Refer to:

http://en.wikipedia.org/wiki/MOESI_protocol

http://en.wikipedia.org/wiki/MESI_protocol

MOESI protocol

In computing, MOESI is a full
cache coherency protocol that encompasses all of the possible states commonly used in other protocols. In addition to the four common
MESI protocol states, there is a fifth "Owned" state representing data that is both modified and shared. This avoids the need to write modified data back to main memory before sharing
it. While the data must still be written back eventually, the write-back may be deferred.

In order for this to be possible, direct cache-to-cache transfers of data must be possible, so a cache with the data in the modified state can supply that data to another reader without transferring it to memory.

As discussed in AMD64 Architecture Programmer's Manual Vol 2 'System Programming',^[1] each

cache line is in one of five states:

Modified

A cache line in the modified state holds the most recent, correct copy of the data. The copy in main memory is stale (incorrect), and no other processor holds a copy. The cached data may be modified at will. Modified cache lines must respond to a snoop
request with data, and change to the Owned state.

Owned

A cache line in the owned state holds the most recent, correct copy of the data. The introduction of owned state allows dirty sharing of data, i.e, a modified cache block can be moved around various caches without updating main memory. The owned state is
similar to the shared state in that other processors can hold a copy of the most recent, correct data. The copy in main memory can be stale (incorrect), and the owner "owns" the responsibility of eventually writing the modifications back to main memory. Only
one processor can hold the data in the Owned state—all other processors must hold the data in the Shared state. The cache line may be changed to the Modified state after invalidating all shared copies, or changed to the Shared state by writing the modifications
back to main memory. Owned cache lines must respond to a snoop request with data.

Exclusive

A cache line in the exclusive state holds the most recent, correct copy of the data. The copy in main memory is also the most recent, correct copy of the data. No other processor holds a copy of the data. The cache line may be changed to the Modified state
at any time in order to modify the data. It may also be discarded (changed to the Invalid state) at any time. In response to a snoop request, the cache line must change to the Shared state, and
may respond with data, or the request may be satisfied by the memory copy.

Shared

A cache line in the shared state holds the most recent, correct copy of the data. Other processors in the system may hold copies of the data in the Shared state, as well. Unlike the MESI protocol, a shared cache line
may be dirty with respect to memory; if it is, some cache has a copy in the Owned state, and that cache is responsible for eventually updating main memory. If no cache hold the line in the Owned state, the memory copy is up to date. The cache line
may not be written, but may be changed to the Exclusive or Modified state after invalidating all shared copies. (If the cache line was Owned before, the invalidate response will indicate this, and the state will become Modified, so the obligation to eventually
write the data back to memory is not forgotten.) It may also be discarded (changed to the Invalid state) at any time. Shared cache lines may not respond to a snoop request with data.

Invalid

A cache line in the invalid state does not hold a valid copy of the data. Valid copies of the data might be either in main memory or another processor cache.

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

(The order in which the states are normally listed serves only to make the acronym "MOESI" pronounceable.)

This protocol, a more elaborate version of the simpler
MESI protocol, avoids the need to write a dirty cache line back to
main memory when another processor tries to read it. Instead, the Owned state allows a processor to supply the modified data directly to the other processor. This is beneficial when the communication latency and bandwidth between two CPUs is significantly
better than to main memory. An example would be multi-core CPUs with per-core L2 caches.

While MOESI can quickly share dirty cache lines from cache, it cannot quickly share clean lines from cache. If a cache line is clean with respect to memory and in the shared state, then any snoop request to that cache line will be filled from memory, rather
than a cache.

If a processor wishes to write to an Owned cache line, it must notify the other processors that are sharing that cache line. Depending on the implementation it may simply tell them to invalidate their copies (moving its own copy to the Modified state), or
it may tell them to update their copies with the new contents (leaving its own copy in the Owned state).