Overclocking Guide - PAGE 3

The Software Factor

Now that the key hardware components are in place you need software to tie them all together and change the variables that will allow you to overclock your system. This is slightly misleading though because the motherboard includes the software. This is the function of the BIOS or Basic Input Output System. The BIOS of your computer is often an enigmatic area that even some experts don't know or understand in it's entirety. Fortunately there are only a few areas that the overclocker needs to be well versed in. We'll break these down and examine each area and its role in achieving a successful overclock. Keep in mind that not all these features will be available with all motherboards or chipsets but the more options in the BIOS the higher the degree of successful overclocking. They are certainly are features to be mindful of when shopping for a motherboard.

• CPU multiplier

• FSB frequency

• Memory frequency

• Vcore

• Vdimm

CPU Multiplier (AKA Clock Ratio)

The CPU multiplier takes the base CPU frequency and and multiplies it by the number you designate (most systems default to the preset CPU settings automatically) For example; if your CPU was say an AMD 1800+ with a base frequency of 133MHz the default multiplier would be 11.5, so 11.5 x 133 = 1530MHz. Changing the multiplier up is one way over overclocking systems because it results in increased frequency i.e. 12.5 x 133MHz now results in a frequency of 1663MHz. Typically you'll see multiplier settings ranging from 4 ~ 20 sometimes and a default option. It should be noted that while many AMD processors are considered "unlocked" that doesn't imply limitless head room and depending on the processor the amount you can increase the multiplier will vary slightly for the most part if you had 11.5 as a default you might be able to go 12, 12.5 and maybe 13 but don't count on getting to 20.

Front Side Bus Frequency (AKA System Clock)

The FSB is the interface between the CPU and northbridge which acts as a gateway to the rest of the motherboard. The higher the frequency of the front side bus, the faster the communication between the processor and the chipset. The FSB frequency in the current crop of CPUs is a bit misleading. The front side bus on the Athlon XP and the Intel P4 boards are actually only clocked at 200 MHz. Intel uses a quad pump while AMD uses a double pump. The double pump on the AMD side is more intuitive - the introduction of DDR memory allowed data transfer on both the rising and falling edge of the clock meaning that two units of data can be transferred per clock cycle. On the Intel side they take advantage of the Dual Channel memory setup of their newer boards and can transfer the equivalent of 4 units of data per clock (rising and falling edge of a clock (2), * 2 channels * actual bus speed (200) = 800 Mhz) resulting in a pseudo 800 Mhz FSB. This is not all smoke and mirrors as performance drops quite significantly with only single channel memory. Of course Intel was claiming a 400 Mhz FSB back in the early days of the P4, but it was a maximum of 4 data fetches per clock cycle instead of that happening in reality with single channel SDRAM.

PCI/AGP Bus lock

A PCI/AGP bus lock is important for the stability of systems being overclocked through the front side bus. On chipsets with an unlocked PCI/AGP bus, stress can be put on peripherals like sound cards and hard drives as those devices end up running out of spec. Some devices are a lot more tolerant to strange PCI bus speeds while other people have horror stories about disk and component problems resulting from an overclocked PCI bus. On chipsets like the Nforce2, the PCI/AGP bus is locked meaning that no matter what FSB frequency is used, the PCI/AGP speed will always remain at 33/66. On VIA KT600 based boards this is not true. The PCI/AGP bus relies on multipliers that determines the speed. On a system with a FSB of 133, the PCI divider is 4, while the AGP divider is 2. The next divider will not kick in till 166, where the divider is 5 and 2.5- this means that for a processor that does not overclock optimally, say it gets stuck at a 160 Mhz FSB, the resulting PCI bus would be 40 Mhz instead of the regular 33 Mhz. Furthermore, some boards will 'protect' end users by not allowing the FSB to hit the next divider plateau so even if a processor can hit the 166 Mhz mark on board A, board B may artificially lock out the ability to set the FSB higher than 165. These are factors to be considered when purchasing a motherboard for overclocking. Some vendors are better about this than others and have a bigger set of dividers that kick in at more reasonable levels but this is not always the case.

Memory frequency

Memory frequency is often preset to change with the FSB to keep all data in sync for optimal performance. Generally it is a good idea to keep memory and FSB frequency in sync- an example being the NForce2 which suffers a performance hit when the memory and FSB are run asynchronously. Higher specced memory may be required if the front side bus is raised a lot otherwise users may have to run things out of sync. Another approach is to change the CAS latency of the memory- a higher CAS latency will reduce performance but the penalty with a higher CAS latency is much less than that of running the memory and FSB out of sync. From testing in the lab, we have found that CAS latency is not as much of a performance factor as memory companies would have you believe and a higher FSB setting is a much higher benefit than having really low CAS latencies.

Vdimm

Vdimm is the setting in the BIOS for RAM voltage. Sometimes increasing the Vdimm will allow your RAM to overclock better. Just remember upping any voltages increases heat and component stress. Some higher end memory modules from Corsair actually require a higher than regular spec Vdimm voltage to run at its advertised speed.

Vcore

Vcore is the setting in the BIOS for CPU voltage. This increases the power to the CPU and is almost always necessary when overclocking any significant amount. Remember to be very careful with these settings, this is where you can do the most damage to your system by overheating the CPU. Good cooling and the monitoring of temperatures are both essential.

A lot of this functionality is also being exposed now with software that runs directly out of Windows. A handful of motherboard manufacturers provide utilties for such purposes, some more sophisticated than others. Nvidia has an utility of their own, Forceware, that supports the adjustment of FSB speeds, Vcore adjustments as well as multiplier locks (on supported motherboards). In general we have found that these utilities are less daunting than working with the BIOS for beginners but they do not work as well as setting BIOS options manually. Sometimes the quality of the programs are not too good although there have been major improvements made in the past year.