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The Basic Input Output System (BIOS) is a series of instructions (software code) which is a part of many modern computers, responsible for basic functions such as controlling the keyboard or booting up an operating system. The term dates back to the original IBM personal computer of the early 1980's.

This firmware typically was written to a CMOS (complimentary metal oxide semiconductor) chip, and some computer enthusiasts used the terms "BIOS" and "CMOS" interchangeably.

Breaking down the BIOS

The BIOS was an API that served as a layer of abstraction, located in non-volatile memory known as CMOS, and thus is said to be firmware. The BIOS chip in a personal computer originally provided an information-hiding layer between the (then) Disk Operating System (DOS) and the hardware.

The contents of an IBM personal computer's firmware include the following essential parts:

  • power-on self test (POST) routines
  • a tiny database about the hardware configuration
  • a setup program for modifying the database (this program could only be run by pressing a certain key, or group of keys, during the boot process; exactly which keys varied per computer manufacturer)
  • a set of routines for accessing standard hardware components such as disk drives and serial ports
  • the boot-strap loader program, which looks at the first sector of the boot drive (the so-called Master Boot Record, or MBR) to locate an operating system, loads the OS into memory, and then transfers control to the OS kernel

The BIOS's role as an information-hiding layer was very important. It allowed many different hardware vendors to create components such as motherboards and add-in cards for the computer, without the operating system code needing to be changed. Because the BIOS was in firmware, it also meant that consumers could upgrade their own disk drives. The latter is an important point, because in 2008, 25 years after the IBM personal computer's BIOS became the de facto standard to booting Wintel computers, an industry consortium is moving to supplant BIOS and MBR booting (admittedly limited in capability) with a newer standard called EFI.

The BIOS has been deprecated

There are many catalogs of the limitations of the long-standing BIOS system in Wintel computers. The proposed standard to replace BIOS, called Extensible Firmware Interface (EFI), was originated by Intel around 2000 and has been converted into an industry consortium (UEFI). It is used in 2008 by the wildly popular Apple Macbook and Macbook Pro notebook computers.

There are many available online resources describing the benefits of UEFI, but very few that discuss the downside of the proposed emerging standard for consumers, which is, that the boot software is no longer firmware but is, instead, at least partly on the boot drive itself in a special, pre-created disk partition. Consumers currently lack the appropriate utilities and software tools to deal well with EFI, though this may be expected to change over time.