What is the job of Bus/Core Ratio

Bus/core ratio is the CPU multiplier if I recall.

Yes the ratio is the CPU Multiplier but the job of the ratio is to derive the clock signal for the CPU based on the speed of the clock signal used by the Front Side Bus.

If your Front Side Bus Reference Clock is 400 MHz and your CPU Multiplier is 8X your CPU Clock Frequency will be 3.2 GHz.

In the CMOS Setup Utility you'll notice that you cannot set the CPU Clock Speed independent of the Front Side Bus Reference Clock and CPU Multiplier settings.