The MESI cache coherency protocol is a popular protocol followed by Intel Pentium 4 and the PowerPC. In this scheme, each cache line can have the four following states:
Modified (M): 'The cache line is modified (dirty) i.e. it is not similar to main memory.
Exclusive (E): The cache line is 'clean' and not present in any other cache memory.
Shared (S): The cache line is 'clean' but may be present in some cache also.
Invalid (I): The cache line does not contain a valid data.
Two bits in the tag are used to indicate the previous four states.