MESI protocol
A coherence protocol in which every cached line is Modified, Exclusive, Shared or Invalid; a core must hold a line alone before writing it.
MESI tracks each cache line as Modified (only copy, dirty), Exclusive (only copy, clean), Shared (clean, possibly in other caches) or Invalid; a write requires M or E, so writing a Shared line first invalidates all other copies (a request-for-ownership), and a read of another core's Modified line forces a write-back or transfer.
Examples
A line one core has just written is Modified in that core's cache and Invalid in every other.
See also