Intel® Pentium® Silver and Intel® Celeron® Processors Datasheet, Volume 1
Port 8xh Decode
The PCIe* root ports will explicitly decode and claim I/O cycles within the 80h – 8Fh range when MPC.P8XDE is set. The claiming of these cycles are not subjected to standard PCI I/O Base/Limit and I/O Space Enable fields. This allows a POST-card to be connected to the Root Port either directly as a PCI Express device or through a PCI Express to PCI bridge as a PCI card.
Any I/O reads or writes will be forwarded to the link as it is. The device will need to be able to return the previously written value, on I/O read to these ranges. BIOS must ensure that at any one time, no more than one Root Port is enabled to claim Port 8xh cycles.