A computer system can be comprised of a number of  BIOS firmware chips. The motherboard BIOS typically is comprised of the code to get the accessibility of  fundamental hardware elements  such as the keyboard, floppy drives, ATA (IDE) hard disk controllers, USB human interface devices, and storage devices. Furthermore, plug-in adapter cards such as SCSI, RAID, Network interface cards, and video boards often cover their own BIOS, superseding  the system BIOS code for the given part.

In few devices that can be utilized by add-in adapters and directly united on the motherboard, the add-in ROM may also be collected as isolated code on the main BIOS flash chip. There is the possibility of making the up gradation of this add-in BIOS called option ROM from the main BIOS code. To manipulate input and output older operating systems such as DOS as well as bootloaders can make the usage of BIOS.

However, most upgraded operating systems will communicate with hardware devices by making the usage of the device drivers to get the accessibility of the hardware. Sometimes, these add-in BIOS are still termed by sophisticated operating systems with the purpose of executing specific tasks such as preliminary device initializations. To trace these memory mapped expansion ROMs at the time of booting, PC BIOS executions make the scanning of real memory from 0xC8000 to 0xF0000 on 2 kilobyte peripheries searching for a 0x55 and 0xAA signature which quickly followed by a byte hinting at the number of 512 byte blocks the expansion ROM possesses in real memory. Last but not the least the BIOS then shifts to the offset with immediate effect after the size byte at which point the expansion ROM code uses BIOS services to give a user configuration interface, register interrupt vectors for the utilization by post-boot applications or show diagnostic data.