BIOS is an acronym for Basic Input/Output System. A BIOS is PC software that:
- Performs the Power On Self Test (POST)
- Loads the operating system from bootable media, such as the installed hard drive, an inserted bootable CD or DVD, or a bootable USB device
- Acts as a standard interface layer between operating systems and the installed hardware
In addition, a BIOS typically has a user interface of some sort that is accessible (only) at power-on; this allows many of the configuration options for the computer to be selected before any operating system is loaded.
The BIOS is typically stored in re-programmable ROM, allowing it to be updated using special reprogramming or “flashing” software (this is unrelated to so-called “flash memory” as used in USB memory sticks and other devices).