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It's been a long time since I built a PC, and I can't remember the methods for over/under clocking it. I have an Intel P4 1.8GHz processor, and PC133 SDRAM memory. However, I've been running this machine headless for a while and after plugging a monitor in and seeing the BIOS initialisation, I noticed it set to PC100.

What's the calculation for setting this up? Should I basically set the FSB to 133, then whatever multiplier required to get the desired 1.8GHz clock speed?

Sorry this question is so n00b, but I've totally forgotten everything I learned when I built this thing.

I don't want to overclock it - I just want the "correct" settings (ventilation isn't great where the tower is situated).


I noticed the BIOS settings were memory speed = "auto" and the multiplier was "x4". I've changed this to 1333 x 13 ( = 1729), but not seen any noticeable difference. The BIOS POST states "Intel (blah blah) 1.8GHz (100 x 1.8). Could it be overriding my settings with something else?

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There may be nothing wrong with your setup. The external clock to the CPU can be different than the clock to the RAM. I would assume that this is case unless something more definite is listed in your BIOS.

The P4's bus in quad-pumped so you'll sometimes see it's bus speed listed as 400 MHz since it transfers data at the same speed as a a standard 400 MHz bus. So in effect it is still running faster than your PC133 RAM.

Your processors multiplier is locked, it won't respect any change you make in BIOS. So if you try to up your processor bus speed to 133 MHz you'll end up with a CPU sped of 133.33 * 18 ~= 2400 MHz, which is a pretty tall over clock. So what may have happened when you made your changes in BIOS is that the computer failed to boot and automatically reset it self back to safe values.

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Standard SDRAM doesn't do anything fancy with the clock signal (like DDR which is edge triggered ticks twice per clock cycle) so the apparent and actual signalling speeds will be the same.

So I would guess that for 1.8Ghz core clock and 133MHz external clock the multiplier wants to be around 13.5 (1800/133).

[obviously make sure your motherboard has a memory controller that can run the RAM and CPU FSB at 133 - but if my memory of RAM and CPU tech advances is right you'd be hard pressed to find a P4 capable board that doesn't support running at that signalling speed - and also make sure that any add-on cards you have can cope with any affect the change has on PCI signalling rates]

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Ahh, great stuff - I actually just tried that and I'm not sure I'm seeing a difference - see my updated question above. – Neil Barnwell Oct 28 '09 at 18:15
Check that there are no BIOS updates available for your board - it could be that you have older BIOS firmware that doesn't correctly accept/recognise/display some combinations. – David Spillett Oct 28 '09 at 18:21

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