Overclocking refers to pushing your computer components harder and faster than the manufacturer designed them to go. The initial pitch is seductive: Buy a slower, lower-cost CPU; juice up the clock speed; and presto! You have a cheap, high-end processor.

Of course, it's not that simple. Overclocking can certainly speed up your system (and save you some cash in the process), but only if you do it right.

We're going to talk about some of the basics of overclocking--what it really is, some of the basic math behind overclocking and how you might push your own system a little harder and faster. The goal here is to get better, stable performance for your money. After all, it doesn't matter how fast your computer runs if you have to wait for it to reboot from a crash every 10 minutes.

Though overclocking is a complex topic, we'll to keep the discussion here as simple as possible. We won't go into great detail on voltage changes or power issues, or examine the intricacies of memory timing. And we're not going to show you how to get your cheap Celeron up to 8.2GHz.

We will, however, talk about core CPU multipliers and memory clocks, and how the two relate to each other. Memory and CPUs are intricately interconnected, and simply pumping up the speed of one or the other may not yield the performance improvements you want to achieve.

What Is Overclocking?

Simply put, overclocking means setting your CPU and memory to run at speeds higher than their official speed grade. Almost all processors ship with a speed rating. For example, an Intel Core i7 860 runs at 2.80GHz out of the box. Overclocking a Core i7 860 means pushing it to a clock speed higher than 2.80GHz. This article will focus primarily on CPU overclocking to illustrate the core concepts (pun intended).

Processors don't instantly melt when you overclock them because a modern CPU's speed rating specifies the speed at which every processor in the same manufacturing batch cano run--a number that's likely to be considerably lower than the maximum speed that your specific processor is capable of.

In general, CPU yields are so good these days that the nominal speed grade locked into a chip may be far lower than the speed that the chip is capable of running a computer at. In other words, since the statistical distribution in the manufacturing process is skewed toward better-quality chips, odds are that your processor is better than its rated speed.

Disclaimers and Myths

Before we start adjusting core multipliers and memory clocks willy-nilly, let's pause for an important disclaimer:

Overclocking will void the warranty of your retail CPU. Overclocking may destroy your CPU, your motherboard, or your system memory. It may corrupt your hard drive. Be careful when overclocking. You have been warned.

After reading this disclaimer, you may be inclined to walk away. Don't. Moderate overclocking is mostly safe.

These days, Intel and AMD don't frown on overclocking as much as they did a few years ago. Both companies now ship CPUs equipped with core multipliers (which we'll discuss shortly) unlocked, and even CPUs that have locked multipliers are fairly easy to overclock.

First, though, let's take a look at a few overclocking myths.

Myth #1: Overclocking requires expensive liquid cooling or very noisy air coolers.

Actually this isn't a myth if you're planning on doing extreme overclocking. But moderate overclocking (one to two speed grades higher than spec) is often achievable without replacing or supplementing the stock cooler supplied with a retail CPU. On the other hand, a better cooler can extend the life of the product at those higher clock speeds.

Photograph: Rob Cardin

Myth #2: Different iterations of the same chip have the same capacity for overclocking.

Because the manufacturing yield is a statistical distribution, you'll probably get a CPU that can run much faster the listed speed, but you might end up with a processor that runs only about 10 percent faster. Consequently, the fact that your buddy down the street can run a Core i5 750 (rated at 2.66GHz) at 4GHz doesn't mean that your Core i5 750 CPU can will be able to run that fast. That caveat is well worth keeping in mind when you attempt to overclock.

Myth #3: Overclocking requires expensive motherboards and memory.

Not necessarily. We'll look at examples involving a fairly high-end motherboard (roughly $250), a $190 board, and a micro-ATX board that's priced around $140. The $300+ motherboards that gave rise to this myth are luxuries for people bent on extreme overclocking (which requires certain special features). Likewise, unless you want to overclock your DRAM to extreme speeds, modestly priced DRAM (which we'll be using in our examples) will work fine.

Bare-Bones Basics

We won't dive deeply into individual CPU architectures, but you do need to know some basic stuff.

All CPUs have a fundamental clock rate, from which all of the other clock rates inside the CPU are derived. Various sections of the processor take this fundamental clock rate, which acts as a kind of standard timekeeper, and multiply it to get an internal clock speed for a particular section of the CPU. In the Intel Core i5/i7 series of CPUs, the fundamental clock rate is called the base clock or BCLK (it's usually 133MHz). For its part, AMD calls this rate the CPU bus frequency; it's commonly set at 200MHz in AMD desktop processors.

Let's look at the Core i5 750 CPU for a moment. Like almost all Intel CPUs in the Core i5/i7 line, the Core i5 750 has a BCLK of 133MHz. On the other hand, the rated speed of the i750 is 2.66GHz. The main processor takes the BCLK number and multiplies it by 20 to get 2.66GHz (2666MHz)--this is the CPU multiplier. Note that Intel's latest CPUs also have a feature called Turbo Boost that allows the CPU to run at clock speeds higher than the default speed under certain conditions. For example, when only one core on the Core i5 750 is in use, the Turbo Boost frequency is 3.2GHz.

Most retail processors are clock-locked, which means that you can't increase the CPU multiplier beyond its rated speed. Some motherboards try to cheat in an effort to unlock the CPU multiplier, but in most instances involving typical retail CPUs, you can't unlock the multiplier. Admittedly, you can usually set the multiplier to a lower number than the maximum rating, but it's unclear why you'd want to do this.

You can usually adjust the setting for an Intel CPU's base clock (BCLK) or for an AMD processor's CPU bus frequency to any value you want. As with all stages of overclocking, though, you have to be careful here: Changing the underlying fundamental clock frequency will change a host of other parameters. Still, it's a useful tool to help with overclocking.

What to Expect From Your Overclocked Processor

Before starting the physical process of overclocking, think about what you're trying to accomplish. If you use your computer to run standard desktop applications--office productivity apps, Web browsers, and so on--overclocking is not worthwhile, since the higher clock speeds won't deliver noticeably better performance.

On the other hand, if you run system-intensive applications such as games that hit all of the different subsystems in your PC--hard drive, graphics, memory, and CPU--you'll see some gain by juicing up the CPU clock, but don't expect too much. Often, even high-end games are not CPU bound, and they may benefit more from a better graphics subsystem than from overclocking. Still, you'll see some increase after adjusting the clock speed.

CPU-intensive apps--particularly multithreaded applications--are likely to see the most significant boost from increased clock speed. Photo editing and video transcoding are examples of these types of programs.

Again, however, remember that the ultimate goal is speed with stability. Extreme clock speeds are merely academic exercises if the overclocked system can't run your applications reliably.