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XT clone's BIOS suddenly breaks keyboard input

Mike Chambers

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Sep 2, 2006
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i have a BMI XT clone, and all of a sudden, after no changes in hardware or software, the keyboard input in DOS stopped working. (this is both on a fresh floppy boot, or a hard drive boot.. any DOS version i try doesn't work with the keyboard, and i've tried it with 3 keyboards)

i considered the mobo dead for quite a while, thinking maybe i could replace the keyboard controller eventually and see what happens. well last night i brought the beast back out and just for kicks burned an EEPROM of that very-tweaked taiwanese XT clone BIOS from phatcode.net and put it on the motherboard. presto, the keyboard magically works again.

now, i can use the machine with this new BIOS firmware, so i'm not complaining -- but, i just have to know.. does anybody have ANY possible explanation for this? it is totally non-sensical, at least to me! some latent bug in the original BIOS maybe? okay maybe, but then why would it just happen all of a sudden after working with the BIOS fine since 1986?

EDIT: i also temporarily swapped the BIOS from my real XT onto the clone as well, and the keyboard worked with that also.
 
Don't know exactly, as I don't know what your XT clone's implementation is. But there's no "controller" per se ion an XT motherboard--just a shift register (74LS322) and the 8255 PPI and a bit of random logic.
 
yeah, i was referring to the PPI loosely as the "controller" ... this is strange. very strange. maybe i'll swap the PPI with one from another board and see what happens with the original BIOS, but i have no idea what the other BIOSes might be doing differently.

the motherboard is a "PC II" motherboard with a copyright date of 1986.
 
i have a BMI XT clone, and all of a sudden, after no changes in hardware or software, the keyboard input in DOS stopped working.
but, i just have to know.. does anybody have ANY possible explanation for this?
The original BIOS ROM is starting to drop bits, and the first bit to go was associated with keyboard processing.

If, and if, the POST is looking for a 8-bit checksum of 00 for the BIOS ROM contents, maybe:
a) The POST only checks part of the BIOS ROM, or
b) Multiple bits have flipped such that the checksum is still 00.
 
good point. ive got an image file i dumped of the BIOS back when it was still working, maybe i should re-dump and compare. if it is indeed losing bits, it must be in the int 16h handler, not int 9. ctrl-alt-del still works.
 
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not sure what i did with it after downloading it, it was a while ago but i should still have it in my e-mail inbox. i'll have a look. how large was it? the largest ROMs i can burn are 8 KB. i have no real dedicated ROM burners, so i cheat and use my XT-IDE card with aitotat's flasher. :)

i bought like 5 blank EEPROMs onoce i realized i could do so! so handy...
 
Sorry for the 'hijack', but can BIOS code be dumped via debug or similar? Are all BIOS chips from XT/AT era likely to have a shelf-life (ahead of the rest of the machine, I mean)?
 
Sorry for the 'hijack', but can BIOS code be dumped via debug or similar?
Yes.

Are all BIOS chips from XT/AT era likely to have a shelf-life (ahead of the rest of the machine, I mean)?
Shelf life or operating life?

On my 'IBM 5150/5160 - common failing components' list, I have number 1 as tantalum capacitor, number 2 as faulty RAM chip, and number 3 as faulty ROM chip.
 
It'd be more than odd if the BIOS chip had actually picked or dropped bits without affecting the checksum. But then there are a few clone BIOS chips that don't even check that the ROM checksums correctly. I remember finding that one out after patching a BIOS and forgetting to recalculate the checksum.

Somewhere I have a "BIOS image getter", if you need it.
 
It'd be more than odd if the BIOS chip had actually picked or dropped bits without affecting the checksum.
True, but he did ask for "ANY possible explanation". I was very tempted to add a possible explanation that aliens had modified his computer as part of psychological experimentation (something difficult to disprove). He really should have asked for "ANY plausible explanation".
 
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