You need to set your bus speed from 100Mhz to 133Mhz. This question has been getting asked every other day lately for some reason.
Here's how it works:
The CPU operates at a multiple of the system bus speed (in fact, everything does!). Now when I say system bus, I mean the memory bus....AMD likes to confuse this issue because it has a faster pipe between the memory hub and the CPU than between the memory hub and the actual memory, so they brag about a faster system bus which means NOTHING to you, the guy setting up his machine. Also, DDR confuses the issue a little bit too, since 266Mhz DDR is actually operating on a 133Mhz clock, but data is sent on both sides of the clock tick thereby doubling performance.
So, you know your XP processor wants a 133Mhz bus (by design), and the XP 1800 wants to run at 1533Mhz which means that the clock multiplier is 1533 / 133 = 11.5. This multiplier is fixed in the CPU itself, and cannot be changed (well, not easily).
So, what happens when you stick your XP 1800 in a machine with the system (memory) bus set to 100Mhz? Let's toss 100 into the equation, knowing that the multiplier is constant at 11.5, and see what CPU speed you end up with:
100 * 11.5 = 1150Mhz
Voila, there's your mysterious number.
In other words, you need to jump your bus speed to 133Mhz.
It's interesting to note that both PCI and AGP also operate at a multiple of this sytem bus, but its multipliers are fractional. The PCI bus is supposed to run at 33Mhz, so modern motherbords (chipsets actually) offer multipliers of 1/2, 1/3, and 1/4 to accomodate bus speeds of 66, 100, and 133Mhz respectively. AGP wants to run at 66Mhz, so chipsets offer 1/1, 2/3, and 1/2 multipliers.
So if your system bus is set to 133Mhz, you know that you're supposed to use a PCI multiplier of 1/4 and an AGP multiplier of 1/2. The reason I bring this up is that a common way to overclock CPU's with fixed multipliers, like the XP and P4, is to bump the bus speed up. This can be somewhat dangerous because small jumps in bus speed result in huge jumps in CPU speed, and therefore people often select non-standard bus speeds.
Let's say you set your system bus speed to 145Mhz. Your XP 1800 has a fixed multiplier of 11.5, so it's going to run at 1667Mhz, which is probably not enough to make the CPU unstable. However, consider that you only have the above multiplier choices for AGP and PCI, and both AGP and PCI have to run at AT LEAST their standard rates. So, you're going to still be using a PCI mulitplier of 1/4 and an AGP multiplier of 1/2. Therefore, your PCI bus will be running at 145/4 = 36Mhz instead of 33 and your AGP bus will be running at 72Mhz instead of 66. This isn't necessarily dangerous, but it can be, and it can lead to overall system instability. I've actually successfully overclocked plenty of times using this method without ever damaging anything, but you never know.
The only reason I bring this up is that it kind of helps to understand what's going on a bit, AND because I know that if you've got a motherboard that allows you to set the bus speed in small increments, you're going to be very tempted to overclock
Slackmaster 2000
