Basic Input/Output System
The Basic Input/Output System (usually known by its acronym, BIOS) is software stored, in binary form, in non-volatile memory on the motherboard of a personal computer, which performs two basic classes of function:
First, when the device is powered on, the CPU starts executing instructions from the BIOS: these check the computer for basic health, and then load a bootstrap from secondary storage which can load and start the operating system.
Second, it provides a design-independent way for user applications to use devices such as disks, displays and keyboards, without having to have code for the specific chips used on that particular motherboard - hence the name. (This functionality is generally not used any more; most modern OS's interact with the devices directly.)
Originally the BIOS was stored in a PROM, but on recent motherboards it is in flash memory, to allow it to be updated without removing the chip it is stored in. (This is a step that should be performed carefully; if it does not work properly, it can 'brick' the motherboard. Some motherboard vendors put the flash chip in a socket, so that it can be replaced without resorting to un-soldering the chip.)