What stands for mesi?

The abbreviation "MESI" stands for Modified-Exclusive-Shared-Invalid, which is a cache coherence protocol used in multiprocessor systems to maintain consistency among multiple copies of the same data in different caches. Here's an explanation of what each of these states represents:

1. Modified (M):

- A cache line is in the Modified state when it has been modified locally and contains the most up-to-date version of the data.

- Other caches that previously had copies of the same data are invalidated when a cache line enters the Modified state.

- The Modified state indicates exclusive ownership of the data, meaning no other cache currently has a valid copy.

2. Exclusive (E):

- When a cache line is in the Exclusive state, it contains a valid copy of the data, but it has not been modified locally.

- Other caches may have invalid or shared copies of the same data, but no other cache has a Modified copy.

3. Shared (S):

- A cache line is in the Shared state when multiple caches have valid copies of the same data, and none of them have modified it.

- In the Shared state, all copies of the data are consistent with each other.

4. Invalid (I):

- The Invalid state indicates that a cache line does not contain a valid copy of the data.

- When a cache line is accessed and found to be Invalid, the processor must fetch it from main memory or another cache that has a valid copy.

The MESI protocol ensures that only one cache has a Modified copy of data at a time, preventing inconsistencies between multiple cached copies. It also ensures that when data is shared among multiple caches, all copies are kept consistent through specific actions taken when state changes occur.