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Why underclock?
Heat and faulty power are the two greatest causes of death in electronics. The certified geek Olympic sport of overclocking usually shortens the life of CPUs and perhaps the mainboard itself simply because running a chip at a faster frequency makes it run hotter. You may have seen some of the extremes that overclockers put into cooling their high-revving Athlons and Pentiums. But no matter. Maybe the overclocking yielded an extra FPS or two in an FPS like Quake or UT2003. (That first FPS is Frames per second, the second is First Person Shooter.)
While many geeks with PCs have been consumed by squeezing more speed out of their system than the law allows, more mature mainframe geeks have long been focused on adding 9s. As in uptime. Speed has been secondary to finding ways to make hardware more reliable and less prone to failure. And that's what underclocking is all about. Usually, that is.
Quiet is cool
Terry Gray wants quiet, not speed. He got into underclocking in his quest to build a quiet PC. Specifically, a PC without fans on the CPU and powersupply. His site documents his experiments with underclocking a Pentium III in order to be able to run it at an acceptable temperature without need of a CPU fan. Other underclockers have done the same.
CPUs are not the only chips being underclocked to attack heat and noise problems: video cards are also game. Leo Velikovich details the hows and whys of such an effort on a GeForce4 Ti4200 in his article in SilentPCReview.