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Dual Graphics Platforms, Part II

Watch Out For CPU And RAM Clock!

The more the system clock is increased, e.g. from 200 to 240 MHz, the more you will automatically overclock both your processor and your RAM. An example would be the Athlon 64 X2 4400+. It defaults to 2,200 MHz, resulting from a 200 MHz system clock speed and a multiplier of 11. Overclocked at 240 MHz, the core clock speed would boost up to 2,640 MHz and this could easily be too much for some of the processors that are available today. Raising the processor voltage usually helps, but we won't go into details here as this article is primarily meant to give you an impression of how overclocking AMD's silicon works.

However, the memory clock is derived from the processor clock: At 2,200 MHz the system would apply a divider of /11 in order to hit the 200 MHz base clock of DDR400 memory. At 2,640 MHz /11, your RAM clock speed would be as high as 240 MHz. Again, this could easily be too fast, especially if you run tight memory timing parameters. Try loosening the RAM timings while determining maximum clock speeds when overclocking.

Every motherboard BIOS will offer you the choice of at least 200 and 166 MHz memory speeds (DDR400, DDR333). You can determine the multipliers that the system will select for each of these memory speed settings by dividing the CPU's default core clock speed by the base memory clock speed the BIOS provides you with. For example, to archive 200 MHz RAM clock (DDR400), the 2,200 MHz processor will divide the clock speed by 11. For 166 MHz (DDR333), it would be 13.5, resulting in 162.96 MHz. Note that multipliers must be a multiple of 0.5, so it won't apply the exact figure (13.2) that 2,200 / 166.66 would indicate.

In order to determine the highest possible clock speeds for both the ATI Radeon Xpress 200 XE and the NVIDIA nForce4 SLI X16 platforms, we extended the memory timings to the slowest settings, in order to find out what the highest system clock speed would be.