BX-133 Video Guide

AGP Bus Divider- A lingering problem

So far, we have eliminated 2 problems out of 3 that prevent a 440BX motherboard from running smoothly at 133 MHz. All we are left with the AGP bus divider, a problem that still remains.

Like the PCI bus, the AGP bus does not operate at full front side bus speed. In a similar manner to the PCI bus's 33 MHz specification, AGP products are designed to interface with the computer via a 66 MHz AGP bus speed. As we noted before, the 440BX was designed around a 100 MHz FSB, a fact which produced all the aforementioned problems. Running at a 100 MHz FSB requires a 2/3 divider to achieve the desired 66 MHz AGP bus speed. Unlike the 1/3 PCI bus divider that was found to switch to a more favorable 1/4 divider when at 133 MHz, the AGP bus divider on the 440BX is set to an unchangeable 2/3. This means that when overclocking, the AGP bus jumps from a stable 66 MHz to an out of spec 88 MHz, speeds which can cause system crashes and failures due to video cards running out of specification. We can therefore see that really the only limiting factor to running a BX board at 133 MHz full time is the locked 2/3 AGP bus divider. Other 133 MHz FSB motherboard solutions, such as the i815 and the Apollo Pro 133A, provide a fix to this problem by using the proper 1/2 divider necessary.

AGP Bus Divider- A solution

Luckily for us, there is a way around this problem. Some have found that certain video cards perform correctly at this grossly out of spec speed, allowing them to run their BX systems at 133 MHz or beyond even with the 88 MHz AGP bus speed. By testing the collection of video cards we have in the lab, we could accurately predict what cards will work on the 88 MHz AGP bus. Be it if you currently use a 440BX running at 133 MHz and are looking to upgrade your video card or if you are looking to push your BX system up to the 133 MHz mark, the following results should help you achieve your goal.