Bus clock multiplier
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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