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In computing, cache coherence (also cache coherency) refers to the consistency of data stored in local caches of a shared resource. Cache coherence is a special case of memory coherence.

Coherency protocol

MESI protocol

In MESI protocol, there are four cache block status:

• 1. Modified (M): the cache block valid in only one cache and the value is like different from the main memory.
• 2. Exclusive (E): the cache block is valid and clean, but only resides in one cache.
• 3. Shared (S): the cache block is valid and clean, but may exist in multiple caches.
• 4. Invalid (I): the cache block is invalid.

MOESI protocol

AMD

AMD Opteron is using MOESI protocol for cache sharing.

References

1. Cache Coherence
2. Cache consistency & MESI Intel
3. A closer look at AMD's dual-core architecture
4. CSE 400 – Related Work: Instructions & Example
5. Trace-Driven Simulation of the MSI, MESI and Dragon Cache Coherence Protocols
6. Understanding the Detailed Architecture of AMD's 64 bit Core
7. MSI,MESI,MOESI sheet