MESI (Modified/Exclusive/Shared/Invalid) extends MSI with an Exclusive state to optimize the common case of reading a line then writing to it without contention. A line loaded as Exclusive can transition to Modified on write without broadcasting an invalidate, saving a bus transaction.

MESI is the protocol implemented (in some variant) by most modern x86 and ARM CPUs, because the uncontended read-then-write pattern dominates real code.

MESI adds Exclusive to avoid redundant bus transactions.

E --[write hit, no invalidate needed]--> M   (no bus traffic)
S --[write hit, broadcast invalidate]--> M   (requires bus)