CPU Multiplier

In computing, the clock multiplier (or CPU multiplier or bus/core ratio) measures the ratio of an internal CPU clock rate to the externally supplied clock. A CPU with a 10x multiplier will thus see 10 internal cycles (produced by PLL-based frequency multiplier circuitry) for every external clock cycle. For example, a system with an external clock of 133 MHz and a 10x clock multiplier will have an internal CPU clock of 1.33 GHz. The external address and data buses of the CPU (often collectively termed front side bus or FSB in PC contexts) also use the external clock as a fundamental timing base, however, they could also employ a (small) multiple of this base frequency (typically two or four) in order to transfer data faster.

Internal frequency of microprocessors is usually based on front side bus (FSB) 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 the actual bus frequency for processors that use dual-data rate (DDR) 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. Some versions of processors have clock multipliers unlocked, that is they can be "overclocked" by increasing clock multiplier setting in the motherboard's BIOS setup program. 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 overclocked by using different technique – by increasing FSB frequency.