mesi protocol

For any given pair of caches, the permitted states of a given cache line are as follows:

	M	E	S	I
M	X	X	X	O
E	X	X	X	O
S	X	X	O	O
I	O	O	O	O

Curstate	Operation	NextState	SOther Core
INVALID	Read	case EXCLUSIVE if other core is INVALID	INVALID (No operation)
		case SHARED if other core contains data	SHARED (no operation) MODIFIED -> Flush -> SHARED EXCLUSIVE -> SHARED
INVALID	Write	MODIFIED	INVALID (no operation) SHARED -> INVALID MODIFIED -> Flush -> INVALID EXCLUSIVE -> INVALID
EXCLUSIVE	Read	EXCLUSIVE	INVALID (No operation)
EXCLUSIVE	Write	MODIFIED	INVALID (No operation)
SHARED	Read	SHARED	INVALID (no operation) SHARED (no operation)
SHARED	Write	MODIFIED	INVALID (no operation) SHARED -> INVALID
MODIFIED	Read	MODIFIED	INVALID (No operation)
MODIFIED	Write	MODIFIED	INVALID (No operation)

Reference: https://github.com/Yefei100/Cache_coherence_protocol/blob/master/main.cc