Cache tag caching

A computer system comprising:

• a processor;
• a main memory;
• a multi-level cache system further comprising:
• a first-level cache comprising a first-level cache tag store, a first-level data store having a plurality of data storage lines for storing data, and a first-level cache controller, the first-level cache controller implementing a replacement algorithm whereby data stored in data storage lines of the first-level data store is selected for replacement, the first level cache residing on a first silicon chip;
• a second-level cache comprising a second-level cache tag store, a second-level cache tag cache, a second-level cache controller, and a second-level data store, the second level cache residing on the first silicon chip;
• and
• a third level cache comprising a third-level cache tag store, a third-level data store and a third-level cache controller, the third level cache being coupled to the second-level cache tag cache and residing on a second silicon chip;
• wherein a line of the second level cache tag cache is updated with tag data from the first level cache tag store when the first-level cache controller selects data storage lines of the first-level data store for replacement.

A method of caching data in a data processing system having a main memory, said method comprising:

• providing a first-level cache comprising a first-level cache tag store, a first-level data store, and a first-level cache controller on a first silicon chip;
• providing a second-level cache comprising a second-level cache tag store, a second-level cache, cache tag cache, a second-level cache controller, and a second-level data store on the first silicon chip;
• providing a third level cache comprising a third-level cache tag store, a third-level data store and a third-level cache controller, the third level cache being coupled to the second-level cache tag cache and residing on a second silicon chip;
• determining if requested data is present in the first or second-level data store based upon a read-request address from a processor of the data processing system and information read from the first or second-level cache tag store;
• evicting data as necessary from the data storage line of the first or second-level cache to make room for the requested data;
• if the requested data is not in the first or second-level data store, fetching a tag from a store of a group consisting of the second-level cache tag cache and the third-level cache tag store to determine if the requested data is in the third-level cache;
• if the requested data is not in the first or second-level data store, and the requested data is found to be in the third-level cache, fetching the requested data to the processor of the data processing system and to a data storage line of the first or second-level cache;
• if the requested data is not in the first or second-level data store, and if the requested data is found not to be in the third-level cache, fetching the requested data from a store other than the third-level cache to the processor of the data processing system and to a data storage line of the first or second-level cache;
• and
• if data is evicted from the data storage line, copying a tag of the first or second-level cache corresponding to the evicted data to the cache tag cache of the second-level cache.