Ther are several you can use on command line. The ones always kicking around on shareware and cover disks used to be fast.com and slow.com, but I have also seen turbo and deturbo ... those you can use from command line or batch. Might have to go looking on a simtel archive or mpoli to dig them up.

Usually this is implemented in the keyboard controller where it detects the keystroke and toggles one of its GPIO pins. So, no

Some turbo XT boards were shipped with utilities to switch CPU speeds, see eg. Juko ST and NEST.
But I'm not aware of anything like that for 286+ boards.

maxtherabbit wrote on 2026-05-09, 00:38:

Usually this is implemented in the keyboard controller where it detects the keystroke and toggles one of its GPIO pins. So, no

The scan codes for turbo on/off were customizable in AMIBCP so it should be controlled by the BIOS.
OP, you could always try stashing Ctrl Alt NumPadMinus in the keyboard buffer to simulate pressing it and see what happens. It probably won't work from a Windows DOS box but a full screen one should work.

jakethompson1 wrote on 2026-05-09, 01:24:

maxtherabbit wrote on 2026-05-09, 00:38:

Usually this is implemented in the keyboard controller where it detects the keystroke and toggles one of its GPIO pins. So, no

The scan codes for turbo on/off were customizable in AMIBCP so it should be controlled by the BIOS.
OP, you could always try stashing Ctrl Alt NumPadMinus in the keyboard buffer to simulate pressing it and see what happens. It probably won't work from a Windows DOS box but a full screen one should work.

All I remember seeing in there is a "speed pin" setting (where you select which GPIO pin it toggles)

The KBC's GPIOs are generally accessible to the CPU though, that's how the A20 gate works after all.

thanks guys for all reply

i tried a small test program, to send keys to keyboard buffer, obviusly did not worl, so i guess its not something you can trigger that easily from normal dos

I found the datasheet for this opti chipset https://theretroweb.com/chipset/documentation … 8e471224329.pdf

and it actually mentions turbo/slow speed control and programmable CPU/bus clock, but nothing how to deal with bios

i may try tracing the bios routine or finding the actual port/register used, someone did? any ideas? thanks

The vague understanding of the utilities I had was that they skipped the KBC hook and went right for a chipset register, so may vary by chipset. Guess it toggles the same bits set when you have turbo on/off settings in CMOS setup.

Been trying to chase down the ones I've seen in the past, but kept pulling up timer fiddling slowdown utils.... and cd.textfiles.com seems to have a lot of bitrot/missing files now.