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Snoopy Bus Protocols

Cache consistency by bus watching mechanism

• Each cache watches bus for write operations
• Write through: forward write immediately to memory
  • Write-invalidate: cache copies are invalidated.
  • Write-update: cache copies are updated.
  • Disadvantage: high bus traffic.
• Copy back: write modified cache line only back if
  1. Line needs to be replaced, or
  2. Another processor reads data.

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