I am more familiar with Award BIOS internal structures, not AMIBIOS but I don't think that much of changes can be done in about 100 bytes of free space you have.
I would suggest either looking for a similar board (possibly with another kind of BIOS) that has the same chipset and all the needed features to adapt it to your board's circuitry - that is usually done by changing the auto-configuration tables that specify the register values for the chipset.
Another way is doing everything in option ROM (and there are existing projects such as PLoP and XT-IDE) - that would require a larger ROM chip. Maybe even some minor board change to make another address line available. But it shouldn't require deep knowledge of BIOS internals - just push the oproms to the lower half of the ROM chip, and the new code will run during ROM scan automatically.

Here is a patch that should enable PS/2 mouse support:

8280: 2F -> 3F
DC5B: 74 -> EB

kmeaw wrote on 2024-10-16, 03:29:

Here is a patch that should enable PS/2 mouse support:

8280: 2F -> 3F

This does not work. The system will just hang in a loop after the memory test and before doing the test seek on the floppies and loading the option ROMs before boot.

kmeaw wrote on 2024-10-16, 03:29:

DC5B: 74 -> EB

That just patches the checksum routine to always act like it passed. The more sensible approach would be to update the complement value. It is in the most obvious place, namely, the last two bytes, right after the version string.

NightmareJoker2 wrote on 2024-10-16, 09:26:

That just patches the checksum routine to always act like it passed. The more sensible approach would be to update the complement value. It is in the most obvious place, namely, the last two bytes, right after the version string.

If this is AMI "color" BIOS, there's a thread on how to manually patch it, calculate and edit checksum: AMI Color BIOS (1993 and earlier) modification in hex editor

analog_programmer wrote on 2024-10-16, 13:19:

If this is AMI "color" BIOS, there's a thread on how to manually patch it, calculate and edit checksum: AMI Color BIOS (1993 and earlier) modification in hex editor

Yes, it was linked in the original post, for being an interesting thread. 😅
Changing 0x2F to 0x3F at position 0x8280 as is described there does however not work. I wish it were that easy.

NightmareJoker2 wrote on 2024-10-16, 09:26:

This does not work. The system will just hang in a loop after the memory test and before doing the test seek on the floppies and loading the option ROMs before boot.

Do you have an idea where does it hang? Or maybe you have a sequence of POST codes before and after that change?

NightmareJoker2 wrote on 2024-10-16, 18:54:

Changing 0x2F to 0x3F at position 0x8280 as is described there does however not work. I wish it were that easy.

It's not just that easy. You have to manually correct BIOS checksum bytes. I gave you the right thread - look at page 2.

analog_programmer wrote on 2024-10-16, 20:07:

It's not just that easy. You have to manually correct BIOS checksum bytes.

Or disable the check. It doesn't work. See the attachment.
If you've got a PET48PN, you can take the BIOS chip out of the socket (, dump its contents so you have a backup), carefully peel the sticker off of the EPROM window, put the chip into one of those cheap Chinese UV erasers and turn on the UV lamp for 10 minutes, and then reprogram it with your favorite EPROM programmer (I used an XGecu TL866II Plus) , and put it back in the socket.

analog_programmer wrote on 2024-10-16, 20:07:

I gave you the right thread - look at page 2.

I have read the entire thing. And I linked to it in the first post... 😔