Introduction

Refer to Figure 53-1 on page 1296. The processor performs an IO Read or IO Write transaction on the FSB due to the execution of an IO instruction (IN, INS, OUT, or OUTS).

There is nothing exotic about IO transactions. Like any other transaction type, an IO transaction consists of a Request, Snoop, Response and Data Phase. The following is a summary of general IO transaction characteristics:

• Since the processors never cache information from IO space, there will never be a hit on a cache line (the caches aren't even checked).

• The only appropriate snoop results are a miss (HIT# and HITM# both deasserted), or snoop stall (both asserted) followed by a miss.

• DEFER# may be asserted by the Response Agent if it intends to issue a retry or a deferred response in the Response Phase.

• In the Response Phase, the only response that may not be issued is the implicit writeback response (because there will never be a hit on a modified IO cache line).