There are basically two main overclocking methods; increasing the front-side bus (FSB) or physically unlocking the processor and changing the multiplier, in effect emulating a higher-end CPU. We'll be taking a look at each, using the following reference system:
AMD Athlon XP 1500+ (1.33 GHz) Chip Markings: AGKGA 0138UPCW Motherboard: MSI KT266 Pro2 (VIA KT266A chipset) Memory: 512MB Crucial PC2100 DDR RAM Video: VisionTek GeForce4 Ti4600 Sound: Creative Audigy Network: Intel Pro/100 NIC Hard Drive: Western Digital 40GB 7200RPM ATA/100
Front Side Bus Overclocking
The first strategy we'll cover is tried-and-true FSB (Front Side Bus) overclocking. This method involves pushing the FSB from the Athlon XP's default 133 MHz FSB (266 MHz DDR memory speed) to whatever amount you choose, up to the limit of your motherboard, CPU or system peripherals. This method is the easiest to attempt, but can pose problems for overclockers as there are a number of variables that need to be taken into consideration.
DDR Memory Speeds and Types
The memory speed is a major factor when overclocking via the FSB. DDR (Double Data Rate) memory takes the standard 133 MHz FSB of the Athlon XP and doubles it to an effective 266 MHz. On most Athlon XP motherboards the FSB also runs synchronous to the speed of the system memory, so an increase in one spells a jump for the other. Even using a lower asynchronous setting, each MHz increase to the FSB still takes the memory speed higher.
If you're going to be overclocking using an FSB higher than 133 MHz, then it's a good idea to pick up some higher rated memory such as PC2400 (300 MHz DDR) or PC2700 DDR (333 MHz DDR) memory. It is always best to give yourself some headroom when overclocking via the FSB, as you want to make the CPU the limiting factor, not your memory. This is not written in stone, as some standard PC2100 can run well above its 133 MHz rating, while other modules may not. It's a hit-or-miss prospect with PC2100, so pick up some higher-grade PC2400/PC2700 DDR memory if you're really serious about overclocking the FSB.
Increasing the DDR memory voltage may also yield significant returns when overclocking the FSB. Not all motherboards have this feature, but those that do will commonly offer DDR voltage increases in 0.05V or 0.10V increments. This option is similar to increasing the CPU core voltage and may result in higher stability at high speeds. This should be used in moderation, as several motherboard manufacturers allow a wide range of voltage options and some (such the 3.2V max on some EPoX boards) are well beyond DDR specifications. The standard DDR voltage is 2.5V and moving it above 2.6V-2.7V is where the real risks start coming in.