Write in invalid state of MESI protocol

Question

How is the write operation for a memory location that's not in the cache handled in the MESI protocol? The state diagrams i have seen mark it as Write Miss but i can't follow what happens in reality.

I think this results in a load operation on the bus to ensure that the processor trying to do the write gets exclusive access to the location and then the block is modified. Is this how it's done in reality or is the handling of write in invalid state implementation defined?

Answer 1

If the policy is allocate on a write miss:

If the block was not present in any other caches but only main memory, the block is fetched into the cache first, marked as M (modified) state, and then the write proceeds.

If the block was present in some other caches, it's copy in the other caches is first invalidated, so that this cache gains the only copy of the block, and then the write proceeds.

If the policy is no allocate on write miss: all write misses go directly to main memory. A copy is not fetched into the cache. If the main memory does not have the only copy of the block (some other cache has a copy), then the other copies are first invalidated and the write takes place in main memory.