RCRB

General

As mentioned in "Root Complex Register Blocks (RCRBs)" on page 765, a Root Port may optionally implement a Root Complex Register Block (RCRB) as a 4KB block of memory-mapped IO registers that can include one or more of the optional PCI Express extended capabilities and other implementation specific registers that apply to the Root Complex. An RCRB must not reside in the same memory-mapped IO address space as that defined for normal PCI Express functions. Multiple Root Ports or internal devices may be associated with the same RCRB (see Figure 24-44 on page 958 for an example).

Firmware Gives OS Base Address of Each RCRB

The spec requires that the platform firmware must communicate the base address of the RCRB for each Root Port or internal device in the Root Complex to the OS. How this is accomplished is outside the scope of the spec.

Misaligned or Locked Accesses To an RCRB

A Root Complex is not required to support memory access requests to an RCRB that cross dword address boundaries or that are accomplished using a locked transaction series. Software should therefore not attempt an access of either type to an RCRB unless it is has device-specific knowledge that the Root Complex supports this access type.

Extended Capabilities in an RCRB

Any Extended Capability register sets in an RCRB must always begin at offset 0h within the RCRBs 4KB memory-mapped IO address space. If the RCRB does not implement any of the optional extended capability register sets, this is indicated by an Enhanced Capability header with a Capability ID of FFFFh and a Next Capability Offset of 0h.

The RCRB Missing Link

The 1.0a version of the spec does not identify any method for discovering the existence of RCRBs that may reside within a Root Complex, nor does it identify any method for associating an RCRB with one or more Root Ports. As of the time of this writing (6/6/03), there is a draft ECN (Engineering Change Notice) to the 1.0a spec that has not yet been approved that addresses this issue. As soon as it is approved, MindShare will immediately include this information in classes taught by MindShare and, of course, this information will be provided in the Second Edition of this book. It is not included in this edition because draft changes have a habit of mutating before they reach their finalized, approved form.