The Secret Of The Multiplier
The multiplier expresses the ratio between the processor clock speed and the FSB clock. For the Pentium D 805, the combination of FSB and processor clocks results in a multiplier value of 20x. By comparison with other CPUs with 200 MHz or 266 MHz FSB, this is a very high value - 12x and 14x are much more common. But this also makes the Pentium D 805 a good candidate for overclocking. Simply by raising the FSB clock rate to 200 MHz, the resulting CPU clock rate rises to 4.0 GHz once the 20x multiplier is applied (20x times 200 MHz equals 4.0 GHz).
You can peruse a list of all currently used multiplier values in our Intel Desktop Processor Overview chart.
An overview of all Intel CPUs back to the Socket 5 era (1993).
By default, the top-of-the-line Extreme Edition 965 uses a multiplier of 14x.
The multiplier for the Extreme Edition 965 can be set in the BIOS anywhere from 12x to 60x, but such capability comes at a high price. This CPU costs around $1,100, which few users can afford to spend just on one component. This is yet another reason why those who buy a high-end CPU should also consider investing the time and effort involved in overclocking.
The 20x multiplier is burned into the CPU and may not be altered using a special BIOS, nor with a hardware jumper on the motherboard.
Since the D 805's multiplier is fixed, there's only one way to overclock it: by increasing the clock speed for the front side bus.