BIOS is an acronym that stands for Basic Input/Output System. It is located in ROM in a PC and contains its standard firmware.

Firmware in the BIOS is run by the computer when it is first powered up. Its primary function is to identify and initialise system component hardware and some other hardware devices. This is to initialise the computer into a known low capability state, so other programs stored on various media can be loaded, executed, and given control of the PC. This process is known as bootstrapping.

The BIOS of a PC class machine contains programs embedded on a chip that recognise and control various devices and a program called the bootstrap loader that handles the bootstrap process. It also provides a library of basic input/output functions that can be called to operate and control peripherals such as the keyboard and display.

Part of the bootstrap process is to search for bootable devices and load the bootloading code from the first bootable device it finds. The order devices are searched is set in the BIOS. In almost all computers, the BIOS can be configured to change the order storage devices are searched to control their priority.