PCI Express System Architecture

Native PCI Express Ordering Rules

PCI Express transaction ordering for native devices can be summarized with four simple rules:

1. PCI Express requires strong ordering of transactions (i.e., performing transactions in the order issued by software) flowing through the fabric that have the same TC assignment (see item 4 for the exception to this rule). Because all transactions that have the same TC value assigned to them are mapped to a given VC, the same rules apply to transactions within each VC.

2. No ordering relationship exists between transactions with different TC assignments.

3. The ordering rules apply in the same way to all types of transactions: memory, IO, configuration, and messages.

4. Under limited circumstances, transactions with the Relaxed Ordering ...