mkarcher wrote on 2020-10-09, 23:30:
No idea why they included a memory test for the non-EGA card into the EGA ROM POST routine, but they did...
On the XT motherboard, you're supposed to set SW5&6 to the "no video card" option when installing the EGA. Those switches tell the XT BIOS not to initialize a video adapter, which means that the XT BIOS also skips past the video card presence test. If you didn't do this, and the only video adapter installed was an EGA, the XT BIOS would fail the video test and beep an error. So when the EGA BIOS gets control, it has to decide (by examining its switch configuration) whether or not to try and initialize an MDA or CGA adapter. If the EGA BIOS doesn't do it, then any MDA/CGA card would remain uninitialized.
Strange things happen when SW5&6 are set to MDA or CGA but yet there is an EGA card installed. In this case, the XT BIOS will initialize the MDA/CGA card and then pass control to the EGA BIOS (because the scan for a video bios in C000-C7FF range is unconditional).
At which point shenanigans might happen because the EGA BIOS doesn't expect the MDA/CGA to already be up and running -- not sure exactly what does / does not go wrong in this scenario.