"None" is the completely normal, correct setting for an ATAPI CD-ROM drive on a 486 machine. The BIOS did not need to know about CD-ROM drives, ZIP drives, etc. until support was added later to boot from those (bloating the BIOS code accordingly) and when the BIOS gained support to do chipset-specific tuning for IDE DMA modes so that the OS could use DMA later.

Your DOS CD-ROM driver, whether that is Oakcdrom.sys or something else (smaller), will talk to the hardware directly, bypassing the BIOS. By the way, ATAPI CD-ROM and HDD on the same interface will cause you troubles if you enable 32-bit disk access in Windows 3.1.

Did you make the appropriate entries for the CD ROM driver in autoexec.bat and confirg.sys? If so you should see the driver post information after DOS starts and report that a device was found (or that none were found). A very simple way to set this up is the MS-Dos starter pack from Phil's Computer Lab

As jakethompson1 said, it shouldn't be visible in BIOS in the first place in such system. Just load the CDROM drivers in DOS and it works.

thanks guys, I feel silly for not remembering this ! All works well now.

another question though relating to HDD.

I have installed a 2GB CF card (in IDE to CF adapter), the BIOS recognises it correctly as 2GB however, the maximum PRI drive I can create in FDISK (DOS 6.22) is 504MB. It simply wont let me go higher. Is that the limitation of the UMC VLB IDE controller ? I thought all VLB controllers were EIDE? or am I doing something wrong?

Do I have to use something like Ontrack Disk Manager to see the full capacity? Does it take up any RAM when loaded?

few photos - https://postimg.cc/gallery/77j5KSH

The 504Mb limit is a limitation of the bios. You may be able to find a patched bios on the retro web which lifts this limitation (or not). Another option is drive overlay software which presents larger drives as multiple drives I belive but I am no expert on those.

IDE CHS (between the BIOS and HDD) can handle 16384 cylinders, 16 heads, and 63 sectors
Int13h CHS (between DOS and the BIOS) can handle 1024 cylinders, 255 heads, and 63 sectors

As you can see, a hack (kludge?) is to trade fewer cylinders for more heads when presenting the drive to DOS, aka "LARGE" or "LBA" mode (the details are a little different). This can get you up to 8.4GB; more than that and you need Win95B or later and an even more capable (Int 13h Extensions) BIOS.
As your BIOS doesn't have this capability built-in, you can either settle for 504MB and ignore the rest, use overlay software, or for a "permanent" solution, stick XT-IDE Universal BIOS on an ethernet card.

Thanks, that all makes sense. I should have remembered all this but then again, it was 30 years ago !

I've 2 VLB systems One of them recognises 8GB drives but hangs on boot, using drives 6GB or below work fine.
The 2nd has the 500MB limit for which I've had no issues with ontrack.

Omarkoman wrote on 2025-05-12, 02:30:

Thanks, that all makes sense. I should have remembered all this but then again, it was 30 years ago !

I read about overlay software in books back when it wasn't obsolete, but never tried to an oversized drive in a pre-LBA system back then, when 300/400/500MB drives were aplenty.
Now of course, a lot of those have failed or still work but sound as loud as an angle grinder, so it's commonplace to use a huge drive. Unless you count CF cards which are still accessible in small sizes.

one more question, this 486 sx-33 machine , I noticed the turbo button is not wired up and on the motherboard, the turbo button is connected with a jumper, so always on. That means its running in slow mode as when I run the benchmark, its super slow. Taking the jumper off its back to its normal speed. However, when the jumper is out, the machine freezes on the spot when its trying to boot from the HDD, the activity light in fully on ( on the cf card reader) but nothing happens. Also for some reason the system doesnt detect the ISA ESS sound card anymore.

is it possible that there are some settings in BIOS that were changed and when CPU is at full speed, it causes issue? why would cpu speed change cause this ?

Image site is 504 for me, do you have model numbers for the Mb and card, check the ISA divider, it should be /4 to 8.25MHz from 33, might also need to set a jumper on the VLB card to waitstate it if it can't hold up at 33, though VLB really should get along at 33

I would also check the cache timing and memory timing in the bios and set these to slow until you have a stable system. As Matth79 mention the bios should also let you set the isa bus clock as a divider from the front side bus clock such as Clk/4. However not all boards actually clock down when turbo is removed but use other tricks to slow performance. The default isa bus speed is 8Mhz back from the IBM days although most isa cards will run fine at higher speeds.

Unfortunately there are no timings to change in bios:

https://postimg.cc/gallery/CVz85r5

This is the motherboard:

https://theretroweb.com/motherboards/s/fic-486-gvt

The flakiest part of a 486 motherboard is the external cache, so you could try disabling it.

Also, it's weird that such an old BIOS has "CPU Write Back Cache" support. Your 486SX CPU definitely doesn't have it. Maybe it refers to one of those Cyrix CPUs that introduced this feature before Intel did.

I installed a dx-33 cpu in and changed the respective jumpers.

Strangely the soundcard that worked ok before now is not recognised. I put another one exactly same and same issue.

Then i tried two different ess cards and they both work and doom is playing fine.

So weird.