Internal frequency of microprocessors is usually based on
Front Side Bus frequency. To
calculate internal frequency the CPU multiplies bus frequency by
certain number, which is called clock multiplier. It's
important to note that for calculation the CPU uses actual bus
frequency, and not effective bus frequency. To determine actual
actual bus frequency for processors that use dual-data rate buses
(AMD Athlon and Duron) and quad-data rate buses (all Intel
microprocessors starting from Pentium 4) the effective bus speed
should be divided by 2 for AMD or 4 for Intel.
Clock multipliers on many modern processors are fixed - it is
usually not possible to change them. "Extreme" versions of processors
have clock multipliers unlocked, that is they can be "overclocked" by
increasing clock multiplier in motherboard BIOS. Some CPU engineering
samples may also have clock multiplier unlocked. Many Intel
qualification samples have maximum clock multiplier locked - these
CPUs may be underclocked (run at lower frequency), but they cannot be
overclocked by increasing clock multiplier higher than intended by
CPU design. While these qualification samples and majority of
production microprocessors cannot be overclocked by increasing their
clock multiplier, they still can be overcloked by using different
technique - by increasing FSB frequency.
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