How to: Overclocking for newbies

Overclocking your processor can give your PC a significant speed boost--but you have to be careful

Myth #3: Overclocking requires expensive motherboards and memory.

Not necessarily. We'll look at examples involving a fairly high-end motherboard (roughly US$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.

The Candidates

We'll be looking at three different CPUs, each on a different motherboard. The first combination is AMD's latest high-end CPU, the Phenom II X6 1090T running on an Asus Crosshair IV motherboard (the high end of the Asus line for AMD processors, typically priced at around $220 to $250).

The second pairing is Intel's new Core i7 875K clock-unlocked CPU--the high end of Intel's socket 1156 processors--running on an Asus P7P55D-E Pro, which costs about $190. The Core i7 875K is easy to overclock and can push to extreme clock frequencies quite easily.

The last processor/motherboard combo is Intel's Core i5 750--a modestly priced, clock-locked processor that nevertheless has substantial overclocking possibilities--running on a Gigabyte GA-P55M-UD4 micro ATX board, which is available for around $140.

Automatic, Semiautomatic, or Manual?

Many motherboards today offer automatic (or semiautomatic) overclocking. We tend to avoid the fully automatic overclocking. When it is enabled, the BIOS setup program runs a few tests and then sets the various clock speeds (CPU and memory) to the highest clock speeds it considers stable. We've found, however, that these high speeds often become unstable when you boot into Windows and try to run applications for a significant length of time.

The semiautomatic approach lets you pick from several overclocking presets and specify limited overclocking. Asus's 'CPU Level Up' feature is one example of this approach.

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