Alright, I found the
original CL-GD5429 VGA BIOS and compared it with the one from Book8088. Purely hex dump at this point. See below.
A few observations:
- First of all, the copyright string starts with 0x00 in the original VGA BIOS, and with 0x80 in the Book8088 VGA BIOS. Perhaps this difference is the flag that tells not to print the copyright string?
- Book8088 VGA BIOS has "STB Systems, Inc" string in it, while the original BIOS doesn't have that. Could be just a vendor-specific modification, it probably wasn't introduced by Book8088 developers
- At 0x103D, the original VGA BIOS has 0x01 and Book8088 VGA BIOS has 0x00, perhaps another flag?! No idea
- At 0x1F97, the original VGA BIOS has the "Cirrus Logic GD-5429 VGA" string, the Book8088 VGA BIOS has "STB Vision" followed by a few garbage?! bytes and then "-5429 VGA". Could be some ID string returned by vendor specific INT 10h call... or not...
- 0x7FFF value is different, but that just used for the checksum correction.
Perhaps an interesting experiment would be to use the original CL-GD5429 BIOS. But don't break your system over that
Regarding extending the UMBs, it will require patching the CPLD, as it generates the RAM chip selects. Also it would need some wires to wire the missing address lines to the second 512 KiB SRAM, that would replace 128 KiB SRAM that is currently on the board... That is unless the designers already routed these signals.