MESI stands for Modified, Exclusive, Shared, and Invalid. It is a cache coherency protocol used in multi-processor systems to maintain cache consistency.
- Modified state indicates that the cache line has been modified by the processor and is not consistent with the main memory.
- Exclusive state indicates that the cache line is only present in the processor's cache and is consistent with the main memory.
- Shared state indicates that the cache line is present in multiple caches and is consistent with the main memory.
- Invalid state indicates that the cache line is not present in any cache and is not consistent with the main memory.
When a processor needs to access a cache line, it first checks the MESI state of the cache line in its own cache. If the cache line is in the Modified state, the processor must write the cache line back to main memory before it can access it. If the cache line is in the Exclusive state, the processor can access it directly. If the cache line is in the Shared state, the processor must first acquire ownership of the cache line before it can access it. If the cache line is in the Invalid state, the processor must first load the cache line from main memory before it can access it.
The MESI protocol ensures that all processors have a consistent view of the memory system. This is important for ensuring that multiple processors can access memory safely and reliably.
