Putting the theory into practice
Now that we have explored the basics of modifying your system's clock speeds, let's talk about putting that theory into practice. Like we said earlier, overclocking is generally an iterative process. You'll want to start with a relatively modest overclock, save the BIOS settings, and then reboot the system to see if it POSTs and makes it all the way to your operating system. If it does, you'll want to confirm that the BIOS changes you made have produced the intended result. Fire up CPU-Z to confirm that the bus speeds you've set in the BIOS are in effect and yielding the correct processor clock speed. CPU-Z can also be used to check on the CPU voltage, which will become more important in a moment.
Our example overclock was a success!
However, we've found that CPU-Z's voltage readouts aren't always quite correct. To get more precise insight into your system's health, you'll want to install your mobo maker's system monitoring application, which is usually included on your motherboard's driver CD. These apps often have flashy or frustrating interfaces, but since they come from the board maker, they should give you correct readings for system voltages, clock speeds, and temperatures. Here's a look at Gigabyte's Easy Tune application on our test system.
The board is (pretty much) honoring our request to lock PCIe at 100MHz, and PCI is at 33MHz, as a result
If you poke around a little in Easy Tune, you'll find a single page that shows voltages, fan speeds, and temperatures all at once.
We'll want to leave this window open to keep an eye on processor temperatures. Core 2 chips start throttling around 72° Celsius, and I wouldn't want to exceed the mid 60s under load.
Speaking of load, it's time to fire up a stability test to ensure that everything is working smoothly. We like to combine a couple of different applications—something like Prime95 to hit the processor and memory in addition to a graphics demo to stress the video card—to ensure that a system is stable. Prime95 will toss out computational errors or even hang your system if the CPU is overclocked too far.
Two instances of Prime95 pinning a pair of processor cores
If you're running a multi-core processor, be sure your stress testing produces a full load on all your processor cores. You can run extra instances of Prime95 by copying the program executable to another directory and launching it from there. The processor affinity for each Prime95 instance can then be set through the app's advanced options.
If Prime95 runs stable for five minutes or so without triggering uncomfortable CPU temperatures, it's probably time to move on to higher clock speeds. If Prime95 throws an error, the system locks or reboots, or you're treated to Windows' dreaded Blue Screen of Death, well, you're going to need some additional help.
You'll also need some additional magic if you can't even get that far. As your processor's clock speed rises with the front-side bus speed, you'll eventually hit a wall where the system will either refuse to POST, refuse to boot your operating system, or otherwise become unstable. You've now exceeded the capabilities of your system, at least with stock voltages.
If you can't get the system to boot far enough to allow you to modify the BIOS, you'll probably start to sweat a little, like I do, as the worry begins. That's never fun. But worry not, because salvation is but a simple step away. You can clear out your overclocked BIOS settings easily. Just find the "clear CMOS" header on your motherboard and slide a jumper onto its two prongs for five seconds or so in order to reset the BIOS to its fail-safe default values. This header is usually clearly marked and located somewhere near the CMOS battery.
Our example board has just two prongs on its header, but many boards have three prongs, with a jumper pre-installed on pins two and three. Moving the jumper to pins one and two will usually do the trick in arrangements like that. However, you'll want to consult your motherboard's manual for advice on your particular board's exact CMOS reset procedures.
After the CMOS has been cleared, the system should happily POST again. You can then go back to your last stable BIOS settings and keep tweaking, if you wish. Here you have the option of calling it a day or pushing on with a little extra juice.
