"Speed pin" refers to the GPIO pin on the keyboard controller which controls the keyboard activated turbo switching (ctrl+alt+plus/minus)

It works like a dream with the keyboard (ctrl+alt+plus/minus). I knew part of that section was about the keyboard, but the bottom section called "Turbo Switch Input" made me think the physical switch connected to the motherboard header would work. Now I can play with some more with the configurations and benchmark them. Thank you very much for your help.

sdw wrote on 2025-05-04, 15:55:

It works like a dream with the keyboard (ctrl+alt+plus/minus). I knew part of that section was about the keyboard, but the bottom section called "Turbo Switch Input" made me think the physical switch connected to the motherboard header would work. Now I can play with some more with the configurations and benchmark them. Thank you very much for your help.

Hey! 😀
Did you by any chance got around to testing what does the "ctrl+alt+plus/minus", actually changes on this particular motherboard??
Does it disable anything or lowers any frequency?

I was kinda mulling over the idea of replacing the Turbo Button with a more complex button (with more contacts), and connect it to the FSB jumpers (with the shortest possible wires) in order to switch the BUS & Ram speed between 100 & 66 Mhz.
Only 2 jumpers need to be switched (with the PC turned off, of course.)

Yes it works with DOS. I never benchmarked it after making that bios change. If you are are still using the AMD K6-2+ there are lots of software you can use to change the clock frequency. Phil's computer lab has them on his site and there lots of videos on youtube.

https://www.philscomputerlab.com/k6-2-2-3-resources.html

Have fun,
SDW

Thanks for the link!
Yes, i'm using a K6/2+... But all the utilities presented by Phil, and that I know of; can't alter the FSB speed. Or disable the L3 Cache (which you have to do through the bios)

So unfortunately, unless you're willing to always mess with jumpers; a compromise has to be made when permanently setting the FSB:
100 Mhz -- and reaching the max potential of the system /or/ 66 Mhz -- and being able to obtain even lower speeds for dos games.

That's why I was hoping that the TURBO feature might do something that the software utilities can't.

It turns off L1 cache. I used modbin to enable "ON/OFF cache for speed change" setting under "Speed Pin Option" to get the ctrl+alt+plus/minus work. I tried enabling "Turbo Switch Input" and "Program Chipset for speed change" with the physical turbo switch on and off, but it did not make a difference. I really didnt know what I was doing. You might get the other settings to work.

I used the same setup as the bios version benchmark thread.
Problem with UDMA, need BIOS fix for Taken TX3 (LGS Prime 3C)

I valued the test results by going into the bios at boot up and turning off the "CPU Internal Cache".

Here is a screenshot from modbin of the settings.

Here is a speedsys screenshot of the same setup I used before with the system run full speed.

Here is a speedsys screenshot after pressing ctrl+alt+minus to slow it down (aka turn turbo off)

DOOM MAX - 15.72 FPS
Quake 360x480 - 2.9FPS
3DBench - 35.3

The 3DBench value puts the machine, per Phil's 136 in 1 CPU Benchmark sheet, the same as a 486 at approximately 50mhz.

I pulled out my K6-2+ 550mhz and clocked it at 6x for a speed of 600mhz. I turned the turbo off via keyboard and got almost identical numbers as above. It turned off L1 and L2 CPU cache off.
3DBench - 36.3

I then used K6INIT to turn off turn off L1, L2, Prefetch, and set the clock 2x, since it is software it sees it as 2 and not 6, for 200mhz.

3DBench - 29.3
Equivalent of a 486 at 33mhz

Have fun,
SDW

Screenshot of speedsys at this reduced speed.

I am not sure how slow you are trying to go, but this is pretty slow.

Have fun,
SDW

Very nice and thorough post, as always SDW 😀 Thank you!

So in short, the bios ROM needs to be edited with modbin, to enable the Ctrl+Alt+plus/minus feature, if I want to disable/enable (only) the CPU Cache(s) quickly via keyboard.
...gonna have to think about that, as I still have PTSD from my last bios adventure on this mobo 😆
By the way, does the keyboard trick also works in Windows? Or just DOS?
Also, I never messed with the "Prefetch" feature... Good to know, thanks!

BTW, I re-read your initial posts about the turbo button, and from what I saw in the manual, there is NO place specified for plugging the turbo button into the mobo. Only the Turbo LED.
So I'm very curious where did you actually connect it... 🤔

And while still being on the subject of the turbo button; I did some testing with a multimeter.
And it seems that the turbo buttons that have 6 pins/contacts, indeed have two independent circuits inside.
Both circuits connect their respective "Common Pin", to either Pin1 or to Pin2, depending on the button's position.
So the button, can in theory be made to comute 2 independent jumpers (each of them, between two positions).

So If we look on the JP9 table printed on the mobo next to the RAM, we can see that between:
FSB=100Mhz - SDRAM=100Mhz - PCI=33MHZ
and
FSB= 66Mhz - SDRAM= 66Mhz - PCI=33MHZ
only 2 jumpers: B & C need to be commuted.

