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Overclocking Basics; HTT, CPU Multiplier, Memory Divider

Tight Timings vs High Clock Frequencies
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As most of you are probably aware, the MHz clock speed of the Athlon 64 and Opteron is the product of the HyperTransport speed (HTT) times the multiplier of the CPU. In the case of the Opteron 165 we used, the default multiplier is 9x and the default HTT speed is 200 MHz, resulting in a CPU speed of 1,800 MHz. The multiplier is only adjustable downwards, while the HTT speed can be adjusted both up and down. There is also a third adjustable factor: the memory speed divider. By default this is set to 1/1, which means that the memory clock frequency will be the same as the HTT speed (200 MHz system base speed equals 200 MHz DDR memory base clock).

Increasing the CPU multiplier would be the easiest way to overclock a processor, but it is factory locked for values higher than the default setting, so you can't. Thus, to overclock, you are going to have to increase your HTT speed. If you increase your HTT speed from 200 MHz to 300 MHz, and lower your CPU multiplier to 6 at the same time, you still get a CPU speed of 1,800 MHz. If your memory divider remains set at 1/1, this means that your system memory will also run at 300 MHz instead of its default speed of 200 MHz. Instead, you should set a memory divider of 2/3, which will bring the memory clock frequency back down to 200 MHz. You pretty much achieve nothing in doing this, but it shows that overclocking is quite flexible nowadays.

As you can see, overclocking your system memory (and not your CPU) requires an increased HTT speed together with a decreased CPU multiplier. Then, all you have to do is play around with memory speed divider settings.

There is one obstacle in the memory scaling benchmark effort, though. If you intend to increase your memory frequency by a small amount - for example, from 200 MHz to 205 MHz - you will have to increase the HTT base clock by this amount. This also means that your CPU clock speed will increase by the clock speed gain (5 MHz) times the CPU multiplier. In this case, CPU clock will rise from 1800 MHz to 1845 MHz.

That makes things a bit more complicated for us. We don't want varying CPU speeds while we are analyzing memory performance, because it will have an impact on system memory performance results. Unfortunately, there is no way to isolate different system memory speeds and settings as the only factor impacting benchmark results.

We selected some benchmarks that are particularly dependent on CPU speed and system memory speed. We then ran them while changing only the CPU multiplier, isolating CPU clock frequency as the sole varying factor.

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