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Origins of Overclocking
Overclocking is popular today in large part due to the fact that modern PC circuitry makes it easy to do. You have motherboards with variable clock speeds on them and processors that respond to signals from the motherboard telling them how fast to run. It wasn't always this way, but overclocking on PCs has in fact been done for over 10 years. On other small microcomputers it goes back over 20 years!
On the PC, overclocking probably goes back to the original IBM AT, which used the Intel 80286 processor. There were two versions of this original system, running at either 6 or 8 MHz (later systems expanded the speed range of the 286 much higher). PC users who wanted more power realized that it was possible to convert the 6 MHz system to 8 MHz by replacing the 6 MHz clock crystal with an 8 MHz one. Like today's overclocking, sometimes this worked, sometimes it didn't, and sometimes it seemed to work but caused "software problems".
I have talked with some serious hardware hackers, guys who were building their own PCs from circuit boards before the PC was even a glimmer in IBM's eye. Apparently the Z80 processor was overclocked by some enterprising hardware experimenters back in the 1970s. This was done in a way similarly to how it was done on the 286, by changing the frequency of the oscillator circuit that drove the CPU clock. Overclockers would experiment with tuning and adjusting the oscillator to precisely control the speed of the machine in a way that cannot be done with modern hardware.
Modern PCs use neither an adjustable oscillator nor a fixed-speed clock crystal; they employ a precision-controlled, variable-speed clock circuit. A single motherboard can typically run at any of several different speeds, depending on how it is configured. This is probably the key step that really allowed overclocking to become popular, because it became much easier to change the frequency of the motherboard--there was no need to tinker with oscillator circuits, and no specialty hardware knowledge was required to overclock.
Next: Who Overclocks?