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).

Figure 53-1. The Execution of an IO Instruction Results in an IO Transaction

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 ...

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