MESI protocol
Beginner
The rules cores use to keep their copies of shared data in agreement. Each copy is marked changed, only copy, shared, or empty.
Novice
A cache-coherence protocol with four states per line: Modified (only copy, changed), Exclusive (only copy, clean), Shared (one of several clean copies) and Invalid. A core must hold a line in M or E to write it, which invalidates other copies.
Expert
Write-invalidate protocol enforcing single-writer/multiple-reader per line. E lets private data be written without a bus transaction; MOESI adds Owned so a dirty line can be shared without writing it back. Real implementations add many transient states.
Explained in Caches and coherence (Architectures).
See also: Cache coherence, Snooping (cache coherence).