Refer to Figure 24-2 on page 897. Otherwise referred to as the PCI Express Capability Structure, implementation of the PCI Express Capability register set is mandatory for each function. It is implemented as part of the linked list of Capability register sets that reside in the lower 48 dwords of a function's PCI-compatible register area. It should be noted however, that some portions of this register set are optional.
Register implementation requirements:
Every Express function must implement the registers that reside in dwords 0-through-4.
The bridge ...