This option can be described many different ways, but it all boils down to the setting of the AGP and PCI dividers. When using a 66 MHz CPU like the Celeron 500, the AGP and PCI dividers are quite different than with a 133 MHz CPU like the Pentium III 833. Also, the available FSB speeds may be limited only to those that apply to a specific AGP/PCI divider setting. Like the FSB speeds, these AGP/PCI divider settings may be present in the system BIOS or through a set of on-board jumpers. The following images represent the most common formats, from jumpered designs to a full BIOS setup.

Here is a chart outlining the basic divider settings at each of the default settings:

By using basic math, you can see that at each of the default FSB speeds, the dividers each arrive at a speed of 66 MHz for the AGP bus and 33 MHz for the PCI bus. When using FSB speeds between the 66, 100 and 133 MHz settings, simply multiply the appropriate divider against the new FSB speed and the AGP and PCI speeds can be determined.

With motherboards based on newer chipsets such as the Intel 815, 810, 820 or VIA Apollo Pro 133A, there should be options for 66, 100 and 133 MHz operation, though the actual user manual may refer to them using different terminology. Older chipsets like the Intel BX only support the 66 and 100 MHz divider settings. Traditionally, AMD motherboards have only used the 100 MHz setting, though upcoming chipsets such as the KT133A will add the 133 MHz option. Some basic motherboards opt for a pure CPU Detect operation (thereby deriving the FSB from the CPU itself), and not even allow users to set these options manually.