PCI-X constitutes an extensive set of enhancements to the basic PCI protocol. These include:

• Revised command protocol emphasizing a distinction between DWORD and burst transactions.

• An Attribute phase in each transaction to identify the initiator of the transaction.

• Split transactions whereby a request is completely decoupled from the response. PCI-X Split Transactions replace conventional PCI's Retry.

• An error correction protocol that allows single bit errors to be corrected automatically.

• Source synchronous data transfers that yield up to 4 Gbytes of bandwidth. Source synchronous transfers implement either two or four data subphases per clock cycle.

• Device ID Messaging that allows a device to talk directly to another device outside of the normal bus address spaces. The target of such a transaction may be explicitly specified by bus, device and function number or implicitly as the host bridge.

• A 16-bit interface optimized for embedded applications.

The next chapter looks at configuration and initialization issues in PCI-X.