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Pc10-iii bios

archeocomp

Experienced Member
Joined
Apr 29, 2012
Messages
397
Location
EU/SK
Hi, did anybody did succesful disassembly of any PC10/20-III BIOSes ? I was basically trying to patch the BIOS and found that the computer is unbootable. At first I tried to get rid of message "Drive A: bad" and "Press F1 to continue". I installed HD floppy on secondary channel and this message is really annoying. But after replacing few JB intrucions with short JMP (trying to avoid the message) and programming EPROM I found the PC is unbootable. Even the VGA card did not start to generate picture/signal, the monitor remains in sleep state. So than I tried to change only one byte. I changed the message to "Drive b: bad" and tried the EPROM. The PC is still unbootable, VGA sleeps. When I program original unmodified BIOS the PC is working again. That makes me think, that at some very early stage the BIOS checks itself for changes and does not proceed any further if the checksum is not correct.
 
Last edited:

archeocomp

Experienced Member
Joined
Apr 29, 2012
Messages
397
Location
EU/SK
I gave it another (succesful) try today. I could not find the routine for consistency check but I found that the 32kB BIOS EPROM is split in three chunks. First chunk is first 16kb, 2nd is following 8kB and third chunk is the last 8kB. Each of them has 8-bit checkum 00. So I have corrected the second chunk's last byte. Well now the modded BIOS boots, the annoying message "Drive A: bad" is gone and the PC boots without interruption to DOS with 1.44MB HD floppy functioning as drive A:.
 
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