Which Memory Clock Speed Is Most Suitable?
The Pentium D 805 works with a front-side-bus clock rate of 133 MHz (533 QDR). Thus, the design of the chipset dictates a maximum memory clock speed of DDR2-533. But overclocking the front side bus also raised memory clock rates; the actual memory clock rate that results from increasing the FSB clock rate may be calculated using a memory multiplier value. Typical Intel platform memory multipliers vary from 2.0 up to 4.0, and may be set as such. Because our overclocking starts from a very low FSB clock speed, a special set of circumstances emerges for the memory multiplier. Namely, for an FSB clock set between 133 MHz and 148 MHz, the only valid memory multiplier values are 3.0 and 4.0 - lower values make no sense.
| FSB (MHz) | Multiplier | Bandwidth |
|---|---|---|
| 133 - 148 | 3.00 | DDR2-400 up to 444 |
| 133 - 148 | 4.00 | DDR2-533 up to 592 |
| 149 - 266 | 2.00 | DDR2-298 up to 533 |
| 149 - 266 | 2.66 | DDR2-396 up to 710 |
| 149 - 266 | 3.00 | DDR2-447 up to 800 |
| 149 - 266 | 3.33 | DDR2-496 up to 888 |
| 149 - 266 | 4.00 | DDR2-596 up to 1066 |
We assume a multiplier of 4.0 is set, which raises memory clock rates for DDR2-533 for FSB clock rates of 133 MHz to the value consistent with DDR2-667, when the FSB clock is raised to 166 MHz. After that, as you increase the FSB clock stepwise up to 200 MHz, you'll also increase the memory speed up to DDR2-800. If you continue to increase FSB clock rates, you'll also set memory clock rates significantly higher as well.
Some motherboard vendors don't lay out their products to use very fast DDR2-1066 RAM; the reasons for this have to do with the components they choose to populate their boards. These must be of higher quality to deliver clean signals to memory at such high speeds, which makes motherboards more expensive. Even printed circuit board designs play a significant role: data lines designed to run at 500 MHz can be placed very close to one another with lengths of up to almost 4 inches (10 cm). A bad design can cause capacitance build-ups and resonance to develop among circuits, which in turn can cause instability or crashes when clock rates are cranked up.
At FSB clock rates below 149 MHz, memory multipliers lower than 3.0 are not available, because they make no sense in terms of the performance they deliver. With an FSB clock rate of 133 MHz, for example, a 2.0 multiplier produces memory speeds of only DDR2-266.
The various memory multiplier values available in the BIOS vary by motherboard vendor. The 4.0 value isn't always selectable, so buying the wrong motherboard can lead to performance degradation. In most cases, vendors refer to a memory multiplier of 4.0 as "Native DDR2-800."
