Overclocking Basics

Changing the multiplier is often difficult. For instance, on most Intel CPUs, the multiplier cannot be changed. However, AMD does produce CPUs that have a multiplier you can modify. You will have to check the specs on your specific motherboard on an AMD product sheet to see if your multiplier can be changed. If you cannot change the multiplier, you will have to change the FSB speed. One trick I found to change the overclockable Athlon CPU is called, simply, "The Pencil Trick."

To change the bridges on your AMD motherboard, get a 0.5mm mechanical pencil with graphite lead and a business card. Remove the processor from your system and locate the L1 bridges on the CPU, which are those responsible for locking out the multiplier. The bridges are the same for both the Athlon and Duron processors. Separate the bridges using the business card and, working from left to right, rub the pencil back and forth across the bridges until the normal gold color is gone and the bridges are black. It sounds outrageous, but the graphite in the pencil is ideal for reconnecting the leads to conduct electricity across the bridge. Reinstall your chipset, and your motherboard is ready for overclocking.

If you've got a motherboard that will allow you to change the FSB speed, there are steps of frequencies to allow you to increase the speed of the CPU to its maximum. Most boards will jump from 75MHz to 83MHz and 90MHz or higher. Because the PCI and AGP bus speeds are directly related to the FSB speed by a different multiplier, you will also need to keep track of their speeds. Some motherboards will automatically set these to the closest available speed to the normal specs: 33MHz PCI bus and 66MHz AGP bus. Most of today's CPUs can handle a 100MHz or 133MHz FSB speed, so that if you change the multiplier jumpers for the AGP and PCI, the bus speeds should be able to adjust accordingly.

Memory bus speeds are also clocked off of the FSB. Often, the FSB speed and the memory bus speed will be the same, and there is no way to change it. Some boards will allow a plus or minus 33MHz alteration, but you will have to check your board specs to determine if this is the case for you. If your board will allow this change and you are running a 100MHz FSB, you can bump your memory to 133MHz without problems. You should get the highest quality memory you can. For serious overclocking, you will want to get PC150 memory, which is memory that runs at 150MHz. Most memory can handle slightly higher speeds than what it is rated at. For example, PC100 memory can run at 110MHz, PC133 can run at 140MHz, etc. PC133 is the minimum you should use.

Now, I'll go into specifics on overclocking a couple of processors you might want to try out: the AMD Athlon and the Intel Pentium 4.

Overclocking An AMD Processor

The Athlon is a great overclocker. It is well-balanced, and its pipeline structure is built for high clock speeds. If you want to increase the FSB, you will need a Slot A motherboard like the ASUS K7M or the ASUS K7V that will allow you to adjust the FSB. These boards run at 100MHz, so, for example, a 7.5 multiplier will give you a 750MHz CPU. Once you have the board set up, all you have to do is change the FSB speed in the BIOS. If it doesn't work, the BIOS will reset to default values after a reboot.

To change the FSB value in the BIOS, reboot your computer and press F2 or DELETE to enter Setup. Find the option that lets you change your FSB, system bus, or clock speed. They should all be the same: between 66MHz and 133MHz. Increase your speed, keeping the capability of your RAM and motherboard in mind. Multiply the new value for FSB speed by your computer's multiplier, and this is your new CPU speed. If you don't know your multiplier, you should check the CPU manufacturer's Web site. If you don't have the option to change your FSB speed in the BIOS, you will need to change the jumpers on the motherboard. Checking the manufacturer's Web site will tell you which type of jumpers you will need to change.

If you're not going to change the FSB, you can change the multiplier. This is more difficult, but it can be done if you're feeling brave and know your way around a PCB and electronics components. In order to change the multiplier, you need to remove the plastic housing of the Athlon module and attach an overclocking device to the Athlon PCB. After you've got the box open and the device attached, you can change the multiplier from 5.0X to 10.5X by flipping a few switches. You can purchase an overclocking device from a local supply store or an online shop, or you can make one yourself using instructions available over the Internet.

Overclocking An Intel Processor

If you want to overclock your Intel CPU, you will have to choose your motherboard carefully, as there are only a few boards that will support overclocking. If you are overclocking a Pentium 4, you will also need a motherboard that will allow you to lock the bus speeds of the AGP and PCI buses because Pentium 4 processors are multiplier-locked. This means that the only way you can overclock a Pentium 4 is to modify the bus speed. With the AGP and PCI busses locked, the only thing you are overclocking is the CPU and memory, but you can even modify or slow down the bus speed of the memory on some boards, so you should only overclock the CPU. Use the method discussed above for AMD overclocking using the BIOS or the jumpers.

After Your Procedure

Once you've made your modifications, you will need to determine system stability. Normally, if you reboot your system after your mods without problems like failure to start, blue screens, or unexplained restarts, you have done it right. Another way to tell would be to run a high-impact software package or 3D video game. If it performs as it should, you're probably OK. If you're paranoid like me, you might want to get a benchmark software package available over the Internet or at your local software supplier (also, check out "Essential Overclocking Utilities" on page 25).


Final Notes

If things don't work out right once you've finished, your system should revert back to the default settings if you've modified your BIOS as I mentioned earlier. If you're brave (or stubborn), you can try it again. If you've modified your motherboard by moving jumpers, you'll have to reset all of the jumpers to their original positions; hopefully you took notes.

Finally, once you've got all of that extra power, you're going to have another problem: heat. All that extra processing power is going to fry your system if you're not careful. For tips on heat removal, check out the "Chill Chat" column on page 158 in this issue for help.

Hopefully this has, at the very least, given you a peek into the world of overclocking your system. It's by no means an end-all of what's possible, but I hope it's helped.

by David Miller