View Full Version : Question about old PCs

May 22, 2005, 08:48 AM
Hi all,
I was just thinking to myself about the old PCs I've seen and used. I remember in some very old PCs I think they were 486 or Pentium 1 or 2, some of the PCs had a "turbo" button. I was very young in those days so I never really used one extensivley so I just wanted to know what was the turbo button used for?

May 22, 2005, 09:13 AM
My old AMD 286 system had a turbo switch. When it was on, the clock speed was 12 Mhz, when it was off it was 8Mhz.

mad jew
May 22, 2005, 09:16 AM
I think they had to incorporate it because there were some programs that couldn't work at the higher clock-speed. But don't quote me on that, I was well and truly a Mac man at that stage and I'm getting this from my memories of a conversation I had with a PC friend a few years ago now.

Anonymous Freak
May 23, 2005, 01:30 AM
mad jew is right. Lots of old DOS programs (mostly games) were clock speed dependent. That is because early PCs did not have an internal (time) clock. Every time you rebooted, you had to manually tell it what the time and date are. It then kept track of time based on processor cycles. Since the first PCs were a fixed 4.77MHz, with very little variation, it was easy. Many programs relied on the processor's clock signal for their timing as well. If you increased the processor speed (I had a PC clone that had a toggle switch on the back to switch between 4.77MHz and 8MHz,) programs would run faster. Including time-based ones. So, a stopwatch program, for example, would run almost double speed, counting time way too fast. The computer's 'time' would also be accelerated. Worst of all, games ran too fast to actually play. (I once tried to play a Donkey Kong clone 'Jumpman' on a 486/66. The game was designed for a 4.77MHz 8088, which it ran fine on. The game was over (all three lives gone) before it had even redrawn the screen once.)

So, when processors started getting significantly faster than 4.77MHz (around the time of the 12MHz 286,) some computer manufacturers shipped them in 'slow' 4.77MHz speed by default, and made you click the 'Turbo' button to turn the speed up to full. Later, the 'Turbo' became the default, and you had to 'un-click' the Turbo button to get to 'Compatible' speed. On even later systems (late 386es,) the turbo button was pretty useless, as even with it off, the system was still way too fast for older programs. Finally, lots of shop-built systems just didn't even bother hooking up the Turbo button, making it do nothing at all. (On lots of systems with 'Tubro', there were small LED character readouts showing the current speed. These were set with jumpers on the back of the LED display, and didn't actually 'read' the speed from anywhere. You could program them to display anything you wanted, within the limits of the '8' shaped LED lines (calculator-style numbers.) You could have it say 'LO' and 'HI' for example, instead of actual speeds, as long as you were only using those 7 little segments.)