The Unabridged Pentium 4 IA32 Processor Genealogy

The FSB is Tuned for Multiprocessing

The Pentium® 4 FSB protocol prevents bus stalls in three basic ways:

1. Up to 12 transactions pending. In a typical Pentium® 4 FSB environment, up to 12 transactions can be currently outstanding at various stages of completion (i.e. the IOQ depth is 12).

2. Retry. If the target of the transaction cannot deal with a transaction immediately, rather than tie up the FSB by inserting wait states, it issues a Retry Response to the initiator. This causes the initiator to wait a little while and then retry the transaction again at a later time. This frees up the FSB for other initiators. A classic example would be an attempted memory write to a full Posted Memory Write buffer in the Root Complex.

3. Deferred transaction