From Wikipedia, the free encyclopedia
BIOS, in computing, stands for Basic Input/Output System or Basic Integrated Operating System. BIOS refers to the software code run by a computer when first powered on. The primary function of 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 computer. This process is known as booting up.
BIOS can also be said to be a coded program embedded on a chip that recognises and controls various devices that make up the computer. 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.
While the name BIOS is an acronym, it may also be a play on the Greek word βιος (bios), meaning life. 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.
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, 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.
BIOS as firmware
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, 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 ("Dual BIOS" boards). 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 a better image. Hardware manufacturers frequently issue BIOS updates to upgrade their products and remove bugs.
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, and 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 BIOSes 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 kilobyte 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 you can dump your BIOS firmware software at http://www.linuks.mine.nu/ree/
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.
The Fall and Rise of the BIOS
Older operating systems such as DOS called on the BIOS to carry out most input-output tasks within the PC; with the introduction of newer operating systems such as Microsoft Windows and Linux, the BIOS was relegated to principally providing initial hardware setup, and bootstrapping. Once it was up and running, the operating system didn't have to rely on the BIOS for much.
In recent years, however, by way of systems such as ACPI, the BIOS has taken on more complex functions such as aspects of power management, hotplug, thermal management etc. This has led to renewed reliance on the BIOS by operating system producers, and an increase in complexity in the BIOS code.
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.
List of BIOS suppliers:
- American Megatrends (AMI)
- Phoenix Technologies
- Award Software International (merged with Phoenix in 1998)
- MicroID Research (MRBIOS)
- Insyde Software (Insyde)
- General Software (General Software)
- Extensible Firmware Interface (EFI)
- LinuxBIOS, a project which aim is to create a free BIOS based on Linux
- Open Firmware
- Input/Output Base Address
- Advanced Configuration and Power Interface (ACPI)
- BIOS boot devices
- BIOS Interrupt Calls
- Power-On Self Test (POST)