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Figure 8.22
Cache state transitions for Illinois cache coherency protocol. (Also see
Appendix F.) |
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Private Dirty state. Cache lines in the Read Shared state do not have a specific owner. The Illinois approach enables the determination of whether or not a cache line is shared when it is loaded. A modified line (Private Dirty) is forwarded directly to the requestor from the cache containing it. This eliminates the need to send out invalidations for cache lines which have not been shared. See Figure 8.22 for the state transition diagram. |
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The Illinois and similar four-state shared protocols are sometimes called MESI protocols, as per the states: modified, exclusive, shared, invalid. |
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When a read miss or write miss occurs, the missing line is first retrieved from other caches, and if none possesses it, from the main memory. If the line is dirty, it is also written back to memory at the same time. If the cache line is shared, the cache with the highest priority supplies the line. On read miss, if the line is supplied by another cache then it and all other caches |
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