BIOS

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For other uses, see Bios.
BIOS Basic input output system

Phoenix AwardBIOS CMOS (non-volatile memory) Setup utility on a standard PC
Stored on:
Common Manufacturers:

BIOS, in computing, stands for Basic Input/Output System also incorrectly known as Basic Integrated Operating System. BIOS refers to the firmware code run by an IBM compatible PC when first powered on. The primary function of the BIOS is to prepare the machine so other software programs stored on various media (such as hard drives, floppies, and CDs) can load, execute, and assume control of the PC. This process is known as booting up.

BIOS can also be said to be a coded program embedded on a chip that recognizes and controls various devices that make up the PC. The term BIOS is specific to personal computer vendors. Among other classes of computers, the generic terms boot monitor, boot loader or boot ROM are commonly used.

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.

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[edit] How the BIOS boots

The BIOS runs off the PROM, EPROM or, most commonly, flash memory when the computer is powered on and it initializes and sometimes performs the Power-on self-test (POST), a set of diagnostic tests on the hard drive, memory, video, chipset and other hardware. Subsequently, it typically decompresses itself from the BIOS memory space into the system main memory and starts executing from there. Nearly all BIOS implementations can optionally execute a setup program interfacing the nonvolatile BIOS memory (CMOS). This memory holds user-customizable configuration data (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 which device boots first: CD, hard disk, floppy disk, flash keydrive and the like. This is particularly useful for installing operating systems or booting to Live CDs, 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.

[edit] BIOS as firmware

ROM with BIOS
ROM with BIOS

BIOS is sometimes called firmware. Before 1990 or so BIOSes were held on ROM chips that could not be altered. As their complexity and need for updates grew, BIOS firmware was stored on EEPROM or flash memory devices. This EEPROM chip sits on a FWH interface, but in some newer boards EEPROM chips are already sitting on a newer, emerging interface named SPI. EEPROM chips are advantageous because they can easily be updated by the user; however, the risk is that an improperly executed or aborted BIOS update can render the computer or device unusable. To avoid BIOS corruption, some new motherboards have a backup BIOS (i.e. they are referred 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 jumping to it. If the boot block detects that the main BIOS is corrupt, then it will typically boot to a floppy so that the user can try flashing again, hopefully with success. Due to the limitation on the number of times flash memory can be flashed flash BIOS are vulnerable to "flash-burn" viruses that repeatedly write to the BIOS permanently corrupting the chip, only replacing the flash ROM with a true ROM can harden a system against such attacks. Hardware manufacturers frequently issue BIOS updates to upgrade their products, improve compatibility and remove bugs.

[edit] Firmware on adapter cards

A computer system can contain several BIOS firmware chips. The motherboard BIOS typically contains code to access fundamental hardware components such as the keyboard, floppy drives, ATA (IDE) hard disk controllers, USB human interfaces, and storage devices. In addition, plug-in adapter cards such as SCSI, RAID, Network interface cards, and video boards often include their own BIOS, complementing or replacing the system BIOS code for the given component.

In some devices that can be used by add-in adapters and actually directly integrated on the motherboard, the add-in ROM may also be stored as separate code on the main BIOS flash chip. It may then be possible to upgrade this "add-in" BIOS (sometimes called an "option ROM") separately from the main BIOS code.

Add-in cards usually only require such an add-in BIOS if they:

  • Need to be used prior to the time that the operating system loads (e.g. they may be used as part of the process which loads (bootstraps) the operating system), and:
  • Are not sufficiently simple, or generic in operation to be handled by the main BIOS directly

Older operating systems such as DOS, as well as bootloaders, may continue to make use of the BIOS to handle input and output. However, most modern operating systems will interact with hardware devices directly by using their own device drivers to directly access the hardware. Occasionally these add-in BIOSs are still called by modern operating systems, in order to carry out specific tasks such as preliminary device initialization.

To find these memory mapped expansion ROMs during boot, PC BIOS implementations scan real memory from 0xC8000 to 0xF0000 on 2 kibibyte boundaries looking for a 0x55 0xaa signature, which is immediately followed by a byte indicating the number of 512 byte blocks the expansion ROM occupies in real memory. The BIOS then jumps to the offset immediately after the size byte, at which point the expansion ROM code takes over and uses BIOS services to provide a user configuration interface, register interrupt vectors for use by post-boot applications, or display diagnostic information.

For UNIX and Windows/DOS systems there is a utility with which BIOS firmware software can be dumped at http://www.linuks.mine.nu/ree/

[edit] The BIOS boot specification

If the expansion ROM wishes to change the way the system boots (such as from a network device or a SCSI adapter for which the BIOS has no driver code), it can use the BIOS Boot Specification (BBS) API to register its ability to do so. Once the expansion ROMs have registered using the BBS APIs, the user can select among the available boot options from within the BIOS's user interface. This is why most BBS compliant PC BIOS implementations will not allow the user to enter the BIOS's user interface until the expansion ROMs have finished executing and registering themselves with the BBS API.

[edit] The Rise and Fall of the BIOS

Older operating systems such as DOS relied on the BIOS to carry out most input-output tasks within the PC. A variety of technical reasons eventually made it inefficient—especially for more recent operating systems written for the Intel 80386 such as Linux and Microsoft Windows—to invoke the BIOS directly. Such operating systems instead used their own better-performing native drivers and were also much easier to extend to support new hardware. As such, the BIOS was mostly relegated to bootstrapping to the point where the operating system's own drivers could take control of the hardware.

There was a similar transition for the Apple Macintosh, where the system software originally relied heavily on the ToolBox—a set of drivers and other useful routines stored in ROM—but later discarded this in favour of software drivers.

In recent years, however, by way of systems such as ACPI, the BIOS has taken on more complex functions such as power management, hot swapping and thermal management. This has led to renewed reliance on the BIOS by operating system producers, and an increase in complexity of BIOS code. This in turn has led to invention of Intel's modern Extensible Firmware Interface (EFI) which in itself incorporates BIOSes extended options. Linux supports EFI via elilo boot loader since early 2000. Microsoft announced that support for EFI in Windows Vista is only available for the 64-bit versions.

The Open Source community increased their effort to develop a replacement for proprietary BIOSes and their future incarnations with an open sourced counterpart through the LinuxBIOS and OpenBIOS/Open Firmware projects. So far, those projects have met with some success, with AMD providing product specifications for a number of fairly recent chipsets, and Google sponsoring the project. Motherboard manufacturer Tyan offers LinuxBIOS next to the standard BIOS with their Opteron line of motherboards. MSI and Gigabyte have followed suit with the MSI K9ND MS-9282 and MSI K9SD MS-9185 resp. the M57SLI-S4 models.

[edit] The BIOS business

The vast majority of PC motherboard suppliers license a BIOS "core", and toolkit from a commercial third party, which creates and maintains such a core. The motherboard manufacturer then customizes this BIOS to suit its own hardware. For this reason, updated BIOSes are normally obtained directly from the motherboard manufacturer.

[edit] List of BIOS suppliers

[edit] See also

[edit] External links

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