For my most modern 486 build, I have a beautiful little board with many names. At POST it identifies as RT-4S3, which seems to be a Rectron part number, but most of the info about it is under the name of Jamicon or Kaimei KM-S4-1. Either way, it's a tiny little BabyAT board with SiS496/497 chipset, ISA, PCI, 256kB cache and 4 72p SIMM slots. BIOS version is 4.2.

According to the manual it supports all 486 CPUs, but in practice it seems not to like Cyrix. I can happily run Am5x86 on it at 4x 33, 3x 40 or even 3x 50MHz, but it refuses to even boot with Cx486DX2-80, and with IBM 5x86C-100 it did POST, but hung immediately afterwards when attempting to access boot device, and did so at any and every speed (even 2x 25MHz...). Both these CPUs came badly battered out of big lots, so given behaviour wasn't the same I assumed they were just two dud CPUs and left it at that...

...until I recently spotted a Cx5x86-120GP for sale. Snapped that one up, installed it and... exactly the same behaviour as the IBM 5x86C: POSTs fine, all the way through to FF, but hangs at the point that it would start booting from a device. Things like keyboard LEDs also stop responding.

Two CPUs with very different history showing exactly the same issue starts to sound a lot more like a motherboard issue.

This is the manual I've been using:
http://www.elhvb.com/mboards/kaimei/kms41.pdf

Settings currently in use are those for the Cyrix M9. Settings work fine for non-Cyrix CPUs.

Anybody know what's going wrong?