iosys wrote on 2026-03-20, 22:20:

Hello, […]
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Hello,

I have a motherboard: https://theretroweb.com/motherboards/s/tk-tk8880f-2066.

I installed an AMD 5x86 P75 processor on it.

The PCI frequency is set to 33 MHz.
I set J27 with a ratio of 2x/4x for 4x33MHz.

I can never get it to boot with this processor once I enable the 2x/4x speed ratio (J27).

This motherboard only boots when J27 is OFF, for a 3x speed ratio. So CPU is set to 100MHz.

Does anyone know if there's a hidden setting or if it's simply impossible to boot with an AMD P75?

Could post a pic of the jumpers?
IIRC it must be set as an Amd cpu, maybe you jumpered for an Intel?

On my PCChips M912 it has to be set as a Cyrix Dx4.
It's P24D.

You could try that too, be careful with voltages.

Check here:
https://en.wikipedia.org/wiki/Am5x86
https://www.cpu-world.com/CPUs/80486/MANUF-AMD.html

hmm so the board supports 3.45V 4V and 5V CPUs, but where is the voltage reg on the board? Im looking and looking and cant see it 😮 .. wait, TO220 mic29150 LDO is either missing altogether and there is only some weird socket for it or is standing directly upright and missing radiator on theretroweb picture?
found another on ebay and im still not sure

rasz_pl wrote on 2026-03-20, 23:15:

hmm so the board supports 3.45V 4V and 5V CPUs, but where is the voltage reg on the board? Im looking and looking and cant see it 😮 .. wait, TO220 mic29150 LDO is either missing altogether and there is only some weird socket for it or is standing directly upright and missing radiator on theretroweb picture?
found another on ebay and im still not sure

Well, this is unexpected.

What it the MX chip doing?

MX is a clockgen

rasz_pl wrote on 2026-03-21, 00:14:

MX is a clockgen

I thought so.

If the OP has an oscilloscope he should check if it is the righ FSB output.

Here is a pic of jumpers.

Strange behavior I think, I can boot OK with others AMD 486 100MHz

And it boot with the 5x86 but only at 100MHz (3x33MHz)

https://theretroweb.com/motherboards/s/tk-tk8880f-2066a2

whatever it is at least iosys board has one unlike ebay and theretroweb ones 😮 😀

rasz_pl wrote on 2026-03-21, 13:07:

whatever it is at least iosys board has one unlike ebay and theretroweb ones 😮 😀

Yes,
seems to be the same board, likely different revision since some jumper locations are populated on OP's board which aren't populated on the TRW photo, as well as some other differences between components/jumperlocations.

@iosys Could you post a full photo of your board?

Hello iosys,

An Am5x86 working in x3 mode, but not in x4 mode, on these older Socket 3 boards is usually due to limited CPU support by the BIOS.

I see only one TK8880F BIOS on TRW with a 12/19/94-UMC-881/886-2A4X5P01-00 ID. This BIOS doesn’t support the Am5x86 in x4 mode and indicates this CPU as Am486DX4PLUS in x3 mode.

Is this the BIOS you are using?
If so, I can try to find a better BIOS for this board.

The jumper settings look fine, although I would set JP20 and JP21 both to 1-2. The Am5x86 often needs this Intel CPU Brand setting to work correctly.
You can also try to set JP11 to 1-2. This puts the Am5x86 in L1 cache Write-Through mode which sometimes helps on an unsupported BIOS.

Until you have a better BIOS, you can also run the Am5x86 in 3x40 mode. This 120MHz mode is no problem for this CPU.

Cheers, Jan