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[ The PC Guide | System Optimization and Enhancement Guide | Overclocking: The Dissenting Opinion | Overclocking Risks and Rewards ]

Risks of Overclocking the System Bus

There are some people that are confused by the notion of overclocking the system bus, and this is understandable. They say "the only way you can run at 75 or 83 MHz system bus speeds is if the motherboard offers them as an option; if they're options then they obviously must be supported, right?" Well, yes and no. The problem is that the motherboard is itself a collection of other components. The motherboard manufacturer can "certify" the board to run at 75 or 83 MHz, but if the components used on it are only guaranteed to 66 Mhz, what does the certification really mean?

The first processor to go mainstream with overclocking the system bus was the Cyrix 6x86-PR200. This chip runs at 150 MHz, and its "PR200" rating is based on being placed into a motherboard running at 75 MHz. At the time it was introduced, the highest standard bus speed was 66 Mhz, so special motherboards were created that had the higher clock speed, and they were "approved" by their manufacturers and/or Cyrix to run at the higher speed. That's all well and good, but most of these boards used Intel's Triton series of chipsets, which Intel only rates to 66 MHz. Therefore, the chipset was overclocked on every one of these boards. The chipset is the brains of the motherboard, by far its most important component.

Note: There are some who claim that Intel knows their chipsets will work at 75 MHz but only rates them at 66 MHz because they want to hurt the Cyrix chip's chance of success. Frankly, I don't think the 6x86 was ever enough of a threat to Intel to matter that much, but it's up to you to decide what you want to believe about the Intel conspiracy theories.

In fact, overclocking of the chipset is only the beginning. The system bus speed is the basis from which the speeds of most of the components in the system are calculated, including the speed of the various I/O buses. The PCI bus is normally half of the system bus speed on a conventional 66 MHz Pentium motherboard. When you change the system speed from 66 MHz to 83 MHz, you increase the speed of your PCI bus from 33 MHz (standard) to 41 MHz (not). (There are some motherboards that employ asynchronous PCI, where the PCI bus is always 33 Mhz independent of the system bus speed, but these are unusual today.)

When you increase the system bus speed, you increase the speed of all of the following:

  • The processor (unless you reduce the processor clock multiplier)
  • The chipset
  • The memory bus
  • The system cache
  • The system memory
  • The integrated IDE hard disk controllers
  • The PCI bus
  • The ISA bus
  • Every peripheral connected to either the PCI or the ISA bus

There's a reason why overclocking the system bus has such a great impact on performance! But as you can see, you are taking a lot of risk here. Most PC components today are only tested on 66 MHz system buses. Most PCI devices are tested only at 33 Mhz. Was your video card tested at 83 Mhz? How about your hard disk? Do you know? Do you think the manufacturers will stand by their products running in an overclocked system? In essence, you run the risk of damage or incorrect performance of any of these components when you overclock.

The newest motherboards out there now officially support 100 MHz system bus speeds. They are equipped with chipsets certified for 100 MHz operation, and are designed to be used with CPUs that have been designed for and tested with 100 MHz systems. They avoid overclocking the PCI bus by dividing the system bus speed by three instead of two, preserving the official 33 MHz PCI bus speed. Ironically, many people find the jump in bus speed from 66 to 100 MHz isn't having as much performance benefits as they expected--probably because the risky overclocking of the PCI bus plays a large part in the benefits of overclocking from 66 to 75 or 83 MHz.

Summary: when you overclock the system bus, you get a lot, but you risk a lot too, at least in my opinion.

Next: The Uncertainty Factor

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