All the PCs I have personally used or encountered which had a "Turbo Mode" (clock frequency switch, with frequencies in the 5-25MHz range) did this via a hardware button similar to the reset button.

However, this question links to a forum thread where somebody claims that this was possible via the keyboard (and on a PC that apparently has no hardware turbo button, judging from the images I googled).

Were there any PCs where "Turbo Switching" was possible via the keyboard, and which ones?

If yes, were the hotkeys intercepted by the BIOS, or was this some DOS extension, driver, or TSR?

If it was intercepted by the BIOS, how did they deal with other programs that wanted to use this particular key combination?

  • The Wikipedia page for Turbo Button claims that the Packard Bell 486ES 3x3 could do this, but doesn't give a source, and also says other models had a dedicated Turbo Button (which begs the question why this particular model didn't) So if someone can verify if this Wikipedia information is correct or not, that would also be interesting.
    – dirkt
    Jan 19 at 9:38
  • 1
    Are you looking for "early PCs" or "any PCs"? I had a 486 with AMI BIOS, bought in late 1992 from German computer chain ESCOM, which had both a physical button and keyboard switching via Ctrl-Alt-+/-. Jan 19 at 10:02
  • 2
    Ctrl-Alt-+/Ctrl-Alt-- support was a standard feature in both AMI and Award BIOSes at least around 1990-1996. On chipsets with software-controllable speed, the chipset support package for that BIOS included the necessary customizations to make speed switching work. Other boards used a general purpose output pin of the keyboard controller to "override" the turbo button. In that case, a simple configuration flag could be enabled in the BIOS to toggle that pin on Ctrl-Alt-+/-. Finally, starting with the 486 processor, toggling the cache instead of the processor clock was another common way. Jan 19 at 19:12
  • Pedantically, it’s supported by the 8088 Book, which is on sale now. So: trivially, yes.
    – Tommy
    Jan 20 at 19:40
  • I used to have a no-name lunchbox form factor 286 that activated turbo mode with <CTRL><ALT>-'+' (and - to de-activate. That switching was built into the keyboard driver in the BIOS.
    – tofro
    Jan 22 at 13:14

3 Answers 3


The Philips P3105 and Commodore PC20-III could be switched with both keyboard combinations and a command line utility according to this thread on Vogons.

I have two 8088 machines with switchable clock speed. Both can be switched on-the-fly either with a keyboard combination or a commandline utility (but no turbo button on the case). One is a Philips P3105, which does 4.77 and 8 MHz. The other is a Commodore PC20-III, which does 4.77, 7.14 and 9.54 MHz.

Regarding the Commodore PC20-III, apparently:

This machine does not have a "Turbo" button. If you use a normal XT keyboard, to set CPU frequency, you have to use SPEED.EXE (see files in Zimmers.net files archive in Links section) program or keyboard shortcuts:

CTRL+ALT+S - Switches to "Standard" mode - 4.77MHz

CTRL+ALT+T - Switches to "Turbo" mode - 7.16MHz

CTRL+ALT+D - Switches to "Double" mode - 9.54MHz

The manual for the Commodore PC20-III is in German but seems to indicate that you would use (page 1-8):

speed -s 4,77 MHz (Voreinstellung)
speed -t 7,15 MHz
speed -d 9,54 MHz


More searching finds the Magitronic 20B33 manual for a "Super 286 Baby Mainboard", which says on page 10:

  1. When switching speeds externally: A mechanical switch is installed onto JP7, JP7 in the OPEN state provides Low speed, when CLOSED, it provides High speed.
  2. When JP7 is OPEN, the clock speed can be switched by using the keyboard. When using AWARD BIOS, keys CTRL, ALT and - (minus) are pressed simultaneous) to switch to High speed. Keys CTRL, ALT and + (plus) are pressed simultaneously to switch to Low speed.
  3. If the power is turned on while JP7 is OPEN, it will turn to Low speed. If JP7 is CLOSED, it will invalidate the keyboard operation and will switch to High speed at all times .
  4. When using PHOENIX BIOS, keys CTRL, ALT, and \ are pressed simultaneously to switch speeds.

So that looks like different BIOSes intercepted different keys, and then could probably manipulate an agreed-upon piece of hardware to set the speed, while also having the option for a dedicated hardware Turbo Key.

It doesn't explain how the BIOSes dealt with other applications wanting to use those keys, though.

Similarly, there's this manual for an AMI BIOS system which says

The 386/32 has a connector for a hardware processor speed switch. If this has been connected to the front panel of your system you can use it to switch between the maximum processing speed (of­ ten referred to as "Turbo") and the secondary slower processing speed. Pushing the switch on the panel changes the speed. If the higher speed is in use and your front panel has the appropriate indicator light, the LED indicator will light up while the high-speed mode is in use.

The processing speed can also be changed using a keyboard command sequence. To use this feature do the following:

  • Hold down the <Ctrl> and <Alt> keys simultaneously and press:

    a + (plus) for the high speed, or
    a - (minus) for the low speed

This looks like a similar way to do that, so maybe there was some kind of unofficial standard?

The Commodore PC10 III manual linked in the other answer is also very interesting, because it contains the full schematics of the motherboard, and the chip responsible to generate the CPU clock (among other things) is a Faraday FE2010A, so we have a description how changing the CPU speed worked:

The Configuration Register (I/O Address 0x63) is a write only register that is used for FE2010A configuration. [...]

Data Bit  Function
5         Fast Mode (0 RAM wait states)
6         7.15 MHz CPU clock
7         9.54 MHz CPU clock

Which means it should be possible to write a TSR with any kind of hotkeys to change speed, as long as this information for the respective motherboard is known.

  • 1
    I find it unfathomably strange that the Magitronics Super 286 used "+" for "slow" and "-" for "fast". Jan 20 at 1:13

There certainly were DOS TSRs that would slow down the system on request. One such is WHOA!.COM, which could be controlled with a hotkey. It worked by hooking the timer interrupt and executing a delay loop on every tick.

I remember using it to cheat at Sopwith.

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