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Phenom II Brief Overclocking Guide

You may think that overclocking AMD processors by raising their clock generator frequency is well-studied approach that has been in successful use since first 64-bit Athlon 64 CPUs appeared in the market. However, it is not quite the case. The launch of quad-core AMD processors together with the introduction of Stars (K10) microarchitecture changed a lot about the way we overclock. It happened due to the introduction of an L3 cache shared among all cores and the modification of the voltage regulator circuitry. That is why Phenom overclocking procedure has certain peculiarities that need to be taken into account when you overclock Phenom II processors based on new 45nm cores.

In order to correctly understand all peculiarities of an overclocked Socket AM2+ platform functioning, you should keep in mind that these systems use several independent frequencies at the same time:

  • Processor frequency, which is usually mentioned among the main CPU specifications. This is the most important parameter affecting the system performance: it describes the actual operational frequency of the computational processor cores.
  • Frequency of the North Bridge built into the CPU. This is the frequency of the L3 cache built into the processor core and of the memory controller next to it. All Phenom and Phenom II processor models have this parameter set at 1.8 GHz or 2.0 GHz, but nevertheless, it also affects the overall platform performance to some extent.
  • DDR2 memory frequency. This is the major memory subsystem parameter, which, however, is also determined by the CPU. This frequency depends on the type of memory modules used in the system. Socket AM2+ systems usually work with DDR2-800 or DDR2-1067 SDRAM clocked at 800 or 1067 MHz respectively.
  • HyperTransport bus frequency. This bus connects the CPU with the chipset North Bridge. Phenom and Phenom II processors working with HyperTransport 3.0 have this frequency set at 1.8 or 2.0 GHz.

All these four base frequencies are connected with one another, they are all set by one base clock generator. However, the exact values are determined by the corresponding multipliers. This dependency formally looks as follows:

  • [CPU frequency] = [CPU multiplier] х [HT Reference clock];
  • [HT frequency] = [HT multiplier] х [HT Reference clock];
  • [NB frequency] = [NB multiplier] х [HT Reference clock];
  • [Memory frequency] = [Mem multiplier] х [HT Reference clock].

All four multipliers are completely independent and may be changed in the mainboard BIOS Setup. The only condition that you should keep in mind when changing the frequencies of different components is that the HyperTransport bus frequency is at all times lower than the frequency of the North Bridge integrated into the CPU.

The base clock generator frequency marked as [HT Reference clock] in the formulas is set at 200 MHz by default. [CPU Multiplier] is determined by the nominal frequency of a specific model but can be adjusted on Black Edition CPUs. [HT Multiplier] and [NB Multiplier] are set to 9x or 10x by default, but in reality they can vary within a much broader interval. As for the supported value range for the memory frequency coefficient, ensures CPU compatibility with DDR2-400/533/667/800/1067 SDRAM at nominal base clock generator frequency.

For example, for the Phenom II X4 920 processor discussed today, working with DDR2-1067 SDRAM:

  • [CPU frequency]: 2800 MHz = 14 x 200 MHz;
  • [HT frequency]: 1800 MHz = 9 x 200 MHz;
  • [NB frequency]: 1800 MHz = 9 x 200 MHz;
  • [Memory frequency]: 1067 MHz = 5.33 x 200 MHz.

As it follows from everything we have just said above, overclocking of contemporary AMD processors is possible not only by changing their multipliers, but also by increasing the clock generator frequency over 200 MHz. Since the clock generator is located on the mainboard, its frequency can be changed in the mainboard BIOS Setup; it is not detected or blocked even in non-Black Edition CPUs.

However, it is important to understand that by raising the clock generator frequency you will not only push up processor clock speed, but will also force the memory bus and HyperTransport bus frequencies to increase appropriately. It will also affect the operation of the L3 cache. All these factors may in the end restrict overclocking success. However, luckily, you can always use special settings in the mainboard BIOS Setup to eliminate these obstacles by lowering the corresponding coefficients.

Moreover, you can also increase the voltage of the main system components in order to improve their frequency potential during overclocking. Although these measures increase not only the frequency potential but also heat dissipation and power consumption, proper cooling will make them an important part of the successful overclocking procedure. There are three voltages that matter most of all for platforms built around processors on Stars (K10) microarchitecture:

  • Processor Vcore. This is the voltage of the CPU cores. Phenom II X4 processor family usually has 1.35 V core voltage. It is considered safe and effective to increase this parameter to 1.5 – 1.55 V with appropriate cooling involved.
  • Voltage of the North Bridge built-into the CPU. Slight increase of this parameter may be good when the L3 cache and memory controller frequencies are increased over their nominal values.
  • Memory voltage. This parameter has indirect effect on the processor overclocking potential. It allows additional overclocking of DDR2 SDRAM installed into the system.

Everything we have just said makes the general rules of Phenom and Phenom II overclocking pretty evident. The most important part of the procedure is increasing the clock generator frequency. The resulting CPU clock speed depends on it linearly via the clock frequency multiplier of the specific CPU model. When you overclock these processors, you should make sure that the frequencies of the integrated North Bridge and HyperTransport bus do not exceed 1.8 – 2.0 GHz, that is why you have to lower the corresponding multipliers as necessary. The same is true for the DDR2 SDRAM frequency multiplier: the actual memory clock speed shouldn’t exceed the physical potential of the memory modules used, which may call for lowering the multiplier determining this frequency setting.

In addition, you may increase the CPU core voltage to improve the overclocking results even more. In this case, however, you should pay special attention to proper heat dissipation and install an efficient cooler.

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