Multiplier Math


I built a computer a month ago that’s running Windows XP on a 2.6GHz Pentium 4 CPU. For some reason the computer thinks it is 1.3GHz. I’ve tried to change it in the BIOS but it will only let me overclock it to 1.54GHz.

—Daichi Tang

The Doctor suspects that you have something set incorrectly in the BIOS. Remember, non-Extreme Intel CPUs are upwardly locked, but can be underclocked. The solution depends on what Pentium 4 you have. There were actually two different .13 micron Northwood 2.6GHz Pentium 4 processors (not counting the numerous 2.66GHz Pentium 4s). The two 2.6GHz P4s include the original running on a 400MHz bus, and a second iteration running on an 800MHz bus. The 400MHz bus chip gets to 2.6GHz by using a 26x multiplier on a 100MHz bus (26 times 100MHz gives you 2,600MHz). The chip that runs on the 800MHz bus has a 13x multiplier with the base clock of the bus running at 200MHz (13 times 200MHz gets you to 2,600MHz). The Doc is guessing that you have the 400MHz version processor, but that the multiplier is set for 13x, which multiplied by 100MHz would yield 1,300MHz, or 1.3GHz. You should go back into the BIOS and try setting the multiplier for 26x on a 100MHz bus. Or try setting it for 13x at 200MHz. If you actually have a 2.66GHz Pentium 4, it uses a 20x multiplier on a 133MHz FSB.

SUBMIT YOUR QUESTION Are flames shooting out of the back of your rig? First, grab a fire extinguisher and douse the flames. Once the pyrotechnic display has fizzled, email the doctor at for advice on how to solve your technological woes.

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