Basic Input-Output System. On the PC architecture, the BIOS is stored on a ROM (usually an EEPROM) on the mainboard. The BIOS performs three tasks: testing the hardware after power-on, loading the OS boot loader, and providing a basic operating environment to real-mode applications (and MS-DOS). The BIOS can be extended in various ways: the VESA BIOS is probably the most common of these, as it is resident on the computer’s video card and provides a uniform interface to SVGA display modes. BIOS extensions also include a SCSI BIOS (allowing MS-DOS to access SCSI drives) and the network boot ROM (allowing the computer to boot from a network server).