MESIprotokollen

MESI is a cache coherence protocol used in multiprocessor systems. It stands for Modified, Exclusive, Shared, and Invalid, which are the four states a cache line can be in. These states ensure that all processors have a consistent view of memory, preventing data corruption.

The Modified state indicates that the cache line has been modified by the current processor and is not present in any other cache or main memory in its up-to-date form. The Exclusive state means the cache line is clean (has not been modified) and is present only in this processor's cache. The Shared state signifies that the cache line is clean and may be present in multiple other caches as well. The Invalid state indicates that the cache line's contents are no longer valid and must be fetched from main memory or another cache before use.

When a processor accesses data, its cache controller checks the state of the corresponding cache line. If it's invalid, the data must be fetched. If it's shared, the data can be read, but if a write operation is intended, the protocol ensures that other caches holding the line are notified and invalidate their copies. If the line is exclusive, the processor can modify it directly. If the line is modified, the processor can also modify it directly, but it must write the modified data back to main memory before another processor can access it, or invalidate other copies.

MESI relies on bus snooping, where cache controllers monitor the system bus for memory transactions initiated by other processors. This allows them to update the state of their own cache lines accordingly. For example, if a processor snoops a write operation to a cache line it holds in the Shared state, it will transition that line to the Invalid state. MESI is a widely adopted protocol due to its efficiency in managing cache coherence.