Item — HN Reader

> Contrary to what it suggests, the "turbo" button was generally intended to let a computer run slower than the speed for which it had been designed when pressed.

Damn these trickster designers and their shitty design decisions. That's exactly the opposite of what I thought it did.

5 comments

I don't know if you already knew why the Turbo button was there in the first place, but back at a time where there is no dedicated monotonic timer, applications (mostly games) rely on the fixed processor speed to determine sub-second time, which while worked on the original IBM PC... made it too fast for new generations of hardware.

Assuming you can detect what clock speed the device is running at, such an approach should still be able to work on different devices. Although with newer systems able to process multiple instructions per clock, as well as having clock speeds change rapidly, it probably wouldn't work as well.

The original applications that were effected were written without knowing there would be IBM PCs (no less clones) that ran at a different clock speed!

See sibling comment. Also, there was no CPUID in those ages!

As far as I remember, the button itself was supposed to let the computers run at their normal, faster, speed. Meaning the computer was supposed to be in Turbo mode, all the time.

You turned the Turbo mode off when you needed to run it slower, say for playing games.

So, technically the button did what it said, run faster in Turbo mode.

There were also clock speed selectors that really did let the CPU run faster. They would let you switch to a CPU clock speed that was higher than the speed the CPU was originally rated for. The cost was a greater possibility of crashes. Depending on what you were doing and how stable the CPU was it was a noticeable improvement.

Some selectors would let you choose between 2 speeds, others had multiple speed selections. Often the higher ones would crash a lot more. There were also variable speed controls that acted like a rheostat with continuous speed changes. Those worked but were even more unstable.

Like 10 years after my 486-DX2 went into the trash after a long long life, I was reminiscing about the turbo button with a coworker, and he mentioned that he always kept his turbo button activated and that there was never a single repercussion from it!

Given that Turbo made it slower, why would that cause repercussions?

Well, slower was the repercussion, but my coworker thought turbo meant faster, and was thinking along the lines of "my CPU didn't catch fire from all the extra speed I was getting"

You misuderstood. When the turbo button is pressed, computer runs on nominal speed. When turbo button is un-pressed, computer would run on half of nominal speed.

So, unpressed button makes computer slower, not pressed one.

Source: actually used the button on my 386 SX 20/40 MHz

Sorry, but I think you're confused in the same manner my coworker was. Activating the turbo button decreases the speed of your computer.

Maybe yours operated differently, but everything I can find about turbo buttons on the internet indicates that they operated as I remember them - your computer gets slower when you turn the button on. I specifically remember playing certain DOS games where, if I didn't activate the turbo button, the entire game would speed through at what felt like double or triple speed.

From wikipedia on Turbo button:

> On IBM PC compatible computers, the turbo button selects one of two run states: *the default normal speed or a reduced "turbo" speed*. It was relatively common on computers using the Intel 80286,[1] Intel 80386 and Intel 80486 processors,[2] from the mid 1980s to mid 1990s. The name is inspired by turbocharger, a device which increases an engine's power and efficiency. *Contrary to what it suggests, the "turbo" button was generally intended to let a computer run slower than the speed for which it had been designed when pressed.*[3]

https://en.wikipedia.org/wiki/Turbo_button

sigh

Look, you read about it in wikipedia, but I actually lived with this computer for my whole childhood. I know for sure how the turbo button worked, because I actually used it to cheat in F1 and Lotus racing games.

Best I can find in 10m I dedicated to this is this video showing switch between 22 and 40 MHz modes with a turbo button (around 8:50): https://youtu.be/TdenDH7S8LM

I've literally told you multiple times that I did too, and my computer behaved in the exact manner that all of the results googling the concept turns up.

i dont know why the summary paragraph is so authoritative about what pressing the button means when a couple paragraphs down basically says “sometimes it’s wired backwards, so really it could be either faster or slower”

Yep. I always had the turbo button pressed, except for occasionally deciding to play Test Drive 3 at speeds lower than "light speed". It was the one game I had that didn't calibrate the delay loop properly and my 486 was waaay too fast for normal playing - something that I didn't know so I learnt to play a very twitchy version...

Vide "macro photography"

Nikon got that one right.