First, I am NOT asking how to use the BIOS Setup utility. I'm asking how to OVERRIDE the built-in settings my BIOS Setup supports for the HD geometry.

I have a 1991 NCR PC with a 386SX, on-board IDE, and IBM ROM with "IBM Setup 4.0.0" built-in. As is common for PC BIOS ROM of the era, it offers a fixed table containing the C/H/S (Cylinder/Head/Sector) geometry for 47 different IDE Hard Disk "Types".

None of the 47 HD Types match the geometry of my IDE DOM (Disk on Module), though all of the options are smaller capacity than the DOM.

  • fdisk.exe seems to work, creating a partition matching the disk type selected in the BIOS Setup.
  • format.exe seems to work, resulting in a volume of the right size.
  • copying system files over, or running the MS-DOS installer seems to work for a few drive types.
  • BUT, in the end, the written files cannot be read back and the drive cannot be booted.

I suspect if the BIOS C/H/S parameters actually matched the C/H/S parameters of the IDE DOM, then things would work fine. The correct settings are 493/16/63 (it's 242MB).

With all that background, my question is whether there is some way to override the geometry settings that are in the fixed table. I'm imagining I could boot from floppy and use a DOS program to change the CMOS to the right HD parameters. That assumes the CMOS actually stores C/H/S, rather than just the type number from the ROM table.

I suppose if editing the CMOS is not possible or would only be useful to change the type number of the drive, then would it be possible to patch my BIOS ROM to replace one of the existing fixed types with the C/H/S values that I actually require?

NOTE: XT-IDE Option ROM is not a possibility.

Someone will probably suggest XT-IDE ROM extension, but I have not been able to get this machine to recognize it when plugged into an ISA card.

  • 3
    There should be a custom setting in the BIOS that lets you enter the parameters manually. Commented Nov 12, 2018 at 4:48
  • 1
    Could dewassoc.com/kbase/hard_drives/… in the section about ATA be part of the explanation? Before LBA the theoretical max limit was apporx 136.9 MB. Fdisk and other DOS programs might work around this...
    – UncleBod
    Commented Nov 12, 2018 at 7:06
  • 1
    @UncleBod IIRC there where also more smaller barriers like ~504MB? and even smaller ones that are probably the problem. But those where usually related to early File Systems
    – Spektre
    Commented Nov 12, 2018 at 8:38
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    "though all of the options are smaller capacity than the DOM" As far as I can remember (and it was a long time ago...) if you have to choose a predefined geometry, you have to make sure that none the C/H/S values chosen exceed the actual parameters of your disk, not just the overall capacity.
    – TripeHound
    Commented Nov 12, 2018 at 10:01
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    If the drive parameters are editable at all, your BIOS should allow you to do this. Typically the first or the last one is editable but the others are not. If none of the parameters are editable then one of the others might work anyway, but your best choice there is something that matches in the last 2 parameters (e.g. anything matching */16/63). If you have a matching option with fewer cylinders than 493 you will only be able to access part of the drive. If you have a match with more than 493 then things will work but make sure in fdisk you don't go past 492/16/63.
    – Ken Gober
    Commented Nov 12, 2018 at 13:32

1 Answer 1


In all the BIOS setups I was dealing with that have similarly 47 drives to chose from the first or the last one was editable directly in BIOS SETUP for manual settings. Sometimes the manual settings was done in different menu entry (near formatting utility ... but beware do not accidentally format IDE !!!).

IIRC DOS uses BIOS routines for HDD access so if BIOS SETUP utility does not support settings you have even overriding geometry data could prove FATAL. I would explore other means:

  1. using smaller HDD size matching compatible geometry

    there might be one of the 47 HDD drive settings that work but produce smaller size of the drive. This is the safest but rather limiting solution.

  2. upgrading BIOS

    this one is very dangerous as if anything goes wrong or you got incompatible BIOS you may render your motherboard unusable (unless have means to revert to original BIOS). So I would do this only as a last resort and also backup the BIOS image. If your BIOS is socketed you might still resurrect it from dead in case of failure if you got a programmer compatible with it. Hot swap is also a possibility Booting from original chip but programing different one but you have to be careful not to short circuit something or blow anything by electrostatic charge.

    btw having a backup copy of the BIOS somewhere is always a good idea as a protection from Černobyl virus ... that one was nasty one

  3. EZ Drive

    This is your best option. Back in the days when the HDDs starts to go bigger utilities like EZ Drive rises. They are installed into Boot sector and overrides the BIOS HDD access interrupts allowing use of bigger HDDs. Not sure if works also without LBA but I think it does (it was ages ago). IIRC it also worked well with win 9x not just MS-DOS.

    There might be also different utilities out there,... But I was using this one and was very happy with it. Beware some BIOSes boot sector protection can prevent you from installing it properly (the same goes for Windows installations) so I would temporarily disable it ... and when working enabling it again.

    Here the EZ-Drive link

  • 2
    I was unaware of EZ-Drive. Worked like a champ and solved my problem. Found it here.
    – Brian H
    Commented Nov 12, 2018 at 15:34
  • @BrianH Yep it was one of the MUST HAVE SW at that time ... I added it to the list as it seems I forgot to add it there ...
    – Spektre
    Commented Nov 12, 2018 at 15:41
  • Back in the days, the older PCs couldn't use a drive bigger than 8GB; however if it was partitioned with a master/boot partition of 8GB, the OS (WinXP) would take over the BIOS and could mount any extra Logical/Extended partition for the remaining disk size.
    – Stavr00
    Commented Nov 15, 2018 at 21:28
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    In a 1991(!!!) PC, there are only a few ways to permanently damage the mainboard while upgrading BIOS: Ripping the EPROM socket off the board, plugging an electrically incompatible, defective or reversed EPROM in, or trying to use a BIOS that is so mismatched to the hardware that it makes something overheat by causing massive bus collisions.... meaning to say: BIOS updates in these days usually were hardware work. Commented Dec 2, 2018 at 4:36
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    @Spektre I was implying EPROM, because that is what you will usually find on a 1991 vintage mainboard. And I was assuming that the starting point for a BIOS upgrade, back then, was acquiring or writing an EPROM with the new version, then swapping it out (EPROM BIOSes were almost always socketed for that reason). What I was REALLY saying was "the instructions about BIOS upgrade are for a much more modern type of mainboard, whoever wrote them lacks a clue about boards that old". Commented Dec 2, 2018 at 15:23

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