The motherboard manufacturer will choose the clock generator chip based upon the chipset, how many PCI and SDRAM slots will be implemented and what system bus speeds need to be supported. Even if a chipset allows for different PCI to system bus divisors, the clock generator chip may not allow for it. Many users have wondered why system and PCI bus speeds are implemented a particular way on a motherboard. The answer lies in the capabilities of the clock generator chip.
In order for a computer to run an operating system, it needs a 'bootstrap' program (derived from the phrase 'pulling yourself up by your own bootstraps'). This boot program loads from a known memory location and provides just enough info to access the devices necessary to complete the operating system load. For example, the program has to load device information for floppy and hard drives, as well as a basic video display.
On the PC, this information is kept in a read-only memory chip called the BIOS (Basic Input/Output System). This chip may have anywhere from 512K to 2MB of memory, which is programmed at the factory and can only be updated by a program that enables a special mode that allows the memory to be overwritten with a new boot program. This procedure is commonly called 'Flashing the BIOS'.
When the PC is powered on, a special process called the Power-On Self-Test (POST) is executed which identifies the processor, how much memory is installed and whether all defined devices are present and functioning. Once this is complete, the boot routine looks for a specific set of instructions at a particular location on each bootable device. The first set of instructions that satisfies the criteria are loaded into memory and are executed. If everything is set up properly, these instructions will complete the boot process by loading the operating system.
In order for the BIOS to know what specific devices need to be supported, there is a special CMOS IC that contains user-specified parameters that are read in right after the processor is identified. This circuitry is actually implemented into the Real Time Clock (RTC) chip, which keeps track of the date and time. The parameters in CMOS are accessed by bringing up a special menu during the POST process, generally displayed by hitting the DEL key while the memory is being counted, and changes are input manually. These changes must then be saved in order to be implemented.
If the devices are not set up correctly, the system may not be able to load the operating system or devices may not be accessible after the operating system loads. The RTC and CMOS retain their information only as long as power is applied, which is supplied by a small battery on the motherboard. If this battery fails or is disconnected, the data is lost in the CMOS, and must be re-entered again at the next boot.