwidth and areas needed for non-inclusive memory hierarchy by using dual tags

摘要

An apparatus and method for optimizing a non-inclusive hierarchical cache memory system that includes a first and second cache for storing information. The first and second cache are arranged in an hierarchical manner such as a level two and level three cache in a cache system having three levels of cache. The level two and level three cache hold information non-inclusively, while a dual directory holds tags and states that are duplicates of the tags and states held for the level two cache. All snoop requests (snoops) are passed to the dual directory by a snoop queue. The dual directory is used to determine whether a snoop request sent by snoop queue is relevant to the contents of level two cache, avoiding the need to send the snoop request to level two cache if there is a "miss" in the dual directory. This increases the available cache bandwidth that can be made available by second cache since the number of snoops appropriating the cache bandwidth of second cache are reduced by the filtering effect of dual directory. Also, the third cache is limited to holding read-only information and receiving write-invalidation snoop requests. Only snoops relating to write-invalidation requests are passed to a directory holding tags and state information corresponding to the third cache. Limiting snoop requests to write invalidation requests minimizes snoop requests to third cache, increasing the amount of cache memory bandwidth available for servicing catch fetches from third cache. In the event that a cache hit occurs in third cache, the information found in third cache must be transferred to second cache before a modification can be made to that information.