Each Function Implements a Set of Configuration Registers

Introduction

At the behest of software executing on the processor, the Root Complex initiates configuration transactions to read from or write to a function's configuration registers. These registers are accessed to discover the existence of a function as well as to configure it for normal operation. In addition to memory, IO, and message space, PCI Express also defines a dedicated block of configuration space allocated to each function within which its configuration registers are implemented.

Function Configuration Space

Refer to Figure 19-3 on page 717. Each function's configuration space is 4KB in size and is populated as described in the following two subsections.

PCI-Compatible Space

The 256 byte (64 dword) PCI-compatible space occupies the first 256 bytes of this 4KB space. It contains the function's PCI-compatible configuration registers. This area can be accessed using either of two mechanisms (both of which are described later):

• The PCI configuration access mechanism (see "PCI-Compatible Configuration Mechanism" on page 723).

• The PCI Express Enhanced Configuration mechanism (see "PCI Express Enhanced Configuration Mechanism" on page 731).

The first 16 dwords comprises the PCI configuration header area, while the remaining 48 dword area is reserved for the implementation of function-specific configuration registers as well as PCI New Capability register sets. It is mandatory that each PCI Express function must implement the PCI Express Capability Structure (defined later) within this area. A full description of the PCI-compatible registers may be found in "PCI Compatible Configuration Registers" on page 769.

PCI Express Extended Configuration Space

The remaining 3840 byte (960 dword) area is referred to as the PCI Express Extended Configuration Space. It is utilized to implement the optional PCI Express Extended Capability registers:

• Advanced Error Reporting Capability register set.

• Virtual Channel Capability register set.

• Device Serial Number Capability register set.

• Power Budgeting Capability register set.

A full description of the these optional register sets may be found in "Express-Specific Configuration Registers" on page 893.