Without modifying the clock divider, you can also see why most people will make their FSB adjustments in 3 MHz increments. With a 1/3
divider, every time you increase the FSB 3 MHz, you increase the PCI bus 1 MHz. Since the PCI bus is the bus that likes being
overclocked the least this is an easy way to overclock the PCI bus 1 MHz at a time until the system fails to POST.
Now we can introduce some of the "non-standard" or "overclockable" clock dividers. Some motherboard manufacturers include different
dividers with their BIOS. The design of the motherboard and the chipset being used will determine the possible clock dividers. For
example, an Abit KD7 motherboard has a clock generator that is capable of supporting a 1/6 divider, but the KT400 chipset on the board
does not.
Some common clock dividers are 3:2:1, 4:2:1, 5:2:1 and 6:2:1. Clock dividers are a valuable tool in an
overclocking arsenal, because they have the ability to keep the buses running close to spec, even at extremely high FSB settings.
For example, let’s take the same P4A motherboard, and compare a FSB adjustment from 100 to 165 MHz using the stock clock divider and
then using a 1/5 divider.
FSB = 165
Clock Divider = 3:2:1
Base Value = FSB/Divider = 55 (165MHz /3)
AGP = 55 MHz * 2 = 110 MHz
PCI = 55 MHz * 1 = 55 MHz
FSB=165
Clock Divider = 5:2:1
Base Value = FSB/Divider = 33 (165MHz /5)
AGP = 33 MHz * 2 = 66 MHz
PCI = 33 MHz * 1 = 33 MHz
The advantage of the 1/5 divider becomes immediately apparent. Even though the front side bus has been pushed significantly higher
than stock speeds, the AGP and PCI buses are still running at their default speeds. Without the 1/5 divider, the PCI bus is running
at 166%; well beyond spec and easily preventing most motherboards from completing a POST.
One word of advice: don’t use the highest clock divider you can. Higher clock dividers coupled with "lower" front side bus speeds
can actually be counterproductive. I’ll use another example to demonstrate why:
FSB = 150
Clock Divider = 5:2:1
Base Value = FSB/Divider = 30 (150 MHz /3)
AGP = 30 MHz * 2 = 60 MHz
PCI = 30 MHz * 1 = 30 MHz
At this front side bus setting and divider, the buses are running under their stock speeds. While this certainly won’t affect
stability, it will limit performance. For example, with the AGP bus running at 60 MHz instead of 66 MHz, a user would get a lower
benchmark score than what the card and the system would typically produce at stock speeds.
Bus Lockdowns
Some motherboards may also have some form of bus lock down mechanism. Obviously, any "overclocking" board should incorporate this,
since it would be a useless feature for systems running at stock speeds.
While there are several different options down there, I’ll present the most common locks available. It’s always wise to check the
manufacturer’s documentation to see what kind of locks and dividers are supported.
FSB Triggered Lockdown
On some older motherboards, if you set the front side bus above and beyond a given speed, the system will automatically lock down
some of the other buses to maintain stability with the higher front side bus speed. This is especially true of motherboards using
PC-133 RAM. Set the FSB higher than 133 MHz and the memory will lock down at 133 MHz and run asynchronously.
Manual Lockdown
Some BIOS features will include a manual lock down for the PCI bus. The most common option is a strict 33 MHz lock, regardless of
the front side bus setting. Other options might include a relatively small PCI overclock, with fixed values like 37 or 44 MHz. For
example, 44 MHz would be the result of a PCI overclock using a 1/3 divider and a 133 FSB setting.
FSB/PCI Pairing
While most systems now support 1MHz increments to the front side bus, older model motherboards may not. Instead, they may provide
3MHz increments, or increments that pair a given FSB setting with corresponding PCI speed. For example, older Asus boards might
have had FSB options of 133/44, 150/50, 150/37, etc. Simply put, these are some preset speeds using 1/3, ¼ or 1/5 clock dividers.
Conclusion
Clock dividers become an invaluable tool in an overclocker’s arsenal, effectively making incredibly huge front side bus adjustments
a much more attainable goal. With motherboards, CPUs and RAM that can handle running at front side bus speeds of 200 MHz and higher,
clock dividers bring stability to a system that can be running twice as fast as it’s factory specification.
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