Unlike the graphics processor, there's no direct way to manipulate the speed of the AGP bus. In fact, the speed of the AGP bus relies on two things - the speed of the system bus and the AGP bus clock divider. With most processors coming with a fixed multiplier, the speed of the processor is usually dependent on the speed of the system bus. So, if you want to overclock the processor, that will usually involve the overclocking of the system bus and consequently the AGP bus as well because the AGP bus speed is slaved to that of the system bus.

You will be more or less restricted in your choice of system bus speeds because with most processors coming with a fixed multiplier, it all depends on just how well your processor can overclock. Needless to say, it's best to overclock the processor as high as you can with the highest bus speed you can use. Don't worry about your AGP card not being able to withstand the higher bus speeds. The AGP bus speed can be reduced using the AGP bus clock divider if the AGP card shows signs of instability with the higher bus speed.

AGP Clock Divider

That brings us to the second factor in overclocking the AGP bus - the AGP bus clock divider. This divider controls how fast the AGP clock runs in relation to the system bus. The purpose of this divider is to reduce the AGP clockspeed to the official clockspeed of 66MHz when the system bus is running over 66MHz. Most motherboard chipsets will only allow 1/1 and 2/3 AGP clock dividers though newer ones will have an additional 1/2 AGP clock divider. Let's just consider the first two AGP clock dividers.

If the AGP clock divider is set as 1/1, the AGP bus will run at the same speed as the system bus. This is the default setting for systems that are running with a 66MHz bus speed because the AGP bus will then be running at the specified 66MHz as well. But if you overclock your system bus to 100MHz, the AGP bus will be overclocked to 100MHz as well. This setting allows you to overclock the AGP bus as high as you can overclock your system bus.

On the other hand, you have the 2/3 AGP clock divider. It is the default setting for systems that are running with a 100MHz bus speed because it makes the AGP clock run at 2/3 of the system bus speed which is 66MHz. This allows you to overclock the AGP bus less than you can with the 1/1 clock divider but the AGP bus speed will be closer to the official speed of 66MHz.

How To Adjust The AGP Clock Divider

It depends on the motherboard you are using. Many motherboards allow you to adjust the AGP clock divider by jumpers or dip switches while others enable you to adjust the AGP clock divider via the BIOS. Quite a few other motherboards will autodetect the proper AGP clock divider for the system bus speed your system is running at. In that case, you will not be able to adjust the AGP clock divider.

If your motherboard uses jumpers or dip switches, you'll need to consult your motherboard manual for the correct jumper or dip switch to adjust. However, if your motherboard uses the BIOS to adjust the AGP clock divider, look for a BIOS function called AGPCLK/CPUCLK, AGPCLK Divider or something similar. It will allow you to set the AGP clock divider. It should have settings of 1/1, 2/3 and even 1/2 for some motherboards.