Hardware

Overclocking 101: An overview

Stepping up your processor speed is both dangerous and fun. It can also help you test software. In this article, James McPherson tells you all you need to know about overclocking before you speed things up on your processor.


“Why should I overclock my processor?” is a question frequently asked by users. The tech addict answers with the classic line, “Because it’s there.” While philosophical, that answer doesn’t help the IT professional who needs a justification to void a warranty. The best reason is that it can make “obsolete” equipment usable again. Low-cost Celeron PCs purchased as backup or for light-use systems can be given significant performance increases. You can also overclock to research software performance on future hardware. As you’ll see in this article, overclocking can make the fastest processor available today as fast as the processors of tomorrow.
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Background
Every processor sold today has a speed rating, indicating the rate that the processor cycles. Think of this rating as the RPM of an engine. Crank up the RPMs, and you can do more work. Do it carefully, and you get the most out of the hardware. Do it wrong, and you risk damaging the device by running it too fast.

“Too fast” is a relative term. The speed limit of a CPU depends on the quality of its processor circuits and associated components. Today’s CPUs differ from those of the past in that they include on-board cache memory. Older CPUs, like the original Pentium and 486, had the majority of their cache memory embedded on the motherboard. Changing the way the CPU operated rarely affected the operation of the cache on the motherboard. Now that all processor caches are attached to the CPU, even if the processor is able to handle a particular speed, unstable cache memory will feed it corrupted data, rendering the CPU useless.

CPU speed: The front-side bus
The speed of a processor is based on two factors. The first factor is the interface between the chip and motherboard. This interface is called the front-side bus (FSB). The second factor is the clock multiplier. The speed of the FSB reflects the amount of data that can be sent between the CPU and the other devices in the computer. More often than not, the FSB is also the speed of the connection to the RAM. In addition, the FSB is used on most motherboards to determine the speed of the accelerated graphics port (AGP), as well as the PCI and ISA busses. In general, the performance of a computer is improved as the FSB increases, whether or not you overclock, because the processor spends less time waiting for data.

The FSB on current Intel processors ranges from 66 MHz for Celerons to 133 MHz for the B-series Pentium III. AMD Athlon and Duron processors use a special 100-MHz bus that communicates twice per cycle, making it essentially a 200-MHz bus. However, I’ll continue to consider it a 100-MHz FSB for reasons that will become clear in the next section.

CPU speed: The clock multiplier
The clock multiplier refers to the ratio of processor speed to the FSB. A 300-MHz Celeron running on its 66-MHz bus would have a multiplier of 4.5 from the factory (4.5 x 66 MHz = 297 MHz). As you can see, a processor’s speed is an approximation. AMD Athlon and Duron processors base the clock multiplier on the cycles of the bus, not the communication rate (which is why I’m considering FSB speeds for these processors 100 MHz instead of 200 MHz). The 1-GHz Athlon processors have a x10 multiplier (100 x 10 = 1,000 MHz = > 1 GHz).

Prior to the Pentium II, switches on the motherboard set the clock multiplier of virtually all processors. However, Intel felt it was necessary to lock the clock multiplier at the factory after having problems with ethically challenged vendors re-marking the processors with higher speeds and increasing the price. Home users cried foul, but the corporate world that buys the lion’s share of computers doesn’t overclock and didn’t care. Intel’s actions were justified recently when AMD was forced to begin locking its processors after a significant relabeling problem with some Australian vendors.

How can overclocking be safe?
The idea of running a device beyond standard operating parameters seems dangerous. And yes, there is some risk. However, you must realize that manufacturers create processors of different speeds on the same day, from the same production line. In many cases, the only differences between processors are the stamp and multiplier lock on the chip.

From a financial standpoint, there are a few reasons to limit the potential performance of the CPUs at the production level. All processors are tested to meet certain standards; CPUs that fail at a given rating are retested at a lower speed. While no manufacturer will detail its performance tests, it isn’t wise to rate any device at the maximum capability it handled at manufacture in case it degrades over time and becomes a warranty liability.

It also makes sense for manufacturers to keep a wide range of CPU speeds available. The low-end components are close to the nominal manufacturing cost while the performance part can command as much as a 30 percent profit margin. It might seem contradictory to downgrade a processor and reduce the profit on that part, but if the sales channel becomes flooded with a particular speed component, prices drop across the board and reduce overall profit.

Thus, due to profit maximizing and engineering safety margins, most processors can be increased by at least one speed rating (between 33 and 50 MHz), assuming the motherboard supports speed changes that small.

Prerequisites to overclocking
Note that manufacturers may or may not honor the warranty on an overclocked CPU. Overclocking may also void the warranty on your motherboard and add-in cards.

Generally speaking, overclocking consists of increasing the FSB and/or clock multiplier, checking to see if the machine boots, and then testing it for stability. The first thing to do is determine whether your motherboard can overclock processors. With the exception of an ingenious device for the Athlon Slot A, called Golden Fingers (discussed later in this article), overclocking will require your motherboard to give orders to the CPU. If you have a computer that came from a major manufacturer, it is almost guaranteed that your motherboard will not provide any overclocking abilities. Even among aftermarket and consumer-level boards, overclocking is a rarity. The following features can help increase the stability of a system while overclocking.

