Old PCs, especially PCs from the 1980s to the mid 1990s, have a button with the curious label “Turbo.” On some PCs, a number on the front changes when you push it. Why did old PCs have a turbo button?

Turbo buttons are a quirk of old PCs, kind of like their beige color that tends to turn yellow, but it served a functional purpose.

Why old PCs had turbo buttons

Speed was always a mixed blessing with early PCs. We all want our computers to work faster, but some software didn’t play well with higher speed. Being faster than the real IBM PC or PC/XT provided a competitive advantage, but some software was sensitive to timing and didn’t like the higher speed.

So to get the best of both worlds, many PC and XT clones, such as the Leading Edge Model D, had a way to run at a turbo speed of 9.54 or 7.16 MHz, and at the push of a button, step back to the same 4.77 MHz of the original IBM XT. Sometimes it was a button on the case and sometimes it was a key combination on the keyboard, like CTRL+ALT and the + or – key on the keypad.

The problems got worse as computers got faster. The Compaq Deskpro 386 had compatibility issues with some older software, even if it ran fine on 286s. Getting as close to 100% IBM compatibility as possible was important.

So a number of PCs, especially inexpensive clones, started sporting turbo buttons. Often these slowed the machine down to 6 or 8 MHz, which was usually enough to get cranky software to run. Pushing the button again restored full speed. Some PCs had an LED, while others had a display that read out the clock speed.

Some brand name PCs had turbo buttons, but they were rarer on the big brands. Most inexpensive clone machines, built from off the shelf parts imported from the Far East, did have them. Machines that enthusiasts bought from the pages of Computer Shopper, or from local independent computer stores, were good examples.

Using the turbo button

To use the turbo button, you pushed the button before you loaded a program that didn’t like high speeds. After you finished, you pushed the button again to restore normal speed. In some instances, you might only have to slow the machine down while the program loaded or initialized, and then at some point you could switch back to full speed and it would work. Versions of Lotus 1-2-3 with copy protection on them were one example. Once you got Lotus loaded, you definitely wanted it running at full speed.

One point of eternal debate is whether pushing the button in made the computer faster or slower. The answer? There was no standard. It depended on your board. In practice, those of us who had PCs with the button figured out which position was faster and we kept the switch in that position most of the time.

Testing speed

One way to test it was to run a command like dir /s c:\ to list a large disk directory and push the button while it ran. As long as you had enough loaded on your hard drive, you’d be able to notice the difference. A better test was to type a long text file with the command type file.txt. That was less dependent on the hard drive and more dependent on the CPU.

An even better, less subjective way to test it was to run the si.exe command from Norton Utilities, if you had a copy of it. Norton SI told you the actual clock speed the computer was running at, as well as an estimate of your computer’s speed in relation to certain other common machines. Norton SI didn’t do a very extensive test, so it wasn’t a thorough benchmark even by the standards of its time. But Norton SI measured CPU speed unless you told it otherwise. For the purposes of testing your turbo button, it worked really well.

On computers that had a digital clock speed readout, that eliminated the guesswork. Assuming the readout was wired correctly, you knew what speed the computer was running at.

About those digital speed readouts

The digital speed readout on old AT cases didn’t actually sense anything. You hard-wired the two things it displayed and it just toggled between the two. Whoever wired up the switch and installed the motherboard had to verify the speed matched the actual performance.

If you put a 200 MHz Pentium motherboard in a case that originally had a 386, it would still display the old 386 speeds unless you rewired the display. If it was only a 2-digit display, “99” was the highest speed you could set it to, even though the system would run at a higher rate. The motherboard had no way of knowing what the case displayed. The other thing you could do, if you were out of digits, was to set the display to read “HI” or “LO” depending on the turbo button’s position. You wouldn’t know what speed you were running, but you’d at least know if you were in fast or slow mode.

For some people, this speed readout as a bit of a show-off option, at least when the PC was in its prime. You could use it to prove your PC was faster than your friend’s. Other people thought they looked cheap. Upscale computer brands didn’t shout their clock rates in gaudy green light-up numbers, after all.

If you didn’t like it, you could buy a case that didn’t have one. Or you could skip hooking up the display.

Why modern PCs don’t have turbo buttons

Software that functions poorly at high speed grew rarer over time. Windows abstracts out the hardware enough that it’s downright difficult to write software that depends on a certain clock speed. MS-DOS didn’t give you that luxury. Besides, software adjusting well to faster computers became a matter of survival. If your software required a turbo button to function properly, it meant you didn’t run on Compaq and IBM computers that didn’t have one. Software publishers rightly began to see turbo buttons as a problem they could solve in software, without having to rely on a hardware function that they couldn’t count on being there. Testing CPU speed and adjusting timings accordingly took additional memory and complexity, but by the time this was a problem, PCs had ample resources to deal with that.

Clock multipliers also messed with turbo buttons. With a clock-doubled CPU, 8 MHz became 16 MHz, and that might not be slow enough to help. It was when CPUs started hitting speeds of 16 MHz that they started having problems, after all. A clock-doubled 16 MHz wasn’t as fast as a true 16 MHz. But in some cases it was fast enough to cause problems. A clock-tripled CPU slowed down to 24 MHz almost certainly wasn’t slow enough to help. So eventually that functionality disappeared from PC cases and motherboards. By the time the Pentium became mainstream, turbo buttons had mostly outlived their usefulness.

But if you want to build an old-school DOS gaming rig, a 386DX-40 with a turbo button can prove very versatile. At full speed, it was fast enough to run early 90s titles. At the slower speed it would run most 80s titles. And that answers the question of why did old PCs have a turbo button as well as any.

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