MESI protocol
The MESI protocol (known also as Illinois protocol due to its development at the University of Illinois at Urbana-Champaign) is a widely used cache coherence and memory coherence protocol. It is the most common protocol which supports write-back cache.
States
Every cache line is marked with one of the four following states (coded in two additional bits):
For any given pair of caches, the permitted states of a given cache line are as follows:
Operation
In a typical system, several caches share a common bus to main memory. Each also has an attached CPU which issues read and write requests. The caches' collective goal is to minimize the use of the shared main memory.
A cache may satisfy a read from any state except Invalid. An Invalid line must be fetched (to the Shared or Exclusive states) to satisfy a read.
A write may only be performed if the cache line is in the Modified or Exclusive state. If it is in the Shared state, all other cached copies must be invalidated first. This is typically done by a broadcast operation known as Request For Ownership (RFO).
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