BIOS guide: how to overclock your CPU

AMD BIOS: main menu

This is the main BIOS menu for an MSI Socket AM3 motherboard. On a board like this, you can access the settings used for overclocking by selecting “Cell Menu” and pressing Return. On other manufacturers’ boards, the option may have a different name, such as “CPU Settings”, “Frequency Control”, “Ai Tweaker” or “MB Intelligent Tweaker”.

AMD Bios: CPU settings

Here we see the various CPU settings, with a summary at the top showing the currently applied settings – a base clock of 200MHz and a multiplier of 18, giving a CPU frequency of 3.6GHz. The multiplier is referred to here as the “Ratio”, and by default it’s set to “Auto”, which will select the fastest multiplier permitted by the CPU.

AMD BIOS: multiplier options

Select the “Auto” setting (by moving down to it and pressing Return) and you’ll see plenty of options. In this example we’re changing the multiplier to 20, for an effective speed of 4GHz. With an unlocked chip you can take this number as high as you like: this board supports speeds in excess of 5GHz, though it’s very unlikely any CPU will run stably at these speeds. If your CPU isn’t unlocked, its multiplier won’t go above its stock setting: you can specify a higher setting in the BIOS, but this will have no effect.

AMD BIOS: new CPU frequency

We’ve selected our chosen multiplier and pressed Return. Now we’re returned to the CPU settings page. The figure below the cursor shows that the new CPU frequency will be. The numbers at the top of the page haven’t changed: the new settings are only applied when we save our changes and exit the BIOS. If you want to try out your new settings, you can do this now (the key commands are shown at the bottom of the screen). If you make a mistake, simply quit the BIOS without saving your changes and they’ll be discarded.

AMD BIOS: base clock

If your CPU is locked, you can raise the base clock instead of the multiplier. On this board the base clock is referred to as the FSB (front side bus). We’ve returned to the regular multiplier setting of 18, but raised the base clock by 20MHz – a 10% increase. This gives us an effective CPU speed of 3,960MHz, but it also raises the NB (north bridge) frequency by 10%. The north bridge – more properly called HyperTransport on AMD boards – is what connects the CPU to high-speed components such as the memory and graphics card. It’s only designed to run a 2.0GHz, so overclocking it in this way could cause problems.

AMD BIOS: north bridge multiplier

We can compensate for this by reducing the north bridge multiplier. Ordinarily this bus runs at 10x the speed of the base clock, but we can change that multiplier in the same way we changed the CPU multiplier. By turning it down to 9x we can get very close to its intended 2GHz speed while keeping our CPU frequency at close to 4GHz. Having done this, we can exit the BIOS and try out our new settings.

Intel BIOS: main menu

This is the main BIOS screen for a Gigabyte Sandy Bridge motherboard. On a board like this, you can access the settings used for overclocking by selecting “MB Intelligent Tweaker” and pressing Return. On other manufacturers’ boards, the option may have a different name, such as “CPU Settings”, “Frequency Control”, “Ai Tweaker” or, as above, “Cell Menu”.

Intel BIOS: main menu

This is the main BIOS screen for a Gigabyte Sandy Bridge motherboard. On a board like this, you can access the settings used for overclocking by selecting “MB Intelligent Tweaker” and pressing Return. On other manufacturers’ boards, the option may have a different name, such as “CPU Settings”, “Frequency Control”, “Ai Tweaker” or, as above, “Cell Menu”.

Intel BIOS: CPU settings

The settings we want to change are the frequency settings: if your motherboard doesn’t show these directly within the CPU menu, they may be within a submenu, as here. On this page you can also see some of the current CPU settings, including a base clock of 100MHz (actually very slightly below, but this isn’t important).

Intel BIOS: advanced frequency settings

In the Advanced Frequency Settings page, you can see that the CPU’s stock speed is set with a multiplier of 33 on a base clock of 100MHz, giving an effective frequency of 3.3GHz. On Intel processors with Turbo Boost you may have to go into CPU Core settings to find the multiplier settings; on older chips, you may be able to change the multiplier directly.

Intel BIOS: adjusting multipliers

Our sample CPU uses Turbo Boost. It therefore applies different multipliers depending on how many cores are in use. Here we can pick maximum multipliers to use when 1, 2, 3 and 4 cores are in use. We can also set power limits, which will govern how hard these cores can work before the CPU automatically cuts its own speed to keep power consumption and heat down.

Intel BIOS: adjusting the base clock

If you want to adjust the base clock instead of the multiplier, that can be done from the main Frequency Settings page. This board lets you adjust the base clock (BCLK for short) in intervals of 0.1MHz, for fine tuning. It’s not recommended to tinker with this on a Sandy Bridge board though, as it affects not only the CPU but many other components that are unlikely to work properly if overclocked.

Once you’re happy with your change, you can exit the BIOS and save your settings to try them out. The key commands are shown at the bottom of the screen.