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.
Let's walk through a semiautomatic scenario using Intel's unlocked Core i7 875K processor. First, press the Delete key during the boot process to get into the BIOS setup program. One of the tabs in the Asus setup is Ai Tweaker, which is a one-stop shop for tweaking system settings. Navigate through the screen with the arrow keys, and press Enter to select a particular setting to change.
The first setting you can change is 'CPU Level Up' (shown in the expandable screenshot at left). The setting options are Auto (which doesn't do much) and three presets, each prefaced by the word 'Crazy'. As it turns out, the first two settings aren't all that crazy; they simply let you boost the clock rate by one or two speed grades.
For the CPU Level Up feature to work with this particular CPU, open the Advanced tab, select Intel SpeedStep Technology (you'll have to scroll down the screen to reach it) and disable it. Note that disabling SpeedStep also disables power management. You didn't think that overclocking would be free, did you?
After you select one of the first two 'Crazy' options in CPU Level Up, the system will boot up and run at the higher clock speed.
Using CPU Level Up carries some risks. For example, though we're using an unlocked CPU, the BIOS setup assumes that the multiplier is locked. So instead of increasing the clock multiplier, the CPU level up boosts BCLK to 160MHz at the 'Crazy-3.52G' setting--and that alters memory timings.
Though our particular CPU was stable at 3.52GHz under stress testing, we recommend sticking with the 3.2GHz setting. That's almost certain to yield a successful overclock with this CPU, and it pushes BCLK to a less aggressive 146MHz. The memory speed isn't much greater than the default 1066MHz, but it is a little faster, at 1170MHz.
Now let's look at manually overclocking an Intel Core i5-750. This quad-core CPU's default clock speed with all four cores running is 2.66GHz. We'll use the Gigabyte GA-P55-UD4 for our exercise. Once you understand how multipliers work, the manual process is pretty easy. Let's take a look at a typical BIOS setup screen.
The expandable screenshot at right shows the advanced frequency settings screen in the Gigabyte P55M-UD4 BIOS setup program. To access this program, press Delete during bootup. We're concerned with three settings: 'CPU clock ratio' (the CPU multiplier), 'BCLK frequency' and 'System Memory Multiplier'. The default values are 20X for the CPU multiplier, 133MHz for the base clock, and 8.0 for the system memory multiplier.
We were able to easily push the Core i5 750 to nearly 3.5GHz by setting BCLK to 166MHz. This also pushed up the memory frequency to 1328MHz. Our more extreme overclock proved successful, but this system had already been running well for months with the following settings:
· CPU clock ratio: 20X
· BCLK: 150MHz
· System memory multiplier: 8X (yielding a memory speed of 1200MHz)
This particular setup has been rock-solid for us at 3.0GHz, running the stock Intel retail cooler.
Now let's look at the AMD system.
The Phenom II X6 1090T processor already runs at 3.2GHz, so pushing it up a couple of speed grades means running the system at 3.6GHz. So we'll set a limit of 3.6GHz using a fairly high-end CPU cooler, the $50 Thermalright Ultra 120 rev C with a fairly stock 120mm fan.
First, let's look at how the BIOS screen appears before we tweak the settings. The expandable screenshot at left shows this screen.
Although the settings for this AMD-based motherboard are similar to those for Intel-based motherboards, they are just a little different. The key items to consider for purposes of overclocking are the CPU ratio and the DRAM frequency.
Our final setting screen is shown in the expandable screenshot at right.
The 1090T is clock-unlocked, so all we had to do was set the Ai Overclock Tuner to manual and change the CPU ratio from the default value of 16 to a new value of 18. In fact, AMD permits you to select incremental settings (for example, 16.5X multiplier), so technically our overclocked system is now running four speed grades faster.
Note that the only other change we made was to manually set the DRAM frequency to 1333MHz. That's it--and the system ran for several hours under stress testing.
If you overclock your system, it makes sense to run a stress test to confirm the system's stability. For serious stress testing, it's hard to beat the combination of FurMark running simultaneously with four instances of Prime 95.
Both of these programs have specific modes for conducting repetitive stress testing. Setting up that test takes a little work and knowledge, but running the two tests concurrently pushes a system hard. If all you want to do is test CPU stability, Prime95 should work fine by itself.
Another testing option is a robust system benchmark like PCMark Vantage. Unfortunately, many such programs cost money these days, as do utilities with stress-testing elements, such as SiSoftware Sandra. Still, you can generally get by with the free software we use for most stress testing.
You can save yourself a little money and gain a little performance by overclocking your CPU. Of course, here we've touched only on CPU overclocking. It's also possible to overclock memory and graphics cards. But pushing all three PC subsystems simultaneously is an advanced topic that requires patience and lots of stress testing.
So check out your motherboard's capabilities and dip your toes in the water. You should find that bumping your system up one speed grade is easy. But be cautious in your approach, and don't try to push too hard. If you're willing to limit yourself to one or two bumps in performance, you should end up with a stable system that's a little faster.