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Configuring Microprocessors and Buses
Most system boards feature autodetection functions as part of the PnP process
that automatically detect different FRU components on the board (processors,
fans, RAM modules, and adapter cards) and synchronizes 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
maximum 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-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 (not to be confused with the
advertised speed rating of the microprocessor).
The BIOS version must support the parameters of the microprocessor so that the
PnP process can correctly configure the device and the chipset. If a
microprocessor upgrade is performed and the BIOS code does not fully support
the new processor, all the problems described earlier can occur.
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, 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
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