Multiple terms used to describe the same thing can confuse or even intimidate a computer user from grasping the fundamentals of overclocking. So, before we move on to the how-to part of this article, it’s a good idea to cover some of the various terminology associated with overclocking.
Speeds, Frequencies, Clocks
Core Speed (CPU Speed, CPU Frequency, CPU Clock Frequency, CPU Clock Speed): The frequency at which the CPU executes instructions (eg.: 3,000 MHz or 3.0 GHz). This is the main frequency we hope to raise out of all the other different components.
HyperTransport Link Speed: The frequency of the link between the CPU and the northbridge (eg.: 1,000 MHz, 1,800 MHz, or 2,000 MHz) This is usually equal to (but must not exceed) the northbridge speed.
Northbridge Speed: The frequency of the northbridge (e.g.: 1,800 MHz or 2,000 MHz). For AM2+ processors, increasing the northbridge speed will boost memory controller performance and L3 cache speed. This must not be lower than the HyperTransport link speed, but it can be raised much higher.
Memory Frequency (DRAM Frequency and memory speed): The speed, measured in megahertz (MHz), at which the memory bus operates. This can be represented by the actual frequency, such as 200 MHz, 333 MHz, 400 MHz, and 533 MHz or the effective speed, such as DDR2-400, DDR2-667, DDR2-800, or DDR2-1066.
Reference Clock: By default, this is set at 200 MHz. As you will see for AM2+ processors, other frequencies are based on this clock speed and are calculated with multipliers and sometimes dividers.
Calculating the Frequencies:
Before moving on to our description of how these frequencies are calculated, we need to mention that most of this guide covers what is involved when overclocking AM2+ processors such as the Phenom II-, Phenom-, or K10-based Athlon 7xxx series processors. But we also want it to apply to earlier AM2 (K8)-based Athlon X2 processors, such as the 4xxx, 5xxx, and 6xxx series. There are some differences in overclocking theses K8 processors, which are highlighted later in this article.
Here are the basic formulas used to calculate the above listed frequencies for AM2+ processors:
Core Speed = Reference Clock * CPU Multiplier
Northbridge Speed = Reference Clock * Northbridge Multiplier
HyperTransport Link Speed = Reference Clock * HyperTransport Multiplier
Memory Frequency = Reference Clock * Memory Multiplier
If we want to overclock our processor (increase its core speed), we can increase either the reference clock or the CPU multiplier. As an example, the Phenom II X4 940 runs a 200 MHz reference clock and a 15x CPU multiplier, resulting in a core speed of 3,000 MHz (200 * 15 = 3,000).
We could overclock this core speed to 3,300 MHz by either raising its multiplier to 16.5 (200 * 16.5 = 3,300) or by raising the reference clock to 220 (220 * 15 = 3,300).
But keep in mind that the other frequencies above are also based on the reference clock, so increasing it to 220 MHz will also increase (overclock) the northbridge speed, the HyperTransport speed, as well as the memory frequency. In contrast, simply raising the CPU multiplier would, in turn, only increase the CPU core speed of the AM2+ processors. In the following pages, we will first take a look at simple multiplier overclocking using the AMD OverDrive utility, and then head into the BIOS for more advanced overclocking by raising the reference clock.
Depending on the motherboard manufacturer, BIOS options for core speed and northbrdige speed will sometimes not use a simple multiplier, but instead a FID (Frequency ID) and a DID (Divisor ID). In this case the formulas would be:
Core Speed = Reference Clock * FID (multiplier)/DID (divider)
Northbridge Speed = Reference Clock * NB FID (multiplier)/NB DID (divider)
By keeping DID at 1, you are back to the same simple multiplier equation above and will see half multipliers 8.5, 9, 9.5, 10, etc. for the CPU. But by setting a larger DID such as 2 or 4, it is possible to end up with much smaller multiplier increments. To complicate matters, the values can be listed as a frequency, such as 1,800 MHz, or as a multiplier such as 9, while you may need to key in a Hex value. For help with this, consult your motherboard manual or do an online search to find the Hex values for keying in your CPU and northbridge FID.
There are other exceptions and you will not always be dealing with the above multipliers. For instance, the memory frequency is often set in the BIOS by directly choosing DDR2-400, DDR2-533, DDR2-800, or DDR2-1066 rather than by selecting a memory multiplier or divider. Also, the northbridge or HyperTransport link speed options may be the actual frequency and not a multiplier. At this point, try not to be too concerned about these variations but instead revert back here if and when the need arises.