PCA L13 Chp4.5
Wu Spring 09 ©USTC
Reporting Snoop Results: How?
° Collective response from $’s must appear on bus
° Example: in MESI protocol, need to know
• Is block dirty; i.e. should memory respond or not?
• Is block shared; i.e. transition to E or S state on read miss?
° Three wired-OR signals
• Shared: asserted if any cache has a copy
• Dirty: asserted if some cache has a dirty copy
- needn’t know which, since it will do what’s necessary
• Snoop-valid: asserted when OK to check other two signals
- actually inhibit until OK to check
° Illinois MESI requires priority scheme for cache-to-
cache transfers
• Which cache should supply data when in shared state?
• Commercial implementations allow memory to provide data
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