MSI protocol

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The MSI protocol is a basic cache coherence protocol that is used in multiprocessor systems. As with other cache coherency protocols, the letters of the protocol name identify the possible states in which a cache line can be. So, for MSI, each block contained inside a cache can have one of three possible states:

  • Modified: The block has been modified in the cache. The data in the cache is then inconsistent with the backing store (e.g. memory). A cache with a block in the "M" state has the responsibility to write the block to the backing store when it is evicted.

  • Shared: This block is unmodified and exists in at least one cache. The cache can evict the data without writing it to the backing store.

  • Invalid: This block is invalid, and must be fetched from memory or another cache if the block is to be stored in this cache.

These coherency states are maintained through communication between the caches and the backing store. The caches have different responsibilities when blocks are read or written, or when they learn of other caches issuing reads or writes for a block.

When a read request arrives at a cache for a block in the "M" or "S" states, the cache supplies the data. If the block is not in the cache (in the "I" state), it must verify that the line is not in the "M" state in any other cache. Different caching architectures handle this differently. For example, bus architectures often perform snooping, where...
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