PCI Express Root Complex

A Root Complex (RC) denotes the root of an I/O hierarchy that connects the CPU/memory subsystem to the I/O.
As illustrated in Figure 1-2, a Root Complex may support one or more PCI Express Ports. Each interface defines a separate hierarchy domain. Each hierarchy domain may be composed of a single Endpoint or a sub-hierarchy containing one or more Switch components and Endpoints.
The capability to route peer-to-peer transactions between hierarchy domains through a Root Complex is optional and implementation dependent. For example, an implementation may incorporate a real or virtual Switch internally within the Root Complex to enable full peer-topeer support in a software transparent way.
Unlike the rules for a Switch, a Root Complex is generally permitted to split a packet into smaller packets when routing transactions peer-to-peer between hierarchy domains (except as noted below), e.g., split a single packet with a 256-byte payload into two packets of 128 bytes payload each. The resulting packets are subject to the normal packet formation rules contained in this specification (e.g., Max_Payload_Size, Read Completion Boundary, etc.). Component designers should note that splitting a packet into smaller packets may have negative performance consequences, especially for a transaction addressing a device behind a PCI Express to PCI/PCI-X bridge.
Exception: A Root Complex that supports peer-to-peer routing of Vendor_Defined Messages is not permitted to split a Vendor_Defined Message packet into smaller packets except at 128-byte boundaries (i.e., all resulting packets except the last must be an integral multiple of 128 bytes in length) in order to retain the ability to forward the Message across a PCI Express to PCI/PCI-X Bridge. Refer to the PCI Express to PCI/PCI-X Bridge Specification, Revision 1.0 for additional information.
A Root Complex must support generation of configuration requests as a Requester.
A Root Complex is permitted to support the generation of I/O requests as a Requester.
A Root Complex must not support Lock semantics as a Completer.
A Root Complex is permitted to support generation of Locked Requests as a Requester.

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