Editing MESI protocol (section)

== States ==
The letters in the acronym MESI represent four exclusive states that a cache line can be marked with (encoded using two additional [[bit]]s):

;Modified (M): The cache line is present only in the current cache, and is ''dirty'' - it has been modified (M state) from the value in [[main memory]].  The cache is required to write the data back to the main memory at some time in the future, before permitting any other read of the (no longer valid) main memory state.  The write-back changes the line to the Shared state (S).
;Exclusive (E): The cache line is present only in the current cache, but is ''clean'' - it matches main memory.  It may be changed to the Shared state at any time, in response to a read request.  Alternatively, it may be changed to the Modified state when writing to it.
;Shared (S): Indicates that this cache line may be stored in other caches of the machine and is ''clean'' - it matches the main memory.  The line may be discarded (changed to the Invalid state) at any time.
;Invalid (I): Indicates that this cache line is invalid (unused).

For any given pair of caches, the permitted states of a given cache line are as follows:
{|class=wikitable style=text-align:center
!   !! &nbsp;M&nbsp; !! &nbsp;E&nbsp; !! &nbsp;S&nbsp; !! &nbsp;I&nbsp;
|-
! &nbsp;M&nbsp;
|      {{N}} || {{N}} || {{N}} || {{Y}}
|-
! &nbsp;E&nbsp;
|      {{N}} || {{N}} || {{N}} || {{Y}}
|-
! &nbsp;S&nbsp;
|      {{N}} || {{N}} || {{Y}} || {{Y}}
|-
! &nbsp;I&nbsp;
|      {{Y}} || {{Y}} || {{Y}} || {{Y}}
|}
When the block is marked M (modified) or E (exclusive), the copies of the block in other Caches are marked as I (Invalid).