Frecuencies are generated by a clock generator (yeah I know, sounds redundant but it's like this). Later the mobo uses dividers and multipliers to achieve different clock frecuencies.
For example, PCI run at 33Mhz; FSB is "Front Side Bus" and run at 100, 133 or 166 for Athlons (depends on model) and P4 runs at 400 or 533 (also depends on model); a given CPU can run at 1330; etc. If the clock generator produces 133Mhz frecuency, a 1/4 divider will be used to run PCI at 33Mhz while a x10 multiplier will be used for CPU.
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