Basic Input/Output System

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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.)

The BIOS on the original IBM PC set the de facto standard for the interface to the BIOS, although other companies now provide BIOS code to various PC manufacturers.