An acronym for "Basic Input/Output System." In standard Intel personal computers, a ROM program responsible for controlling low-level access to system devices. In most modern operating systems, the BIOS is used mostly to perform the POST and then boot the operating system.

The BIOS is a reminder that your computer continues to pay tribute to the great IBM PC XT in the sky. Nowadays, though, BIOSes still have very useful functions; assuming your BIOS isn't stored in a separate partition on the hard drive. BIOSes also serve as useful reminders as to how recent your computer is; simply look at the copyright date to see how new your computer is. Example:

Award BIOS v7.00PG, An Energy Star Ally
(c)1984-2002, Award Software Inc.

The BIOS's Setup menu is one of the most important things in your computer; it is here where you can view legacy settings like 'shadow Video RAM' and 'Mem Hole 15-16M'. Generally, these menus' 'Help' files will be in broken English, reflecting the brilliant programming skills but poor language of the Taiwanese developer of your motherboard. The only times you'll ever have to deal with BIOS setup is when you've installed a new hard drive or to disable integrated onboard cruft.

A BIOS is not just limited to the motherboard, though. Video cards have their own BIOSes, and so do SCSI controller cards.

Generally speaking, the fastest way to destroy your computer is to try and flash a BIOS from a protected mode environment such as Win32. Assuming your BIOS doesn't have any write protection enabled, attempting to do this will have no immediate effect - until you reboot the computer, when the computer must enter real mode again. The proper way to do this is to use a bare-bones Win95/98 floppy disk created using FORMAT A: /S, rebooting, and running the executable zip archive from the floppy. ASUS's LiveUpdate was created to address this problem but the program can be a bit flaky at times, and of course it was created only for ASUS motherboards.

The original intent of the BIOS on PC compatible computers was to be a simple hardware abstraction layer to assure the operating system that no matter what strange hardware was installed in a PC there would be some uniform minimum functionality available so that the operating system (usually PC-DOS at the time) and application software could perform basic tasks in a hardware independent manner.

At the time this was a very solid design decision, as it allowed DOS to leave all low-level disk operations to the BIOS. When the IBM AT came out and the standard entry level hard disk controller moved from RLL to IDE there was very little adjustment to be done on the operating system and application levels because the new controllers came with a ROM chip containing a new set of BIOS services that presented the same software interface (or API if you wish) to the OS and apps despite the fact that the computer addressed the new controller through a totally different protocol.

The basic interfaces provided by the BIOS of your PC/AT compatible system were the following:

  • Secondary Storage (tapes and floppies, later hard disks as well):
    • Reading
    • Writing
    • Formatting
    • etc...
  • I/O ports: (Serial, Parallel)
  • Console functions:
    • Keyboard Input
    • Video Mode Control
    • Cursor Positioning
    • Beeping the Speaker

The system became outdated at some point and began to unravel as people started to outgrow the BIOS designers wildest guesses as to what a PC could be used for. For example Sysops outfitted their BBS machines with four port serial boards that overran the number of devices provided for by the BIOS calls. As CPUs advanced to provide memory protection, flat addressing, and multithreading the old Real Mode device drivers provided by the BIOS became less useful and the basic role of the BIOS became that of a rudimentary bootstrap loader with the main task of initializing all the peripherals and then loading the kernel of the OS, which will contain faster, more feature-packed Protected Mode drivers for any installed hardware.

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