>Hi,
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>Most modern motherboards have BIOS settings that let you control CPU speed. Most of the time these settings are "Auto" which runs CPU, front side bus and RAM at maximum rated values as determined by the components themselves.
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>It may be that on that machine the BIOS speed settings have been set manually to a lower-than-maximum figure. This could be because:
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>- machine doesn't work at all, or is flaky at "rated" speeds. Basically someone hasn't bothered to track down the hardware problem, they just de-tuned the speed until things work again
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>But if they've de-tuned it can I not do the opposite (at least to find out what happens). I've been through the BIOS and tried pretty much all of the combinations that I thought might affect this. It's an AMI BIOS. The machine seems to run fine whether I let the BIOS set up to optmimum performance itself or manually max all the options - except that the processor is *always* at 2.1GHz.
>
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>- there have been a few cases of builders buying chips that failed rigourous testing, then installing them in systems with de-tuned speed settings. These sometimes show up in "too good to be true"-cheap desktop PCs. Hopefully this is not your case, otherwise the whole machine is questionable.
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>You may have hit the nail on the head here. The machine in question was just a white box job intended as a replacement for the family at home and bought on EBay. In fact I'd already rejected it because other parts of the system were not as described. But I'd still, just out of curiosity, like to know exactly why the CPU is running below its rated speed.

Even motherboards with BIOS speed settings also have hardware jumpers that can manually control speed settings. This is to overcome the chicken-and-egg situation where you set the speed too high in the BIOS; now the machine won't even start, to let you get back into the BIOS to set it lower again. It may be that the motherboard jumpers have been set to a "manual" speed setting, not letting you set the speed in the BIOS at all (or so that those settings have no effect).

If you don't have the mobo manual, as long as you can find its exact model and revision # you can usually download mobo manuals from the manufacturer, which will fully explain jumper settings.