MESI Cache Coherence

Cache-line state machine — Modified / Exclusive / Shared / Invalid.

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stateDiagram-v2
[*] --> Invalid
Invalid --> Exclusive: read miss (no other copy)
Invalid --> Shared: read miss (other copy)
Exclusive --> Modified: write
Exclusive --> Shared: snoop read
Shared --> Modified: write (invalidate others)
Shared --> Invalid: snoop write
Modified --> Shared: snoop read (write-back)
Modified --> Invalid: snoop write (write-back)