GOP

The second part of the BIOS is called the (UEFI 2.0) GOP driver (EFIRom compressed) and it is (in our context) mainly responsible together with OpenCore to enable the internal LCD of an iMac on boot to show an emulated (OC or OpenCore) boot picker (after fixing the OBJ_INFO connector information).

You can leave the GOP part unchanged as long as it works with OpenCore as described, otherwise one can replace it completely with another GOP from a known to work GPU (even from a different chip type). Rarely you will find a third part containing microcode after the 0x20000 offset. Do not delete this data, do not change it. You may just make the complete BIOS unusable and brick the card flashing such a crippled vBIOS. The S7100X is such an example.

In case the AMD card comes without a GOP one can use this tool to add a GOP automatically or one can copy and paste a GOP using a hex editor from a similar generation AMD card (in fact I believe we can use the very same GOP for all cards listed).

So very first step will be adjusting the output connector table (OBJ_INFO module) within the legacy BIOS part. This is in general necessary because the PC cards have been used in Laptops with different hardware and different video ports. We need at least one connector of type eDP with correct parameters, and another of type DP to drive the external (miniDP) connected monitor. Only in a single case the BIOS came already correctly configured.

More details to follow ...