BIOS
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Motherboards contain a special set of memory chips quite separate from the main memory made up of SIMMs or DIMMS used for loading and running applications.
The extra memory is the BIOS - Basic Input Output System. BIOS information may be stored partly in ROM and partly in CMOS RAM.
ROM - (READ ONLY MEMORY) - keeps information intact even when the PC is powered off. Whereas, information held in CMOS RAM (Random Access Memory) is maintained by a backup battery when the PC is turned off.
The BIOS in ROM - the ROM BIOS chips usually contain the POST or Power - On-Self -Test sequence and an option which can allow the user to enter the BIOS SETUP program by pressing key combinations.
BIOS in CMOS RAM stores specific information regarding the size and quantity of hard disk, floppy disks, memory, date and time. The BIOS chips can auto detect hard drives.
The BIOS chip stores software instructions that are used when the system is first powered up .
These instructions scan the attached hardware.
If the hardware present complement the CMOS settings the operating system is loaded, otherwise an ERROR message is generated.