BIOS BIOS (/ˈbaɪɒs/ BY-oss; an acronym for Basic Input/Output System and also known as the System BIOS, ROM BIOS or PC BIOS) is non-volatile firmware used to perform hardware initialization during the booting process (power-on startup), and to provide runtime services for operating systems and programs. The BIOS firmware comes pre-installed on a personal computer's system board, and it is the first software to run when powered on. The name originates from the Basic Input/Output System used in the CP/M operating system in 1975. The BIOS originally proprietary to the IBM PC has been reverse engineered by companies looking to create compatible systems. The interface of that original system serves as a de facto standard.

History The term BIOS (Basic Input/Output System) was created by Gary Kildall and first appeared in the CP/M operating system in 1975, describing the machine-specific part of CP/M loaded during boot time that interfaces directly with the hardware. (A CP/M machine usually has only a simple boot loader in its ROM.) Versions of MS-DOS, PC DOS or DR-DOS contain a file called variously "IO.SYS", "IBMBIO.COM", "IBMBIO.SYS", or "DRBIOS.SYS"; this file is known as the "DOS BIOS" (also known as the "DOS I/O System") and contains the lower-level hardware-specific part of the operating system. Together with the underlying hardware-specific but operating system-independent "System BIOS", which resides in ROM, it represents the analogue to the "CP/M BIOS". With the introduction of PS/2 machines, IBM divided the System BIOS into real- and protected-mode portions. The real-mode portion was meant to provide backward compatibility with existing operating systems such as DOS, and therefore was named "CBIOS" (for "Compatibility BIOS"), whereas the "ABIOS" (for "Advanced BIOS") provided new interfaces specifically suited for multitasking operating systems such as OS/2.

User interface The BIOS of the original IBM PC and XT had no interactive user interface. Error codes or messages were displayed on the screen, or coded series of sounds were generated to signal errors when the power-on self-test (POST) had not proceeded to the point of successfully initializing a video display adapter. Options on the IBM PC and XT were set by switches and jumpers on the main board and on expansion cards. Starting around the mid-1990s, it became typical for the BIOS ROM to include a "BIOS configuration utility" (BCU[10]) or "BIOS setup utility", accessed at system power-up by a particular key sequence. This program allowed the user to set system configuration options, of the type formerly set using DIP switches, through an interactive menu system controlled through the keyboard. In the interim period, IBM-compatible PCs‍—‌including the IBM AT‍—‌held configuration settings in battery-backed RAM and used a bootable configuration program on disk, not in the ROM, to set the configuration options contained in this memory. The disk was supplied with the computer, and if it was lost the system settings could not be changed. The same applied in general to computers with an EISA bus, for which the configuration program was called an EISA Configuration Utility (ECU). A modern Wintel-compatible computer provides a setup routine essentially unchanged in nature from the ROM-resident BIOS setup utilities of the late 1990s; the user can configure hardware options using the keyboard and video display. Also, when errors occur at boot time, a modern BIOS usually displays user-friendly error messages, often presented as pop-up boxes in a TUI style, and offers to enter the BIOS setup utility or to ignore the error and proceed if possible. Instead of battery-backed RAM, the modern Wintel machine may store the BIOS configuration settings in flash ROM, perhaps the same flash ROM that holds the BIOS itself.

Extensions Peripheral cards such as some hard disk drive controllers and some video display adapters have their own BIOS extension option ROMs, which provide additional functionality to BIOS. Code in these extensions runs before the BIOS boots the system from mass storage. These ROMs typically test and initialize hardware, add new BIOS services, and augment or replace existing BIOS services with their own versions of those services. For example, a SCSI controller usually has a BIOS extension ROM that adds support for hard drives connected through that controller. Some video cards have extension ROMs that replace the video services of the motherboard BIOS with their own video services. BIOS extension ROMs gain total control of the machine, so they can in fact do anything, and they may never return control to the BIOS that invoked them. An extension ROM could in principle contain an entire operating system or an application program, or it could implement an entirely different boot process such as booting from a network. Operation of an IBM-compatible computer system can be completely changed by removing or inserting an adapter card (or a ROM chip) that contains a BIOS extension ROM. The motherboard BIOS typically contains code to access hardware components necessary for bootstrapping the system, such as the keyboard, display, and storage. In addition, plug-in adapter cards such as SCSI, RAID, network interface cards, and video boards often include their own BIOS (e.g. Video BIOS), complementing or replacing the system BIOS code for the given component. Even devices built into the motherboard can behave in this way; their option ROMs can be stored as separate code on the main BIOS flash chip, and upgraded either in tandem with, or separately from, the main BIOS. An add-in card requires an option ROM if the card is not supported by the main BIOS and the card needs to be initialized or made accessible through BIOS services before the operating system can be loaded (usually this means it is required in the bootstrapping process). Even when it is not required, an option ROM can allow an adapter card to be used without loading driver software from a storage device after booting begins – with an option ROM, no time is taken to load the driver, the driver does not take up space in RAM nor on hard disk, and the driver software on the ROM always stays with the device so the two cannot be accidentally separated. Also, if the ROM is on the card, both the peripheral hardware and the driver software provided by the ROM are installed together with no extra effort to install the software. An additional advantage of ROM on some early PC systems (notably including the IBM PCjr) was that ROM was faster than main system RAM. (On modern systems, the case is very much the reverse of this, and BIOS ROM code is usually copied ("shadowed") into RAM so it will run faster.)

Initialization After the motherboard BIOS completes its POST, most BIOS versions search for option ROM modules, also called BIOS extension ROMs, and execute them. The motherboard BIOS scans for extension ROMs in a portion of the "upper memory area" (the part of the x86 real-mode address space at and above address 0xA0000) and runs each ROM found, in order. To discover memory-mapped ISA option ROMs, a BIOS implementation scans the real-mode address space from 0x0C0000 to 0x0F0000 on 2 KiB boundaries, looking for a two-byte ROM signature: 0x55 followed by 0xAA. In a valid expansion ROM, this signature is followed by a single byte indicating the number of 512-byte blocks the expansion ROM occupies in real memory, and the next byte is the option ROM's entry point (also known as its "entry offset"). A checksum of the specified number of 512-byte blocks is calculated, and if the ROM has a valid checksum, the BIOS transfers control to the entry address, which in a normal BIOS extension ROM should be the beginning of the extension's initialization routine. At this point, the extension ROM code takes over, typically testing and initializing the hardware it controls and registering interrupt vectors for use by post-boot applications. It may use BIOS services (including those provided by previously initialized option ROMs) to provide a user configuration interface, to display diagnostic information, or to do anything else that it requires. It is possible that an option ROM will not return to BIOS, pre-empting the BIOS's boot sequence altogether. An option ROM should normally return to the BIOS after completing its initialization process. Once (and if) an option ROM returns, the BIOS continues searching for more option ROMs, calling each as it is found, until the entire option ROM area in the memory space has been scanned.