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BIOS
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In computing, the BIOS is an initialism stands for the Basic Input/Output System according to the IBM Personal Computer Technical Reference manual or the punctuational variants Basic Input Output System, or Basic Input-Output System.
BIOS in part refers to the firmware code run by a PC when first powered on, which is a type of boot loader. The primary function of the BIOS is to identify and initialize system component hardware (such as the video display card, hard disk, and floppy). This is to prepare the machine into a known low capability state, 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. 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 alternatives for Legacy BIOS in the x86 world: Extensible Firmware Interface, Open Firmware (used on the OLPC XO-1) and coreboot.
The BIOSes of IBM PC class machines 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, and provides a small library of basic Input/Output functions that can be called to operate and control the peripherals such as the keyboard, primitive (800 x 600) display functions and so forth. Computers designed to run Windows ME or Windows 2000, or later, supersede this basic monitor functionality by taking over direct control of the interrupt table and replacing the monitor routines with faster and more robust low-level modules that, unlike the BIOS function set, are re-entrant. Various BIOS functions in ROM were left in control in earlier windows versions, and the BIOS only comes into play today in the alternate shell CMD.exe, or if the machine is booted into a legacy DOS version.
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 term is incorrectly known as Binary Input/Output System, Basic Integrated Operating System and occasionally Built In Operating System.
The BIOS Chip and BIOS Recovery| Boot Block | | DMI Block | | Main Block |
Prior to the early 1990s, BIOSes were stored in ROM or PROM chips, which could not be altered by users. As its complexity and need for updates grew, and re-programmable parts became more available, BIOS firmware was most commonly stored on EEPROM or flash memory devices. The first flash chips attached to the ISA bus. Starting in 1997, 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 be easily updated by the user; hardware manufacturers frequently issue BIOS updates to upgrade their products, improve compatibility and remove bugs. However, this advantage had the risk that an improperly executed or aborted BIOS update could render the computer or device unusable. To avoid these situations, more recent BIOSes use a "boot block"; a portion of the BIOS which runs first and must be updated separately. This code verifies if the rest of the BIOS is intact (using hash checksums or other methods) before transferring control to it. If the boot block detects any corruption in the main BIOS, it will typically warn the user that a recovery process must be initiated by booting from removable media (floppy, CD or USB memory) so the user can try flashing the BIOS again. Some motherboards have a backup BIOS (sometimes referred to as DualBIOS boards) to recover from BIOS corruptions. In 2007, Gigabyte began offering motherboards with a QuadBIOS recovery feature.
Due to the limitation on the number of times flash memory media can be flashed, a flash-based BIOS is vulnerable to "flash-burn" viruses that repeatedly write to the flash, permanently corrupting chip content. 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.
The size of the BIOS, and the capacities of the ROM, EEPROM and other media it may be stored on, has increased over time as new features have been added to the code; BIOS versions now exist with sizes up to 8 megabytes.
Firmware on adapter cardsA 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 interface devices, 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 the operating system), and:
- Are not sufficiently simple, or generic in operation to be handled by the main BIOS directly
PC operating systems such as DOS, including all DOS-based versions of MS Windows, as well as bootloaders, may continue to make use of the BIOS to handle input and output. However, other 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 these operating systems, in order to carry out specific tasks such as preliminary device initialization.
To find these memory mapped expansion ROMs during the boot process, PC BIOS implementations scan real memory from 0xC0000 to 0xF0000 on 2 kibibyte boundaries looking for the ROM signature bytes of 55h followed by AAh (0xAA55). For a valid expansion ROM, its signature is immediately followed by a single byte indicating the number of 512-byte blocks it occupies in real memory. The BIOS then jumps to the offset located immediately after this size byte; at which point the expansion ROM code takes over, using the BIOS services to register interrupt vectors for use by post-boot applications and provide a user configuration interface, or display diagnostic information.
There are many methods and utilities for dumping the contents of various motherboard BIOS and expansion ROMs. Under a Microsoft OS, DEBUG can be used to examine 64 KiB segments of memory and save the contents to a file. For UNIX systems the dd command can be used by a user with root privileges: "dd if=/dev/mem bs=1k skip=768 count=256 2>/dev/null | strings -n 8".
The BIOS boot specificationIf 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 BIOSes 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.
Evolution of the role of the BIOSSome operating systems, for example MS-DOS, rely on the BIOS to carry out most input/output tasks within the PC. A variety of technical reasons makes it inefficient for some recent operating systems written for 32-bit CPUs such as Linux and Microsoft Windows to invoke the BIOS directly. Larger, more powerful, servers and workstations using PowerPC or SPARC CPUs by several manufacturers developed a platform-independent Open Firmware (IEEE-1275), based on the Forth programming language. It is included with Sun's SPARC computers, IBM's RS/6000 line, and other PowerPC CHRP motherboards. Later x86-based personal computer operating systems, like Windows NT, use their own, native drivers which also makes it much easier to extend support to new hardware, while the BIOS still relies on a legacy 16-bit runtime interface. As such, the BIOS was relegated to bootstrapping, at which point the operating system's own drivers can 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 based on Motorola's 680x0 CPUs. These Apple ROMs were replaced by Open Firmware in the PowerPC Macintosh, then EFI in Intel Macintosh computers.
Later BIOS took on more complex functions, by way of interfaces such as ACPI; these functions include power management, hot swapping and thermal management. However BIOS limitations (16-bit processor mode, only 1 MiB addressable space, PC AT hardware dependencies, etc.) were seen as clearly unacceptable for the newer computer platforms. Extensible Firmware Interface (EFI) is a specification which replaces the runtime interface of the legacy BIOS. Initially written for the Itanium architecture, EFI is now available for x86 and x86-64 platforms; the specification development is driven by The Unified EFI Forum, an industry Special Interest Group.
Linux has supported EFI via the elilo boot loader. 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 coreboot and OpenBIOS/Open Firmware projects. AMD provided product specifications for some chipsets, and Google is sponsoring the project. Motherboard manufacturer Tyan offers coreboot 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 modems.
The BIOS businessThe vast majority of PC motherboard suppliers license a BIOS "core" and toolkit from a commercial third-party, known as an "independent BIOS vendor" or IBV. 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.
Major BIOS vendors include American Megatrends (AMI), General Software, Insyde Software, and Phoenix Technologies (which bought Award Software International in 1998).
See also
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