[emily-c]

You're still relying pretty heavily on the BIOS to write to this region that you otherwise wouldn't be. Getting the legacy framebuffer region set up to be decoded to the graphics card is using some hacks to get it to work over PCI[e]. The memory controller/PCI root complex needs to be configured to forward transactions to the the legacy A and B segments to the PCI bus using a proprietary bit to enable that MMIO hole. To enable correct decoding of that, all PCI bridges on the path to the graphics device need to be specially marked in their config space bridge control as being "VGA compatible" so that legacy framebuffer hole requests and VGA IO ports can ultimately be forwarded to the graphics device. This needs to exist because the root ports/host bridges in the system base and limit need to encompass the addresses of their children as well as needing to exist in a hole that the memory controller/root complex knows to forward requests down to PCI.

[jng]

Thanks for the very complete explanation. It makes sense that this would work this way.