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Increased stress on system components, due to overclocking via the front-side bus, often generates instability. Components built with low-quality manufacturing or design standards are most likely to suffer, though even the best components have limits. Hard drives, video cards, and memory are the most susceptible to instability introduced by overclocking.
Overclocking the front-side bus affects the operating speeds of all system buses, as discussed in Chapter 4. Each bus rate is affected differently, according to the front-side bus speed. The exact deviation in operating speed depends on the chipset being used with each motherboard's specific design. In general, all system bus speeds increase as front-side bus rates increase, although most x86 architectures employ multiplier division to derive bus speeds. Devices attached to each of the various buses must be able to sustain operation at extended speeds introduced through the overclocking process.
Drive arrays are the most susceptible to problems in an overclocked system. Loss of data integrity is a real concern. PCI bus speeds that exceed the normal specification of 33 MHz can cause problems, especially above 40 MHz. Drives based on the IDE standard are most likely to suffer data corruption, while SCSI-based devices are less worrisome.
Decreasing the drive controller's transfer rates through the BIOS Setup interface will often prevent data corruption problems. You'll have to experiment to find optimum stability, but most drive problems cease when the controller's transfer rate is lowered by one or two levels. For example, ATA/66 DMA 4 could be taken down to ATA/33 DMA 2. While you can lower drive transfer rates without devastating performance, you should never disable direct memory access (DMA) transfers in current generation systems. DMA bypasses the processor during data transfers; this valuable feature should be enabled at all times.
Examine any changes in performance that result when drive transfer rates are lowered. System performance could be degraded due to decreases in drive data throughput. Bandwidth gained through overclocking the bus rate may be negated by reduced signaling rates. You must maintain a balance between maximum operating frequency and available bandwidth. A quality benchmarking utility, such as SiSoft Sandra, will help you assess drive bandwidth as you make overclocking decisions.
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