At a certain point in time the major CPU manufacturers — Intel and AMD — decided to produce their CPUs with locked multipliers. This meant that we were no longer able to manipulate our CPU frequency by simply increasing the multiplication factor of the system bus. But all was not lost. Motherboard manufacturers started to produce motherboards with incremental adjustments for the system bus itself, designed for overclocking. Once limited to a handful of standard bus speeds such as 60, 66, 100 and 133MHz, modern motherboards now have the capacity to generate any bus speed over a wide range in 1MHz increments. This gives us an alternative method of increasing the overall CPU frequency, but it also introduces another layer of complexity in understanding the process.
Increasing the system bus frequency also increases the system memory frequency, as the memory frequency is also generated from the system bus frequency. To overcome this limitation it was necessary to either use memory with a high frequency rating to cope with the speed increase, or select a motherboard with memory frequency multipliers. This gave a range of multipliers which could be utilised to bring the memory frequency back down to as close to its normal speed as possible. In some cases, to achieve a desirable CPU overclock it was necessary to run the memory at slightly below its rated frequency.
Armed with this knowledge of how our system frequencies are generated there are two more issues we need to address.
In order to generate more power from our motor car engine, we need to feed more fuel into it. Similarly, when we start pushing our CPU beyond its factory frequency setting, it will most likely require some added juice. Modern motherboards allow us to increase the voltage to the CPU. This will allow the CPU to operate at even higher frequencies than might otherwise be possible at the default voltage settings, and help to stabilise the CPU when we reach our final frequency during the stability testing phase. We may also need to add voltage to the Northbridge chipset on Intel based systems, particularly as the bus speed is pushed towards and past the 200MHz mark.
Please note, however, that increasing the voltage to any component in your PC increases the risk of that component failing. There are many forum groups and hardware review sites on the internet where you can research “safe” limits for voltage increase on specific CPU and Motherboard models, and if you are an inexperienced overclocker it is always wise to stay well within these limits.
Overclocking your CPU generates extra heat. Overvolting your CPU generates extra heat. The fan and heatsink combination which ships with your CPU has been specifically engineered to cope with the thermal load your CPU will create while under normal conditions at its factory default settings. It won't be able to effectively dissipate the extra heat generated during any moderate to extreme overclocking attempts. This is why overclocking enthusiasts will always replace their CPU Fan and Heatsink with specially developed third party solutions. These heatsinks use fluid filled pipes which rapidly migrate heat away from the CPU surface and quite often dissipate more than twice the heat load of the original unit.
Some overclockers move beyond the realm of air cooling and use water cooling setups, which work in much the same way as the cooling system in your motor car. Beyond this there is an extreme fringe of overclockers who will introduce water chillers or phase change refrigeration into their cooling setup and push their CPU to the absolute limit.
I have been overclocking my own PCs since the mid-1990s using either high-end air cooling or water cooling, and still enjoy the feeling of knowing that I have a system capable of outperforming any default configuration on the market. Generally, however, I am willing to sacrifice a little bit of bleeding-edge speed if it allows me a significant reduction in voltage and heat output. For example, my current gaming rig has an Intel Core i7 875K 2.93GHz CPU running at 4.2GHz. This chip will run stably at just over 4.4GHz but requires quite an extra push in voltage to get there, with a significant rise in temperature being recorded when the chip is under heavy processing load. The loss of 200MHz is pretty much unnoticeable from a performance perspective and the chip is obviously a lot less stressed at this speed.
The final step in the overclocking experience is the testing phase. It is always wise to push your CPU speed up in small increments. Going to high too soon could result in damage to your CPU or motherboard. Nobody wants to blow up a $500 CPU or motherboard on their first day of testing. The way I approach my overclocking is to do some research first. I like to get an idea of the potential of a particular CPU model from the many hardware review sites on the internet and use that as a starting point for my testing. Every single site I checked was able to achieve at least 3.8GHz from the 875K CPUs they tested. All but two of the tests hit 4.2GHz or more, so I decided to begin my testing at 3.8GHz. If the PC posts, and boots successfully into windows, I then run a stress testing program called prime95. This runs all CPU cores at 100% whilst generating prime numbers and checking the results for accuracy. You can download the latest version of Prime95 from our Web site here: Prime95 download.