Still hoping for someone intimately familiar with modifying AMI BIOS to chime in since it's possible that this can be done and I'm just not using AMIBCP correctly.

IMO, it will be difficult to add. It's not just an item in a bios settings you can edit using AMIBCP. It also a set of data to program a PLL (clock generator).
It might be easier to send data into PLL from Windows using PLL soft utilities (clockgen, setfsb, speedfan). Of course only if a datasheet of the PLL is available somewhere in Internet. Although the utilities may know the chip.
What model of PLL is set on the board ?

Some clock gens I believe in that time accept old style pull up/pull down. Or is this a north bridge ratio kinda deal? Any chance of intercepting the signal on the mobo and forcing it?

Have you tried a donor bios from a similar board?

Sphere478 wrote on 2022-09-02, 19:44:

Some clock gens I believe in that time accept old style pull up/pull down.

Do you mean the clock is set by jumpers? I think "pulling up/down"is used only for early start of the system. Further a BIOS procedure sets the clock programming a PLL

Sphere478 wrote on 2022-09-02, 19:44:

Have you tried a donor bios from a similar board?

ECS P4S5A looks promising: very similar design. But what's about its BIOS settings ? Who knows...

Roman555 wrote on 2022-09-02, 19:00:

IMO, it will be difficult to add. It's not just an item in a bios settings you can edit using AMIBCP. It also a set of data to program a PLL (clock generator).
It might be easier to send data into PLL from Windows using PLL soft utilities (clockgen, setfsb, speedfan). Of course only if a datasheet of the PLL is available somewhere in Internet. Although the utilities may know the chip.
What model of PLL is set on the board ?

The brand and model number are:
ICS UT180021
0138
9325AF

A Google search brings up exactly nothing.

I tried a BIOS for the nearly identical P4S5A some time ago, and at first it seemed to have been exactly what I wanted. It POSTed, and I was able to set the CPU to 133/166 just as I wanted. I thought it was strange that I had to set the multiplier as well, but it booted into Windows 98, and everything worked. But when it was powered off it would lose the CPU settings, and you had to manually enter the settings on every boot up. So I gave up on that and went back to the stock BIOS.

maybe you can copy and paste some code?

Roman555 wrote on 2022-09-02, 20:34:

Do you mean the clock is set by jumpers? I think "pulling up/down"is used only for early start of the system. Further a BIOS pr […]
Show full quote

Sphere478 wrote on 2022-09-02, 19:44:

Some clock gens I believe in that time accept old style pull up/pull down.

Do you mean the clock is set by jumpers? I think "pulling up/down"is used only for early start of the system. Further a BIOS procedure sets the clock programming a PLL

Sphere478 wrote on 2022-09-02, 19:44:

Have you tried a donor bios from a similar board?

ECS P4S5A looks promising: very similar design. But what's about its BIOS settings ? Who knows...

no, I was figuring a hardware mod. humpers stopped being a thing around pentium 2

Repo Man11 wrote on 2022-09-02, 21:12:

The brand and model number are: ICS UT180021 0138 9325AF […]
Show full quote

The brand and model number are:
ICS UT180021
0138
9325AF

A Google search brings up exactly nothing.

I think one digit is missing in the last number. I guess it should be 93725AF and then it's a clock buffer in fact, not a PLL.
Is it near a DIMM ?
The PLL should be between AGP and PCI slots as I can assume looking at a poor google picture of the mainboard.
I believe it may be something like ICS952001AF.
To distinct one from another - a PLL always has a Crystal Oscillator 14.318M near.

About the foreign BIOS.
It was important to flash it the whole including a Boot Block (BB).
If it was done using a hardware programmer that is all right : the BB was changed.
Because flashers usually write a BB for sure only if you select the option 'force write BootBlock' .
And obviously 'Clear CMOS' is obliged after.

Good luck!

You are correct, I was mistaken. Yes, it's an ICS 952001AF, the same as the P4S5a according to this post: https://www.overclockers.com/forums/threads/o … nitions.189212/ . IIRC I had to use Uniflash, I don't have a programmer - but it's been months ago, I may have renamed the BIOS file so that the flash software would work.

Repo Man11 wrote on 2022-09-03, 07:10:

***
IIRC I had to use Uniflash, I don't have a programmer - but it's been months ago, I may have renamed the BIOS file so that the flash software would work.

Uniflash is fine:

1Since v1.33, flashing is done including bootblock by default

P.S. Also there're plenty of BIOSes on theretroweb.com

Interesting thing about the BIOS files they have for this board; they are all Award, and this board (and the BIOS I found for the P4S5A that I tried) are AMI. I just tried one the other day, setting myself up for a possible hot swap flash if it didn't work, but Uniflash couldn't complete the flash, and I assumed that it was the boot block that was causing the fault.