In the POST of the 08/16/82 and 10/27/82 BIOS ROM's are two separate routines.
- One scans for BIOS expansion ROM's in the C0000 to C7FFF range, i.e. video ROM's (e.g. ROM on EGA cards, ROM on VGA cards).
- One scans for BIOS expansion ROM's in the C8000 to F5FFF range, e.g. ROM on MFM hard drive controller, ROM on XT-IDE, etc.
Yes, with an MDA card and the 08/16/82 BIOS ROM fitted, the ROM on my XT-IDE card displays its banner.
But something is wrong in regard to video ROM's.
As an experiment, I modified the BIOS expansion ROM on my IBM EGA card to do only only one thing, to send 55h to a
LPT/parallel reader device (then halt the CPU).
As expected, with the 10/27/82 BIOS ROM fitted to the motherboard, I saw '55' appear on the LPT/parallel reader device (a few seconds after power-on).
I pulled out the 10/27/82 BIOS ROM then put in the 08/16/82 BIOS ROM - no '55'.
I put the 10/27/82 BIOS ROM back in, and '55' appeared.
Then, an accidental discovery. At some point in further testing the 08/16/82 BIOS ROM, I accidentally had the motherboard video switches set to MDA. About seven seconds after power-on, I saw an underline cursor appear on my EGA display.
So the EGA video ROM is being detected, and its initialisation code called, if the motherboard video switches are set to MDA (which is expected), but
not if the motherboard video switches are set to EGA.
Later, I will take a look at the 08/16/82 BIOS ROM code to see where the bug is.