BIOS

BIOS (pronounced /ˈbaɪoʊs/), 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 a personal computer when first powered on. The primary function of the BIOS is to identify and initiate component hardware, (such as hard drives, floppies, and CDs). 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 on the x86 world: Extensible Firmware Interface, Open Firmware (used on the OLPC XO-1) and LinuxBIOS.

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.

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 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 Technical Reference Manual.

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 LiveCD 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.

Before 1990 or so BIOSes were held on ROM chips that could not be altered. As its complexity and need for updates grew, BIOS firmware was subsequently stored on EEPROM or flash memory devices. The first flash chips attached to the ISA bus. Starting in 1998, the BIOS flash moved to the LPC bus, a functional replacement for ISA, following a new standard implementation known as "firmware hub" (FWH). In 2006, the first systems supporting a Serial Peripheral Interface (SPI) appeared, and the BIOS flash moved again.

EEPROM chips are advantageous because they can easily be updated by the user; hardware manufacturers frequently issue BIOS updates to upgrade their products, improve compatibility and remove bugs. However, the risk is that an improperly executed or aborted BIOS update can render the computer or device unusable. To recover from BIOS corruption, some new motherboards have a backup BIOS (i.e. they are referred to as "Dual BIOS" boards, Gigabyte even offers a motherboard with quad BIOS). Also, most BIOSes have a "boot block" which is a portion of the ROM that runs first and is not updateable. This code will verify that the rest of the BIOS is intact (via checksum, hash, etc.) before transferring control to it. If the boot block detects that the main BIOS is corrupted, then it will typically initiate a recovery process, by booting to a removable media (floppy, CD or USB memory) so that the user can try flashing again.

Due to the limitation on the number of times that flash memory can be flashed, a flash-based BIOS is vulnerable to "flash-burn" viruses that repeatedly write to the flash, permanently corrupting the chip. Such attacks can be prevented by some form of write-protection, the ultimate protection being the replacement of the flash memory with a true ROM.