All transactions, including non-memory transactions, have a Snoop Phase. For non-memory transactions, however, there are only three valid snoop responses:
A snoop miss (HIT# and HITM# both deasserted).
Defer (DEFER#) asserted, indicating that the currently-addressed target will issue a Retry or a Deferred response in the transaction's Response Phase. In this case, the snoop result is irrelevant (unless HITM# is asserted, cancelling the deferral).
A snoop stall (HIT# and HITM# both asserted), indicating that the Snoop Phase is to be extended by two clocks. A non-memory Response Agent is permitted to stall the snoop response to give itself more time for internal operations to complete before presenting a miss or defer as the snoop result.
The non-memory transaction types are the Deferred Reply, Interrupt Acknowledge, Special, Branch Trace Message, IO Read, and IO Write transactions. For more information on these transaction types, refer to “Pentium® 4 FSB Transaction Deferral” on page 1277, “Pentium® 4 FSB IO Transactions” on page 1295, and “Pentium® 4 FSB Central Agent Transactions” on page 1301.
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