Disk controller probably just needs more clock speed than 2MHz

Boot at the lowest speed you can and then use a cache tool or disable the internal cache in BIOS. That will significantly slow the things down. You might be able to slow the system to a 6MHz AT level that way

carlostex wrote:

Boot at the lowest speed you can and then use a cache tool or disable the internal cache in BIOS. That will significantly slow the things down. You might be able to slow the system to a 6MHz AT level that way

Oh my main objective was not trying to hit lower score in LM6, but just curious if there is way to boot DOS at 2MHz.
Anyway I will give it a shot!

The slowest CPU supported by DOS 6.22 should be the 8086 5MHz. It's possible that it just doesn't like low MHz numbers in CPUs, even though that 486 should be way faster than even a 8086 at 10MHz. There is a 286 at 4MHz which should work with DOS, so it's possible that some part of the OS is looking at the clock speed of the CPU during the boot process, which would explain why your 486 works at 4MHz but not 2MHz. This is all speculation though.

carlostex wrote:

Boot at the lowest speed you can and then use a cache tool or disable the internal cache in BIOS. That will significantly slow the things down. You might be able to slow the system to a 6MHz AT level that way

It turned out to be a 5MHz AT 😁

Well I finally did it. 486SX @ 2MHz

486SX @ 2MHz w/o L1 Cache

The way I made it work was a BIOS swap. The board came with an ancient NCR/IBM BIOS and I flashed it with the latest BIOS (3276gn1.bin) provided by FIC. The latest BIOS won't boot into DOS at 4MHz. Later I flashed it with another BIOS (att409be.bin) provided by FIC and that's where the story begins. Now I flashed the original BIOS back and it boots at 2MHz.
Here are some other facts I observed. The PS/2 keyboard stopped working at 8MHz and lower. The floppy drive sounds weird at 4MHz and lower.
Some of the issues might be CMOS settings related since the att409be BIOS refuses to boot at 4MHz if BIOS and VGA BIOS shadowing are turned off, but the system is too slow for me to check every possible combination of settings.

Congratulations! You probably have the slowest 486 in use today 😁 If this was 1995 I'm sure it wouldn't be something to be proud of.

What you could do is try to play some games that were speed sensitive and meant for machines like the 5150 or the XT.

iirc disk-booter Lode Runner wont start-up at 10Mhz XT, but you can safely change the speed from 4.77Mhz to 10Mhz after the game displayed on monitor.