Very often, the pci and agp bus speeds are a division of the FSB clock.
So for instance if the nominal FSB speed is 133MHz, the pci speed would be FSB/4 (22MHz) and the agp speed would be FSB/2 (66MHz). If the FSB speed is increased to 150MHz, this would change the pci speed to 37.5MHz and agp speed to 75MHz. This can be enough to cause numerous faults on pci and agp buses, and consequently cause instability. This is often called the southbridge multiplier.
Often the memory speed is linked to the FSB speed too, this is called the Northbridge multiplier.
Some motherboards have the ability to use a separate clock for the Northbridge and Southbridge speeds, which is useful in determining maximum stable processor speeds, but the result is that memory and peripheral transfers are out of sync with the processor, which reduces performance slightly, but as we have seen from 3GN's posts the end result is still better than standard.
And while this is usually done in bios settings, motherboard manufacturers often supply software to adjust FSB and multiplier speeds 'on the fly', which can be extremely useful for testing system stability - if the settings have been locked into the bios, and the system won't boot because the speed is too high, the only solution is to clear the cmos and start again, and hope you can remember what all the settings were.
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