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 coherency and memory coherence protocol. It is the most common protocol which supports write-back cache. Its use in personal computers became widespread with the introduction of Intel s Pentium processor to "support the more efficient write-back cache in addition to the write-through cache previously used by the Intel 486 processor"[1].