BIOS

BIOS (pronounced), in computing, stands for Basic Input/Output System.

The term is incorrectly known as Binary Input/Output System, Basic Integrated Operating System and occasionally Built In Operating System. BIOS refers to the firmware code run by an IBM PC when first powered on. The primary function of the BIOS is to identify and initiate component hardware (such as hard disk, floppy and optical disk drives). This is to prepare the machine so other software programs stored on various media can load, execute, and assume control of the PC. This process is known as booting, or booting up, which is short for bootstrapping.

BIOS can also be said to be a coded program embedded on a chip that recognizes and controls various devices that make up x86 personal computers. Among other classes of computers, the generic terms boot monitor, boot loader or boot ROM were commonly used. Some Sun and Macintosh PowerPC computers used Open Firmware for this purpose. There are a few proposed alternatives for Legacy BIOS in the x86 world: Extensible Firmware Interface, Open Firmware (used on the OLPC XO-1) and coreboot.

The term first appeared in the CP/M operating system, describing the part of CP/M loaded during boot time that interfaced directly with the hardware (CP/M machines usually had a simple boot loader in ROM, and nothing else). Most versions of DOS have a file called "IBMBIO.COM" or "IO.SYS" that is analogous to the CP/M disk BIOS.

The BIOS runs from the PROM, EPROM or, most commonly, flash memory when the computer is powered on. It initializes several motherboard components and peripherals, including:

* The clock generator.
* The processors and caches.
* The chipset (memory controller and I/O controller).
* The system memory.
* All PCI devices (by assigning bus numbers and resources).
* The primary graphics controller.
* Mass storage controllers (such as SATA and IDE controllers).
* Various I/O controllers (such keyboard/mouse and USB).

Finally, it loads the boot loader for the operating system, and transfers control to it. The entire process is known as power-on self-test (POST). On the original IBM PC, the hardware only needed minimal configuration and POST was indeed used for testing; on modern systems, most of POST actually consists of hardware configuration.

Once system memory is initialized, the BIOS typically copies/decompresses itself into that memory and keeps executing from it.

Nearly all recent BIOS implementations can optionally execute a setup program interfacing the nonvolatile BIOS memory (CMOS). This memory holds user-customizable configuration data (passwords, time, date, hard drive details, etc.) accessed by BIOS code. The 80x86 source code for early PC and AT BIOS was included with the "IBM Personal Computer Technical Reference Manual".[4]

In most modern BIOS implementations, users select where the BIOS obtains its boot image: CD, hard disk, floppy disk, USB device or via a networked connection. This is

particularly useful for installing operating systems or booting to a Live CD or flash keydrive, and for selecting the order of testing for the presence of bootable media.

Some BIOSes allow the user to select the operating system to load (e.g. load another OS from the second hard disk), though this is more often handled by a second-stage boot loader.