The Unabridged Pentium 4 IA32 Processor Genealogy

The Data Transfer Length

Behavior Permitted by the Spec

When an IO read or write transaction is initiated, the data transfer length is output by the Request Agent in request packet B (see Table 49-8 on page 1219). The spec permits IO data transfer lengths of:

• A qword or less. Any combination of byte enables are valid, including none.

• Two full qwords. All byte enables must be asserted in request packet B.

• Four full qwords. All byte enables must be asserted in request packet B.

• Eight full qwords. All byte enables must be asserted in request packet B.

On a 0-byte read, the response must be the no data response (unless DEFER# is asserted by the Response Agent, indicating that it intends to retry or defer the transaction).

On a 0-byte write, the ...