What did the turbo button on old PCs do?
From Wikipedia's turbo button article:
The button was generally present on older systems, and was designed to allow the user to play older games that depended on processor speed for their timing.
Older games often would run programs as fast as the processor allowed. Since the developer designed the game for a 33MHz processor, as long as the user had a 33MHz processor, everything functioned as designed. Once the user upgraded to a 66MHz processor, though, the game now ran twice as fast, making it unplayable. The turbo button would slow the computer down to deal with this effect.
It would switch the processor between 4.77Mhz (the stock speed for an IBM XT) and whatever faster speed the processor was capable of - often 8Mhz. My hunch is that it was switching between two different crystals on the motherboard, but I don't have an old XT mobo lying around to follow the traces to be sure.
Nice picture, it depicts exactly the same casing I had on my 386DX :)
[Stolen from Wikipedia]
The turbo button changes the effective speed of the system. It usually accomplishes this by either adjusting the CPU clock speed directly, or by turning off the processor's cache, forcing it to wait on slow main memory every time. The button was generally present on older systems, and was designed to allow the user to play older games that depended on processor speed for their timing. Systems could also use the keyboard combination of ctrl-alt-+/-, '-' switching turbo off and '+' switching it on (ctrl-alt-\ to toggle between turbo/normal mode on ITT Xtra machines). Of course, calling it a "turbo" button when its function slows the system down can be a bit misleading, but the button was usually set up so the system would be at full speed when the button was "on". The turbo button was often linked to a MHz LED display on the system case, or to a "hi"/"lo" LED display. While the implementation of the turbo button by manufacturers has all but disappeared, Software Developers have compensated with software replacements. One example is DOSBox, which offers full turbo button functionality. Some modern PCs that support ACPI power management provide an on-screen control for the user to switch a PC's performance state between low- and high- performance modes.