In the Forums...
Posted: January 25, 2000
Written by: Keith "Farrel" McClellan
System Bus Multiplier and Multiplier Locks
The system bus multiplier takes the system bus * whatever the multiplier is to determine the speed at which the processor is running. That means that a computer running at the 100 MHz bus speed with a 4.5 bus multiplier would be running at 450 MHz. Simple enough to overclock your computer without messing with the system bus, right? Wrong. Why is that? It is because Intel had the audacity to lock the clock multiplier on its processors. That means that your computer HAS to use the 4.5 bus multiplier to derive the processor's clock speed, and dramatically limits the speed range of most processors. This, combined with the fact that most computer components don't function properly on non-standard bus speeds, makes overclocking most computers difficult (to get a completely stable system you have to jump up to the next standard bus speed - a mighty task for most processors).
Of course, AMD has (sort of) come to the rescue by not locking the system multiplier. However, to change the setting, you have to break the chip's casing open, hence voiding the warranty (overclocking voids your warranty anyway - so no big deal). They were even so nice as to include an edge connector to allow the connection of third-party jumpers to make overclocking a snap. Of course, you get the best results using a soldering iron... but that's an entirely different article.
Chip Voltage and Stability
Chip voltage can turn a not-quite-so-stable chip into rock hard granite. Most CPU's have some sort of way to change the voltage of the chip. Raising (and in some rare cases, lowering) the chip's voltage can create a much stabler chip, at the cost of more heat. Heat, of course, alternately lowers the overclockability of a chip, but it doesn't lower the chip's overclockability as much as upping the voltage raises it. And besides, there is always cooling. But more on that later.
The basic theory on chip voltage and how it affects the processor is this: a higher chip voltage increases the signal strength between transistors within the chip, allowing the signal to ignore greater discrepancies within the silicon core itself. You see, the silicon wafers used to make the chips aren't always pure, and they definitely aren't all of the same quality. A chip with a higher clock rate is generally going to have a core made of a higher quality silicon wafer (something that can't be determined until after fabrication, due to the fact that all the wafers are as pure as they can make them).
Now, the processor signal has two choices as to how to deal with a chip impurity (how it is dealt with has to do with quantum physics and really isn't imperative to this discussion). It can either jump the gap, or go around it. When the processor frequency is lower, the signal has the time to go around the defect if need be, but if the frequency is too high and the signal must go around, the signal doesn't get to its destination in time or at all (remember we are dealing with millionths or billionths of seconds), causing a miscalculation that usually will cause some form of software error (commonly it causes a crash).
However, upping the core voltage is like giving the signal a running start, it allows the signal to jump gaps within the chip with relative ease (sort of like a lightning arc), and the signal gets to it's destination in time.