MESI protocol

The MESI protocol (known also as Illinois protocol) is a widely used cache coherency and memory coherence protocol, which was later introduced by Intel in the Pentium processor to "support the more efficient write-back cache in addition to the write-through cache previously used by the Intel 486 processor".

Every cache line is marked with one of the four following states (coded in two additional bits):

M - Modified: Indicates that this cache line was modified and therefore the underlying data (e.g. the main memory) is no longer valid.
E - Exclusive: Indicates that this cache line is only stored in this cache and hasn't been changed by a write access yet.
S - Shared: Indicates that this cache line may be stored in other caches of the machine.
I - Invalid: Indicates that this cache line is invalid.