Cache-coherency protocol with recently read state for data and instructions

摘要

A method of providing instructions and data values to a processing unit in a multi-processor computer system, by expanding the prior-art MESI cache-coherency protocol to include an additional cache-entry state corresponding to a most recently accessed state. Each cache of the processing units has at least one cache line with a block for storing the instruction or data value, and an indication is provided that a cache line having a block which contains the instruction or data value is in a "recently read" state. Each cache entry has three bits to indicate the current state of the cache entry (one of five possible states). A processing unit which desires to access a shared instruction or data value detects transmission of the indication from the cache having the most recently accessed copy, and the instruction or data value is sourced from this cache. Upon sourcing the instruction or data value, the cache that originally contained the most recently accessed copy thereof changes its indication to indicate that its copy is now shared, and the processing unit which accessed the instruction or data value is thereafter indicated as having the cache containing the copy thereof that was most recently accessed. This protocol allows instructions and data values which are shared among several caches to be sourced directly (intervened) by the cache having the most recently accessed copy, without retrieval from system memory (RAM), significantly improving the processing speed of the computer system.