Research into PCI express device's configuration space on PC platform

When developing BIOS or device driver on PC Platform, it's necessary to access PCI configuration space and PCI Express extended configuration space. The register organization in PCI Express configuration space was detailed, PCI capability structure and PCI Express Extended Capability structures were emphasized here. A way of accessing device configuration space utilizing flat memory addressing was given, and its raw code was obtained on a Intel G31 chipset based PC.