AGPCLK / CPUCLK from The Tech ARP BIOS Guide!

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AGPCLK / CPUCLK

Common Options : 1/1, 2/3, 1/2, 2/5

 

AGPCLK / CPUCLK Quick Review

The AGPCLK / CPUCLK BIOS feature allows you to set the ratio between the AGP clock speed and the CPU bus (also know as front side bus or FSB) clock speed.

This allows you to keep the AGP bus speed within specifications (66 MHz) while using a much faster CPU bus speed.

When the ratio is set to 1/1, the AGP bus will run at the same speed as the CPU bus. This is meant for processors that use the 66 MHz bus speed, like the older Intel Celeron processors.

The 2/3 divider is used when you use a processor running with a bus speed of 100 MHz. This divider will cut the AGP bus speed down to 66 MHz.

The 2/5 divider is used when you use a processor running with a bus speed of 166 MHz. This divider will cut the AGP bus speed down to 66 MHz.

Generally, you should set this feature according to the CPU bus speed you are using.

This means using the 1/1 divider for 66 MHz bus speed CPUs, the 2/3 divider for 100 MHz bus speed CPUs, the 1/2 divider for 133 MHz CPUs and the 2/5 divider for 166 MHz CPUs.

 

AGPCLK / CPUCLK Details

The AGP bus clock speed is referenced from the CPU bus clock speed. However, the AGP bus was only designed to run at 66 MHz while the CPU bus runs anywhere from 66 MHz to 133 MHz.

Therefore, a suitable AGP bus to CPU bus clock speed ratio or divider must be selected to ensure that the AGP bus won’t run way beyond 66 MHz. This is where the AGPCLK / CPUCLK BIOS option comes in.

When the ratio is set to 1/1, the AGP bus will run at the same speed as the CPU bus. This is meant for processors that use the 66 MHz bus speed, like the older Intel Celeron processors.

The 2/3 divider is used when you use a processor running with a bus speed of 100 MHz. This divider will cut the AGP bus speed down to 66 MHz.

The 1/2 divider was introduced with motherboards that provide 133 MHz bus speed support. Such motherboards need the 1/2 divider to make the AGP bus run at the standard 66 MHz. Without this divider, the AGP bus would have to run at 89 MHz, which is more than what most AGP cards can withstand.

The 2/5 divider was introduced with motherboards that provide 166 MHz bus speed support. Such motherboards need the 2/5 divider to make the AGP bus run at the standard 66 MHz. Without this divider, the AGP bus would have to run at 83 MHz, which is more than what most AGP cards can withstand.

Generally, you should set this feature according to the CPU bus speed you are using. This means using the 1/1 divider for 66 MHz bus speed CPUs, the 2/3 divider for 100 MHz bus speed CPUs, the 1/2 divider for 133 MHz CPUs and the 2/5 divider for 166 MHz CPUs.

If you are overclocking the CPU bus, you are supposed to reduce the divider to ensure that the AGP bus speed remains within specifications. However, most AGP cards can run with the AGP bus overclocked to 75 MHz. Some would even happily run at 83 MHz! However, anything above 83 MHz would be a little iffy.

In most cases, you can still stick with the original AGP bus / CPU bus clock divider when you overclock the CPU. This means that the AGP bus will be overclocked as well. But as long as the AGP card can work at the higher clock speed, it shouldn’t be a problem. In fact, you can expect a linear increase in AGP bus performance.

Be warned though – overclocking the AGP bus can potentially damage your AGP card. So, be circumspect when you overclock the AGP bus. 75 MHz is normally the safe limit for most AGP cards.

 

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