Multiplier settings
Earlier, I indicated that the FSB was used to set the PCI, AGP, and memory bus speeds. You should be aware that just like a processor, some components will not like working at the higher speeds. Even if you have a magic processor capable of a 50 percent or more speed increase, it is unlikely that your video card or memory will enjoy running that much faster. A good overclockable motherboard will either provide multiple PCI/AGP and memory bus ratios or allow them to be locked at their preferred operating parameters. Of course, if you use high-speed memory that is rated to operate faster than your FSB settings, you’ll have one less headache.

Don’t expect a big section in the manual labeled “Overclocking,” even on the boards that do. Instead, look for the CPU installation section. If you’re lucky, you’ll find either a BIOS setup menu or a set of switches or jumpers on the motherboard that controls the FSB, voltage, and, we hope, the clock, PCI/AGP, and memory bus multipliers. The only real necessity is the FSB adjustment, but without the others, your options decline significantly.

Power
You will also need to know how much power your processor uses. Just as a motor running at a higher speed needs more fuel, you’ll often need to give more power to your faster CPU. This is one of the “risky” aspects of overclocking: Use too much power, and you could burn out your processor.

These voltages change frequently. A shift in manufacturing process can change the operating voltage of a CPU by a significant amount. Rather than provide a list of each voltage, I recommend that you check the manufacturer’s Web site for your CPU’s specs. I do not recommend running a processor at a voltage more than 10 percent above one listed for your type of processor at the new speed. Even then, you’re relying on the engineering design overhead—not something I recommend. Individuals determined to squeeze out every last iota of performance can take that route, but for those who prefer long-term stability, it should be used only for testing.
What’s your opinion on overclocking? Do you believe it’s appropriate for the office? Post a comment below or send us a note and voice your opinions.
Cooling
The last thing you will need is something often neglected: a potent cooling solution. Overclocking causes the circuits to cycle faster, generating more heat. Insufficient cooling can result in permanent heat damage to your processor. Before you ask, let me answer this question: “Doesn’t my computer come with a cooling system?” The heat sink or fan that came with your computer is a cheap component, intended to keep the processor within accepted tolerances while keeping costs low. Overclocking is almost guaranteed to exceed the capacity of that part.

Look for a good heat sink/fan combination. You’ll want to use a large heat sink that connects tightly to the processor. Use a thermal compound (easily acquired at electronics shops) to improve heat transfer. The fan should cover the heat sink to maximize airflow into the case.

If you’re using a cartridge-based processor, you’ll more than likely have to remove the plastic cover. This is not hard—and the process should be documented in the heat sink’s manual—but it will thoroughly void your warranty. Simply remove the four screws located at the corners. Then, insert a flathead screwdriver in the seam and twist. Repeat that step at all four corners. Carefully pull the covers off the cartridge to reveal two plastic shells and the circuit board. Attach the heat sink and go.

Don’t neglect the airflow in your case; your processor is still at risk if the processor’s heat cannot escape the case. Do not rely on the power supply’s exhaust fan to handle your cooling needs. At the very least, you should have an additional exhaust fan at the top of the case. Adding a second fan at the bottom of the case to draw air in greatly improves the airflow and costs only about $10 per case fan.

A number of Web sites review heat sink/fan combos for current processors. Be sure the cooling solution you choose is specifically designed for your processor. Improperly mounting a heat sink, or using the wrong model, can damage your processor.

The overclocking process
As you’ve already gathered, overclocking consists of increasing the FSB and/or clock multiplier, checking to see if the machine boots, and then testing it for stability. The process is repeated until the maximum stable speed is identified.

Stability testing is essential to prevent problems from cropping up in the future. The exact tests you’ll need to perform will vary, depending on your operating system and hardware. The goal is to apply a heavy workload to every aspect of your system to ensure there are no hidden problems, especially when the FSB has been changed. Hardware testing is fairly easy; simply use every peripheral as you normally do. Pay particular attention to CD-Rs and CD-RWs because changes to the FSB can cause problems with drive controllers.

For PCs with Microsoft operating systems, I recommend using a full system test suite, like the freely available WinBench. Any program you depend on should be run stressed; load the largest and most complex files you have. Linux systems can compile software to test the new CPU speed; compiling the Linux kernel is a well-documented process that puts a significant load on the computer. Games like Quake 3 and Unreal Tournament are also capable of identifying many overclocking problems when put into demo mode and left running in loops. If you use a game to test stability, be sure to test the stability of the unmodified machine to establish a baseline. You should be able to run a game in a loop for at least two hours after a reboot.

WinBench, kernel compilation, and games are also capable of determining the performance increase of your system. WinBench generates a performance value for various aspects of your system, not all of which are affected by processor speed. The kernel compilation process reports the time required to complete; shorter times reflect increased performance. Games can be set to report the frame rate, which is the rate the system can update the screen. You’ll need to run the test on the unmodified machine and record the results to make the comparisons.

Conclusion
In this article, I’ve given an overview of the concepts behind overclocking and discussed the general procedures you need to follow for speeding up your CPU, including setting the clock multiplier, cooling, and adjusting the power settings. I’ve also covered some of the problems you might encounter. My next Overclocking 101 article will cover how you can speed up specific processors, including Intel Celeron, Pentium II and III, AMD Slot A Athlon, and AMD Socket A Duron and Athlon chips. I’ll also provide a list of online resources.

James McPherson has served his time in the trenches of technical support, honed his skills as a network administrator, and still managed to complete a B.S. in Engineering. After working for four different companies without changing offices, he is currently a freelance consultant and the bane of computer salesmen everywhere.

The authors and editors have taken care in preparation of the content contained herein but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for any damages. Always have a verified backup before making any changes.
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