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Configuring Microprocessors and Buses

Most system boards feature autodetection functions as part of the PnP process that automatically detect different field replaceable unit (FRU) components on the board (processors, fans, RAM modules, and adapter cards) and synchronize the different bus speed configurations. For example, the autodetect feature examines the installed microprocessor and the installed RAM modules to configure the front-side bus for optimum microprocessor-memory operations.

Similarly, the chipset may detect an advanced video adapter card in one of the expansion slots and adjust the expansion bus speed to maximize the performance of the video display. Likewise, the system autodetects the installed hard drives and CD/DVD-ROM drives and adjusts the IDE bus speed to provide the best drive-system performance based on what it finds.

Finally, the system evaluates the information it has acquired about its components and buses and configures the North and South Bridges to provide synchronization between their other buses and the PCI bus that connects them. The PCI bus speed (and by default its AGP video slot derivative) does not change to accommodate different installed components. Its speed is established as a derivative of the microprocessor clock speed (not to be confused with the advertised operational speed rating of the microprocessor).

Some BIOS versions actually provide a user-definable clock divider setting for the operation of the PCI bus. In these systems, you can set the PCI clock divider at one-half (for example) and the PCI bus will run at half the speed of the system's FSB clock frequency. This option is generally provided to keep the PCI bus running within specification when the processor is being overclocked. The setting options should be used to keep the PCI bus speed near the specified maximum speed for the standard PCI bus and its adapter cards, which is 37.5MHz.

The BIOS version must support the parameters of the microprocessor so that the PnP process can correctly configure the device and the chipset.

Key microprocessor and bus configuration settings typically included items such as the following:

  • Microprocessor Type—This setting tells the system what type of processor is installed. If this setting is incorrect, the system will assume that the installed processor is the one specified by the setting and try to interact with it on that basis. Depending on which microprocessor is indicated, the system POST might identify the processor incorrectly and still run, but not properly. In other cases, the processor might lock up during the POST or not run at all. In either case, the processor could be damaged.
  • Core-to-Bus Speed Ratio—Again, depending on the exact mismatch, the system might overclock the processor and run, but erratically. If the overclocking is less than 20%, the system might run without problems. However, the processor's life expectancy will be decreased over time. If the deviation is greater than 20%, the system might not come up at all, and the processor might be damaged.
  • Bus Frequency Setting—Configuring this setting incorrectly will cause the processor to run faster or slower. This is a common method employed by users to increase the operating speed of their older systems. If the variation is less than 20%, the system will probably work with a shortened processor life over time. Greater levels of overclocking the bus might cause the system to have random lockups.
  • Core Voltage Level—This setting establishes the voltage level at which the microprocessor core will operate. The setting is linked to the processor's speed and power dissipation. Normally, the microprocessor will not operate at all if the voltage level is more than 20% too low. Conversely, if you operate a processor at a voltage level that is higher than its specified value, this can cause physical damage to it.

The processor configuration settings must be correct for the type of microprocessor installed in the system. If the core voltage level is set too high, the microprocessor will probably overheat slowly, or burn out, depending on the amount of voltage applied. Conversely, if the voltage level is configured too low for the installed processor, the system will most likely refuse to start. Likewise, setting the speed selection incorrectly can cause the system to think that a different processor is installed in the system.

For example, if an 850MHz Pentium III processor is installed in a system whose BIOS-supported processor speeds only up to 600MHz, the BIOS will report a processor speed of only 600MHz during the POST portion of the startup. The system will be limited to running at 600MHz. For this reason and others, the capabilities of the system BIOS should always be examined when performing microprocessor upgrades.

However, as described earlier in this chapter, newer processors possess speed step capabilities that enable them to reduce their operating speeds in steps depending on their usage levels. This is a power-saving feature and must be considered before assuming a newer system is incorrectly configured.

As mentioned earlier, different groups of PC enthusiasts, such as gamers, make a practice of overclocking the processor to squeeze additional performance out of the system.

Because the microprocessor is running faster than designed, both the front-side bus and the PCI bus run faster than their stated values by a factor directly proportional to the amount that the microprocessor is overclocked. The additional speed also generates additional heat from both the processor and its supporting devices. This requires the installation of additional fans and cooling systems to prevent damage from the additional heat generated.

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