BX-133 Video Guide

PCI Bus Dividers- A problem no longer

There are three main external systems that interact with and are dependent upon the front side bus. These are the PCI clock, the AGP clock, the memory, and the processor. From the beginning, it was well known that the processor would not be a limitation to higher FSB speeds.

So, while the processor was left out of the equation for FSB overclocking success, 3 items still remained. Of these problems, the first one to be solved was the PC clock divider.

Ever since the introduction of the PCI bus standard, it has been known that cards are produced around a 33 MHz specification. Due to this fact, the FSB speed always has to be divided down in order to interface with the PCI bus. With the 66 MHz FSB speed that was predominate in the original Pentium era, the PCI bus divider was set to 1/2, reducing the FSB speed from 66 MHz to the 33 MHz needed for the PCI bus.

The 440BX brought along the 100 MHz FSB speed. The increase in FSB speed resulted in a divider increase for the PCI bus. No longer could the FSB be divided in half to provide a PCI bus frequency in spec. As a result, a 1/3 PCI bus divider was introduced. By dividing the FSB speed by 3, the PCI bus on a 100 MHz FSB system would be running at the desired 33 MHz..

It was thought that since the PCI divider was "set" to 1/3 on 440BX motherboards, running at a 133 MHz front side bus speed would result in numerous problems. It was assumed that the 44 MHz PCI clock that would result from a 133 MHz FSB would cause the malfunction of PCI cards, and in fact, this PCI bus speed did. It was not until the automatic 1/4 PCI bus divider was discovered by a hardware tweaker named Mark Booth that we came to realize that the "133 MHz FSB [is] a viable alternative to 100 MHz, 103 MHz, and 112 MHz front side busses." Armed with his oscilloscope, Mark showed that his ASUS P2B 440BX based motherboard would automatically choose the most desirable PCI bus divider in order to attempt to maintain the 33 MHz PCI bus standard. It was proven that at speeds up to 133 MHz, the motherboard chose a 1/3 divider (causing PCI bus problems) but at the 133 MHz speed the chipset would use the desired 1/4 clock divider.

It was later shown that it was not only Mark's ASUS P2B that had this property, but all newer 440BX based motherboards. With this discovery, we could rest assured that an out of spec PCI bus would not be a problem and that we could safely remove the PCI divider problem from our list of road blocks.