So if the cables aren't too long, and the switch is nice&clean so as not to add extra electrical resistance (impedance); the turbo button could very easily be made to directly switch the FSB & RAM speed, between 100Mhz and 66Mhz, without opening the case!
Resulting not only in a lower BUS speed; but also reducing the CPU even further, from 200Mhz to 133Mhz.

When I made my cousin's PC with this mobo and a K6/2+, i tried every speed combination possible (except for the Prefetch thing). And some games still waren't completely happy.
Either too fast or too slow, or some sound issues.. And in the case of the original (unpatched) version of The Secret of Monkey Island, the game refused to start, throwing out a runtime error integer divide by 0 (which is a symptom of a "too fast" CPU).

Now.. I am aware that by doing this mod, and then accidentally pressing the turbo button while the PC is running, could yield disastrous consequences...
But I'm still mulling this over, how to make it safer. Maybe somehow include the electrical wire from the cylindrical key-lock that some AT cases have, to somehow enable/disable some of the turbo wires...
I'll have to probe a bit on the motherboard to see how the jumpers are actually wired up.
I'll post results if anything comes out of this ideea.

P.S.
I don't remember if I already mentioned, but I discovered an undocumented 2.0 Volt jumper setting for the CPU on this mobo:
on the JP6 jumper cluster, all 4 jumpers must be disconnected
This brings the CPU voltage down to 2.0 Volts, and this is what I've been using and testing with the K6/2+ ever since.

Maybe this info will be useful - taken from MODBIN USER'S GUIDE V4.50.58 rev. 1.5:

For making the physical "turbo" switch to work you'll have to fiddle with "Output" and "Input" sections settings from "Speed Pin Option" menu. And it seems like these settings are chipset, KB-controller and BIOS specific.

This is what I got so far:

Wired like this, it even gives the possibility to switch between the two modes of a Turbo Display (or Turbo LED), together with the FSB.
Also, the jumpers must be permanently removed from sections B and C (the picture erroneously shows them as still being on)

It goes without saying, that basic multimeter skills are absolutely MANDATORY, in order to check the pins of the Turbo Button (as each button may differ in pin arrangement).

And don't ask me what happens if you press this button while the computer is ON.
Might explode.... might just freeze.... or nothing may happen.
I have no clue.

Using the KeyLock's electrical switch to provide a layer of protection against accidental presses, will most likely remain a dream, as it would involve a lot of extra complexity with a bespoke electronic circuitry to be done properly.
As it is, the KeyLock could maybe be used to "cut" the GND connection, so as to leave the common pins(green & yellow) floating, in case of one accidental press of the TB Button without turning the key first... and only if it happens while the PC would be at FSB 100 (where you wouldn't need the key to be in the ON position).
But this may also present a problem if the green and yellow pins are touching (like in my drawing), because the electrical current may have nowhere to go to, and if one of them may have higher potential than the other, then it might result in damage...(but then again, this would inadvertently happen for a split second if accidentally pressing the switch, even if the ground is not cut). So in this situation, it would be needed to keep the circuitry separate, and use all of the pins of the TB Button.... but you would then loose the TB Display control possibility, just for some very questionable "protection".

So I'm gonna leave things at this stage; as I believe that the risk can be more easily mitigated by mechanical means and user's ingenuity... like having the switch mechanically locked somehow, or hard to press, or even hidden somewhere at the back of the PC, or behind one of the bay panels.
Some rare PC cases even have that little shitty reset button that you can only press with the tip of a pencil... and you could pretty easily replace the reset switch behind it, with a turbo switch.

So yeah... This would be my ideal take, on the TB Button.
If everything else can be managed through software; I would assign the TB Button a task that exempts us from opening the case 😄

So in short, the bios ROM needs to be edited with modbin, to enable the Ctrl+Alt+plus/minus feature, if I want to disable/enable (only) the CPU Cache(s) quickly via keyboard.
...gonna have to think about that, as I still have PTSD from my last bios adventure on this mobo 😆

It is easy to do. Just take the J4 version change that one field and save it. Here is the one I did and loaded on my machine.

By the way, does the keyboard trick also works in Windows? Or just DOS?

I only got it to work in DOS. I think Win98SE was trying to interpret the key sequence.

BTW, I re-read your initial posts about the turbo button, and from what I saw in the manual, there is NO place specified for plugging the turbo button into the mobo. Only the Turbo LED.
So I'm very curious where did you actually connect it... 🤔

You are correct. I looked on the motherboard and the way they wrote "TB-" over "LED" next to the pin header I took that to be two different pin sets. One for the turbo switch and one for the led. My eyesight going downhill. That is why I could never get the turbo button to do a thing when hooked up to the led. I never checked if the keyboard switching caused the LED to come on and off. Maybe one day. I unhooked the rig.

What you did with the turbo button is very cool.

Keep having fun.
sdw