Older CPU in "turbo mode"?

General Questions
#1

I remember, not too long ago, when one of my best friends got his new computer. It was one of the fastest machines on the market. It was a blazing beast. It was a true workhorse. It was the most perfectest game machine. It was a 286.

Anyway, it had a button on the front to turn the machine in “turbo mode”. This was to become a relatively standard feature, but I never realy understood what it did, or rather, why you would want to turn it off if it did make your computer run faster.

Am I wrong to believe that it might have been over-clocking the CPU? Or did it just light up an LED on the front pannel?

#2

Well, some older games were dependant on the CPU clock speed for timing; on the newer machines the games would run way too fast; the turbo button allows one to slow the machine down so older software could run, and activating the turbo would give you a faster clock speed for more modern apps.

#3

It was overclocking the CPU, and that has its downsides.

If you overclock on a program that has brittle timing rotutines hardcoded in, you can make the program run way too fast. This is a serious problem when you’re playing old games (the most common programs that depend upon realtime interaction) on a faster but compatible chip (very common in the PC world, since Intel hasn’t scrapped an opcode since the 8086). There are numerous programs that can slow down the CPU to make the games playable.

If you overclock too far, you can literally bake the chip and make it unusable. Overclocking (roughly) sends more electrons down the pipe to the chip, and more electrons generate more heat in all those little etched-on gates. Serious overclockers invest in serious cooling systems for their overworked CPUs, and you shall know the unsuccessful by the smell of fried silicon and plastic. But I doubt the overclocking button on those old PCs could possibly cause a hardware fault, unless something else conspired with it to overheat the CPU.

(If you want to overclock your own CPU, Google for ‘overclocking FAQ’. There’s a lot of info out there.)

#4

Heh. I’m in a computer lab at Uni right now, and some young rapscallion has scrawled “TURBO” on the front of the computer case. Right next to the reset button.

#5

“Overclocking” usually refers to the practice of pushing a chip past its rated speed. The “Turbo” switch wasn’t overclocking in this sense; it let you choose between the faster clock speed (which the CPU is rated for) for best performance, and a slower speed for running older timing-sensitive software. It should have been labeled “SLOW” but who wants to put a “slow switch” on his/her computer?

#6

I beg to differ, Derleth. As DreadCthulhu said, it was actually a method of underclocking the CPU to provide backwards compatibilty with hard-coded games.

[On preview, I see that scr4 has beaten me to the “its not overclocking” punch. Still, this post is going out because it has numbers, dagnabbit].

Gather round, young’uns, for some early PC history.

The original IBM PC and XT used 8088 CPUs running at 4.77MHz. There soon appeared clones running at 8MHz. This was not, however, an example of what is now known as “overclocking”. The CPUs used were specified to run safely at the higher clock frequency. After the IBM AT was introduced in 1984 with a 6MHz 80286, clones appeared with 8MHz/10MHz/12MHz etc CPUs.

The “Turbo” button found on clone PCs was, IIRC, a normally-closed toggle switch. If the switch was not connected to the motherboard, the CPU ran at its maximum speed – as also occurred if the switch was connected and pushed in. However, if the switch was connected and released, the CPU ran at a clockspeed of 4.77MHz, to provide backward compatibility with games that were hardcoded for the original IBM PC/XT.

An example of such a program was the original Microsoft Flight Simulator. This – again IIRC - was not even a DOS program. It booted up from floppy, but was its own operating environment. The floppy disk did not have a DOS-readable file system. Its peculiarities were such that it was used as the “acid test” for PC-compatibility. If a PC could run Flight Simulator, it was a true “clone”.

#7

After a certain point, the difference between full and (roughly) 1/2 speed became indistinguishable, and the turbo button was dropped. I think there were a bunch of Pentium machines with one, but those would have been the last.

Nowadays hardware is so diverse that not checking the clock in a simulation-type game would be insane. As late as the Pentium era though there were still nutbars releasing software that ran out of control, expecting that maxing out the CPU would still run okay. (i.e. - Bullfrog and some of their racing games :stuck_out_tongue: ).

Those hard-coded old games are still a PITA to run… you can find stuff to slow your current computer down though (search for “CPU Killer” or the like).

#8

I seem to remember that Control-Alt-Minus toggled turbo/non-turbo mode on my XT. I think it boosted it from 8 to 12 Mhz! Wow! 50% increase!

As far as non-speed controlled games, the last time I tried to play Vette! or Test Drive (1) on my 2Ghz machine, all I saw was a blur - then I lost!

#9

Man, I loved those games. Especially Vette! My copy was on 5 1/4" floppies, which sadly died long before I could transfer them to a better medium. :frowning:

#10

Test Drive! Was that the one with the train? And the 747 landing at the airport (as well as an X-Wing?!) And where you could run over the chicken?

#11

Me too! I’ve never been to San Francisco, and yet I feel like I’ve lived there all my life.

#12

Up thru the 286 era, the turbo button (or keyboard combination) changed the clock speed. Then on some later systems, it turned on/off caching, which then caused a performance hit on most programs. It was also possible to set things up to change the bus speed.

With the intro of the BF jumpers in Pentiums, people started using their otherwise useless turbo button to switch the CPU multiplier without taking the case off. (For overclocking.) Note that the button had to be set while the machine was off.

I hack old AT cases into ATX cases. I take apart the turbo button to make it a momentary switch and use it for soft power. The computer I am on now is set up exactly this way.

I have an old Toshiba 8086 laptop that I still power up on occasion to play old games. It is 8MHz by default but has a Fn+slow key to set it for 4MHz. (It was within arm reach so I just checked it.) So some of them actually did say “slow”.