There are several cache coherence protocols used in computational architecture:
1. MESI (Modified, Exclusive, Shared, Invalid): This is a widely used protocol that maintains cache coherence by defining four states for cache lines: Modified, Exclusive, Shared, and Invalid.
2. MOESI (Modified, Owned, Exclusive, Shared, Invalid): A modification to MESI that adds the Owned state. In this protocol, a cache line that is owned by a processor may still be cached by other processors in the Shared state.
3. MOSI (Modified, Owned, Shared, Invalid): A variation of the MOESI protocol that eliminates the Exclusive state.
4. MSI (Modified, Shared, Invalid): A simplified version of MESI that does not have an Exclusive state. In this protocol, a cache line is either Modified, Shared, or Invalid.
5. MESIF (Modified, Exclusive, Shared, Invalid, Forward): This is an extension of the MESI protocol that adds a Forward state. In this state, a cache line is forwarded to another processor without waiting for a cache miss.
6. Dragon (Directory-based Read and Atomicity Generation): A directory-based protocol that keeps track of the state of cache lines in a central directory. This protocol is designed to reduce the amount of traffic on the interconnect by forwarding updates only to processors that need them.
7. Firefly: An extension of the Dragon protocol that adds support for speculative execution. In this protocol, a processor can make speculative updates to cache lines without waiting for confirmation from the directory.
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