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PCI Express System Architecture by MindShare, Inc, Ravi Budruk, Don Anderson, Tom Shanley

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Relaxed Ordering

PCI Express supports the Relaxed Ordering mechanism introduced by PCI-X; however, PCI Express introduces some changes (discussed later in this chapter). The concept of Relaxed Ordering in the PCI Express environment allows switches in the path between the Requester and Completer to reorder some transactions just received before others that were previously enqueued.

The ordering rules that exist to support the Producer/Consumer model may result in transactions being blocked, when in fact the blocked transactions are completely unrelated to any Producer/Consumer transaction sequence. Consequently, in certain circumstances, a transaction with its Relaxed Ordering (RO) attribute bit set can be re-ordered ahead of other transactions. ...

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