Acronym for Basic Input/Output System, which is a software program built into a computer and is the first program to run when the computer is started. The messages that appear on the screen when the computer starts are, in fact, from this software program.

On personal computers (PCs), the BIOS contains all the code (on a ROM or a flash memory chip) required to control the keyboard, the disk drives, the display screen, a number of functions, and serial communications. After BIOS finishes testing the memory and configuring the system, it “boots” the operating system installed on the hard drive by loading an executable loading program from the boot block of the hard drive, CD-ROM, or, in some instances, the network.

See Also: Code or Source Code; Computer, Network.