Multipler or Bus Speed?
Multiplier and bus speed
How is your CPU clock speed determined? There are 2 factors that are multiplied together to determine what the MHz speed is that your CPU is marked at: the multiplier setting, and the front side bus speed.
The multiplier setting is the internal clock oscillator which determines at what ratio the CPU's internal speed runs at, based off of the system's bus speed. To put that simply, the CPU's internal speed (clock speed) is derived by taking the multiplier and multiplying this number by the system bus speed. For right now, the standard bus speeds are 66 and 100 MHz, although 133 is around the corner with Intel's "Camino" chipset, and 200 MHz is going to be the K7's frequency. Wow.
So to get the clock settings of a CPU, you multiply those two values together. For instance, let's take a Celeron 300A, the honey of the overclocking community. It is supposed to run at a multiplier setting of 4.5, on a 66 MHz bus. When you multiply these two numbers together, you get 297. Wait, 297?! It's supposed to be 300, right? Well, the discrepancy arises because the 66 MHz is actually 66.666666666666666666666 MHz, (the .6 goes on forever). This is just one of those electrical things that I can't explain. So, let's just see if you take 4.5 x 66.666666666666666666….you get 299.9999999999999999999999999. That's how the 300 MHz is derived.
Now this [multiplier x bus] calculation can be used to derive every speed of processor. Some processors are tricky, because versions were released with both 66 and 100 MHz versions. This shouldn't be a problem though, because most resellers/stores will let you know what the bus speed the CPU is at, and any good store would be able to tell you if you asked them (if it wasn't printed somewhere).
What does this have to do with overclocking?
Well, to get more performance out of your CPU, you have to change one of these 2 numbers, since the two values determine the clock speed of your CPU. Changing the multiplier setting is possible only on some CPU's however, as Intel locked the multipliers of their CPU's with their PII Deschutes core processors, which include Celerons as well. AMD K6-2's are not multiplier locked, so I used to run a K6-2 system that was supposed to be 350 (3.5 x 100) but had a bit of overclocking to 448 (4.0 x 112).
The more common method of overclocking (and only option in most Intel scenarios) is changing the bus speed that your system and CPU run at. The bus speed is the speed at which your processor "communicates" with the motherboard and some of the other system components, specifically the AGP and PCI buses.
I'd like to point out a differentiation between raising the multiplier setting and raising the bus speed. Raising the bus speed will give you better overclocking results than raising the multiplier setting. Why is this? Because the multiplier setting is internal to the CPU, raising it affects the CPU's operations only. Undoubtedly, this will still improve system performance, but not as much as raising the bus speed, which will speed up the CPU's operations as well as increasing the speed of the rest of the system. The main components that would help out would be the RAM and sometimes the video card, depending on what applications you're running. So hypothetically, if you have a CPU that's 366 MHz (5.5 x 66) and you bump it up to 400 by doing (6.0 x 66), this 400 will not be faster than if a 350 MHz (3.5 x 100) was bumped up to 400 by going (4.0 x 100).
This is probably a moot point, however, since most of you will probably not be able to change your multiplier settings anyway. This is because Intel started locking the multiplier settings around 3rd quarter (fall season) of 1998. With this in mind, if you plan to overclock, it's best to go with a CPU that has a lower "factory" bus speed setting. That is why Celerons are so popular, because they all come with 66 MHz as their default clock setting which means there are several bus speed settings they can choose from, all which increase the overall speed. Newer PIIs are harder to overclock, because they come at 100 MHz, which means in order to overclock you have to go to 112, which is usually a do-able mild overclock, but going higher than that may cause problems.