The BIOS is the first piece of code executed at powerup of a PC, used to load and execute an OperatingSystem. It does things like update CPU microcode, set up the PCI and ISA busses, initialize the IDE and keyboard controllers, etc.
If you are interested in programming using services provided by the BIOS, you may want to look for Ralph Brown's Interrupt list.
The BIOS usually has a built in utility for configuring features of the motherboard chipset. This would include things like setting the time, telling it which IDE devices you want to enable (and possibly what they are), whether or not you have a floppy drive, access timings for RAM and Busses, etc...
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