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Overclocking a PC: A Short Primer
by Jim Lai

The modern home PC is founded on sequential logic, circuits whose output values depend upon the values of inputs and memory. To increase that stability of designs and simplify testing, timing circuits, i.e. clocks, are employed to synchronize the various components. These circuits are said to generate clock pulses. Overclocking is the act of running various such digital clocks at a faster rate than recommended by the manufacturer. When measuring the speed of such clocks, you will typically encounter the term megahertz (MHz), which is defined as one million cycles per second.

Often, circuits complete their tasks and end up having to wait for the next clock signal before continuing. By reducing the time between clock pulses, such circuits can be made to operate faster. The lure is the promise of increased performance at minimal cost.

Reasonably reliable clock speed increases of 10% are not uncommon. As an example of an extreme case, people have successfully overclocked an Intel Celeron 300A, normally running at 300 MHz, to speeds of up to 450 MHz, giving the more expensive Pentium II/450 a run for its money. Overclocking has been particularly tempting for the PC gaming community, faced with rapid obsolescence of computer hardware by faster and more powerful components.

If the clock is made to run too quickly, the circuits won't have enough time to complete their operations and provide reliable outputs. Early warning signs would be occasional instability. More serious cases might involve circuits malfunctioning and locking up outright, and evem data loss.

Longer term, heat is an issue. When components are run faster, they use up more power and thus generate more heat. Too much heat over an extended time could damage the circuits. It's still possible that it might be cheaper to buy and replace an inexpensive CPU (central processing unit) than to get the more reliable high speed counterpart, though this isn't typically the case. Additional cooling fans can be installed to reduce the temperature of components.

Intel has been trying to reduce the ability to overclock their CPUs. It's not the hobbyist that concerns them, but overclocking fraud. Some CPU distributors and PC assemblers are tempted to increase their profit margins by remarking slower CPUs as their faster counterparts, and thus charge more to the unsuspecting consumer. When the CPU fails prematurely, Intel typically gets blamed, not the actual guilty party.

Overclocking is not for the timid. That being said, there typically are up to three types of clocks that can be tweaked in modern PCs: the front-side bus (FSB), the CPU, and the video card. It's often best to scout out web pages and other sources of information when deciding how far to overclock a system component. To assure a minimum level stability, an overclocked system should be "burned-in" for a period of, say, 24 hours. and then be subjected to some programs that serve as stress tests.

The front-side bus is the backbone of the PC. It carries data between the CPU, RAM, and all other peripherals, such as hard drives and video cards. Increasing the FSB clock speed will increase the speed which data transfer occurs. In Pentium class systems, the FSB is either 66 MHz or 100 MHz, depending on the chipset present on the motherboard. When increasing (or slowing) the FSB speed, the ISA, PCI and AGP bus speeds are typically affected proportionately. Failures here tend to result in system lockups or some components not operating. In the case of hard drives, it's possible that data may be lost.

CPU speeds have risen dramaticaly in the past few years. As a result, most CPUs now run at a multiple of the FSB clock speed, called the clock multiplier. For example, an AMD K6-2 chip rated for 333 MHz might be set to run at 5.0 times the FSB clock speed of 66 MHz. Thus, CPU speeds can be altered by adjusting the FSB speed or the clock multiplier. For example, said CPU could be run at 4.5 times an FSB clock speed of 75 MHz, resulting in a net speed around 338 MHz.

High performance video cards offering 3D acceleration have recently provided designs where the clock speed of the video RAM and/or processor may be tweaked. These parameters can usually be set in software. It might involve adding environment variables to the AUTOEXEC.BAT file loaded at startup. Some manufacturers even provide convenient utilities to adjust the clock speeds.

Overclocking may improve your system's performance, but keep in mind that like any other complex piece of technology, there may be other cheap and/or inexpensive ways to speed up your system. For example, hard drive defragmentation will reduce the amount of time spent between disk reads; purchasing sufficient RAM to eliminate the creation of swap files will also speed up your machine. Software such as Norton Utilities can help keep things running smoothly.

Tom's Hardware Guide stays up to date with the performance of up and coming hardware. His site is one of several which has access to test equipment for review before it hits the market. CPUs, graphics cards, and motherboards... you name it.

Anandtech.com is run by a teenager. But don't let that put you off the value of his reviews.

Sandpile.org is a repository of more technical aspects of PC CPUs.

MaximumPC.COM has an archive which contains more detailed articles on overclocking and system tuning.

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Jim Lai is a senior programmer/analyst at a major Canadian bank and lives in Toronto.

The writer of this article welcomes your comments: jwtlai@io.com