MeSi

MESI is a cache coherence protocol used in shared-memory multiprocessor computer architectures to maintain coherence of data stored in multiple CPU caches. It extends the MSI protocol by introducing an Exclusive state, allowing a cache line to be clean and owned by a single cache, reducing unnecessary bus traffic. MESI is widely used in modern multi-core and multi-processor systems and is commonly implemented with a snooping bus or a directory-based coherence mechanism.

The four states in MESI are Modified, Exclusive, Shared, and Invalid. Modified means the cache line is dirty and the only copy resides in that cache; the data differs from memory and must be written back before eviction. Exclusive means the line is clean, matches memory, and is the only copy in the system. Shared means the line is clean and may be stored in several caches. Invalid means the line is not present in the cache.

State transitions are driven by processor requests and bus transactions. A read miss in I may bring the line into E (if no other cache has it) or S (if another cache holds it). A read in E or S generally leaves the line in E or S, respectively, with other caches informed as needed. A write in E moves the line to M. A write in S requires an invalidate broadcast and typically upgrades to M in the requesting cache. If another cache requires a line currently in M, that cache must write back the dirty data and invalidate the M copy, allowing coherence to be maintained.

MESI is implemented in many systems through snooping or directory-based mechanisms and is related to other coherence schemes such as MSI and MOESI.