PC Hacks 100 Industrial-Strength Tips and Tools

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Hack 27 Set the CPU Multiplier

Changing the CPU's clock multiplier values can dramatically boost its performance without affecting other components.

The general nature of this hack applies only to AMD CPUs, all of which support multiplier value changes. Intel CPUs, with the exception of many Celerons, do not usually provide a means to change or even allow changing the CPU speed multiplier. Your system board must support the ability to change the processor clock multiplier value (the number the FSB or main clock speed is multiplied by within the CPU to arrive at the true operating speed).

The ability to change the multiplier value has significant merit in overclocking a system. First, it affects only the CPU, so neither the FSB nor the I/O bus clock values change. Second, it allows greater flexibility in altering the CPU speed; a small change of the multiplier value yields big changes in CPU clocking.

For example, if you have determined that your system will not boot or is unstable with an FSB/main clock value greater than 110 and your current CPU multiplier is 13 (yielding 1430 MHz), changing the multiplier value to 15 could yield a functional CPU speed of 1650 MHz. If 1650 MHz is too fast for your CPU chip to run stably, back down the FSB speed to 105 MHz and see if the CPU will survive at the resulting 1575 MHz, a 21% improvement from 1.3 GHz. AMD processors typically use odd-number multiplier values because of the way the CPU's internal clock works, so don't be surprised if trying x12, x14, or x16 leaves your system temporarily dead. Simply reset the multiplier value to an odd number and try again.

Though many system boards can self-recover from erroneous settings, a "dead" system may have to have its CMOS settings reset to factory defaults [Hack #3].

If you've tried changing the multiplier value and nothing happens or the value returns to default during P.O.S.T., then it is likely the multiplier value is locked in your CPU and you may be able to unlock your CPU multiplier [Hack #28] .

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