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. Some CPU engineering samples may also have the clock multiplier unlocked. Some versions of processors have clock multipliers unlocked that is, they can be 'overclocked' by increasing the clock multiplier setting in the motherboard's BIOS setup program. 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.Ĭlock multipliers on many modern processors are fixed it is usually not possible to change them. For calculation, the CPU uses actual bus frequency, and not effective bus frequency. To calculate internal frequency the CPU multiplies bus frequency by a number called the clock multiplier. The internal frequency of microprocessors is usually based on FSB frequency. The external address and data buses of the CPU (often collectively termed front side bus (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) to transfer data faster.

For example, a system with an external clock of 100 MHz and a 36x clock multiplier will have an internal CPU clock of 3.6 GHz. A CPU with a 10x multiplier will thus see 10 internal cycles (produced by PLL-based frequency multiplier circuitry) for every external clock cycle. In computing, the clock multiplier (or CPU multiplier or bus/core ratio) sets the ratio of an internal CPU clock rate to the externally supplied clock. ( Learn how and when to remove this template message) JSTOR ( October 2019) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification.

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