The BIOS (, an acronym for Basic Input/Output System and also known as the System BIOS, ROM BIOS or PC BIOS) is a type of firmware used to perform hardware initialization during the booting process (power-on startup) on IBM PC compatible computers, and to provide runtime services for operating systems and programs. The BIOS firmware is built into personal computers (PCs), and it is the first software they run when powered on. The name itself originates from the Basic Input/Output System used in the CP/M operating system in 1975. Originally proprietary to the IBM PC, the BIOS has been reverse engineered by companies looking to create compatible systems and the interface of that original system serves as a de facto standard.

Bios or BIOS may refer to:

• BIOS (Basic Input/Output System), a de facto standard firmware interface of IBM PC compatible computers
  • Award BIOS, manufactured by Award Software
  • Phoenix BIOS, manufactured by Phoenix Technologies
  • AMI BIOS, manufactured by American Megatrends
• BIOS (CP/M), the BIOS in the CP/M family of operating systems
• DOS-BIOS, the equivalent of the CP/M component in some DOS operating systems, IBMBIO.COM in PC DOS
  • IO.SYS, in MS-DOS