The BIOS stands for Basic Input/Output System and also known as the System BIOS, ROM BIOS or PC BIOS) is a type of firmware used during the booting process (power-on startup) on IBM PC compatible computers. The BIOS firmware is built into PCs, and it is the first software they run when powered on. The name itself originates from the Basic Input/Output System used in the CP/M operating system in 1975. Originally proprietary to the IBM PC, the BIOS was reverse engineered by companies looking to create compatible systems and the interface of that original system serves as a de facto standard.
The fundamental purposes of the BIOS are to initialize and test the system hardware components, and to load a boot loader or an operating system from a mass memory device. The BIOS additionally provides an abstraction layer for the hardware, i.e. a consistent way for application programs and operating systems to interact with the keyboard, display, and other input/output devices. Variations in the system hardware are hidden by the BIOS from programs that use BIOS services instead of directly accessing the hardware. MS-DOS(PC DOS), which was the dominant PC operating system from the early 1980s until the mid 1990s, relied on BIOS services for disk, keyboard, and text display functions. MS Windows NT, Linux, and other protected mode operating systems in general ignore the abstraction layer provided by the BIOS and do not use it after loading, instead accessing the hardware components directly. Every BIOS implementation is specifically designed to work with a particular computer or motherboard model, by interfacing with various devices that make up the complementary system chipset. Originally, BIOS firmware was stored in a ROM chip on the PC motherboard; in modern computer systems, the BIOS contents are stored on flash memory so it can be rewritten without removing the chip from the motherboard. This allows easy updates to the BIOS firmware so new features can be added or bugs can be fixed, but it also creates a possibility for the computer to become infected with BIOS rootkits.
Unified Extensible Firmware Interface (UEFI) was designed as a successor to BIOS, aiming to address its technical shortcomings. As of 2014, new PC hardware predominantly ships with UEFI firmware.
History.
/* C P / M B A S I C I / O S Y S T E M (B I O S)
COPYRIGHT (C) GARY A. KILDALL
JUNE, 1975 */
[...]
/* B A S I C D I S K O P E R A T I N G S Y S T E M (B D O S)
COPYRIGHT (C) GARY A. KILDALL
JUNE, 1975 */
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The term BIOS (Basic Input/Output System) was invented 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" (aka "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 analogous to the "CP/M BIOS". With the introduction of PS/2 machines, IBM divided the System BIOS into real-mode 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..
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The first commercial licensing of CP/M took place in 1975 with contracts between Digital Systems and Omron of America for use in their intelligent terminal, and with Lawrence Liver more Laboratories where CP/M was used to monitor programs in the Octopus network. Little attention was paid to CP/M for about a year. In my spare time, I worked to improve overall facilities [...] By this time, CP/M had been adapted for four different controllers. In 1976, Glenn Ewing approached me with a problem: Imsai, Incorporated, for whom Glenn consulted, had shipped a large number of disk subsystems with a promise that an operating system would follow. I was somewhat reluctant to adapt CP/M to yet another controller, and thus the notion of a separated Basic I/O System (BIOS) evolved. In principle, the hardware dependent portions of CP/M were concentrated in the BIOS, thus allowing Glenn, or anyone else, to adapt CP/M to the Imsai equipment. Imsai was subsequently licensed to distribute CP/M version 1.3 which eventually evolved into an operating system called IMDOS.
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The BIOS of the original IBM PC 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 POST had not proceeded to the point of successfully initializing a video display adapter). Options on the PC and XT were set by switches and jumpers on the main board and on peripheral cards. Starting around the mid-1990s, it became typical for the BIOS ROM to include a"BIOS configuration utility" (BCU) 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 boot-able 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.
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.
Operation.
System Start-up.
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Intel had hired him a few months earlier to write a control program monitor to run on their little demo system for 8008 and now 8080. Glenn knew this and he would be talking with Gary, and he started twisting Gary's arm. He said, "Hey Gary, why can't we run this in this IMSAI?" "The I/O's all different, won't run." But Glenn persists and finally makes a deal with Gary. He says, "Okay Gary, if you split out the I/O, I'll write the BIOS, basic I/O's system," and Glenn named it then. "We'll split it out separately. I'll write that part, as long as you can make a division in the program there." And he got Gary to do that and Glenn put those two pieces together and was running Gary's CP/M on an IMSAI. Glenn let us know that, and it wasn't too much later than Bill was down there making arrangements with Gary Kildall to license CP/M. Now that the BIOS is separated out, anybody could write a BIOS for their machine, if it was 8080-based, and run this, so he started selling that separately under the company Digital Research that he formed and did quite well."
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When an x86 microprocessor is reset, it loads its program counter with a fixed address near the top of the one-megabyte address space assignable in real mode. (Depending on the microprocessor model, it may also assert all of the address lines above the first 20, causing the real-mode address space to be mapped into the last megabyte of the physical address space, until the first FAR jump or call.) The address of the BIOS's memory is located such that it will be executed upon processor reset. There are only a few bytes in memory after the reset startup address, so a jump in struction near the end of the BIOS ROM directs the processor to start executing the BIOS startup code at an earlier address.
If the system has just been powered up or the reset button was pressed ("cold boot"), the full power-on self-test(POST) is run. If Ctrl+Alt+Delete was pressed ("warm boot"), a special flag value is stored in nonvolatile BIOS memory before the processor is reset, and after the reset the BIOS startup code detects this flag and does not run the POST. This saves the time otherwise used to detect and test all memory. The non-volatile RAM (NVRAM) is in the real-time clock (RTC).
(POST) is run. If Ctrl+Alt+Delete was pressed ("warm boot"), a special flag value is stored in nonvolatile BIOS memory before the processor is reset, and after the reset the BIOS startup code detects this flag and does not run the POST. This saves the time otherwise used to detect and test all memory. The non-volatile RAM (NVRAM) is in the real-time clock (RTC).
The POST checks, identifies, and initializes system devices such as the CPU, RAM, interrupt and DMA controllers and other parts of the chipset, video display card, keyboard, hard disk drive, optical disc drive and other basic hardware. Early IBM PCs had a little-known routine in the POST that would attempt to download a program from into RAM through the keyboard port. (Note that no serial or parallel ports were standard on early IBM PCs, but a keyboard port of either the XT or AT / PS/2 type has been standard on practically every PC and clone.) If the download was apparently successful, the BIOS would verify a check sum on it and then run it. This feature was intended for factory test or diagnostic purposes. While it was of limited utility outside of factory or repair facilities, it could be used in a proprietary way to boot the PC as a satellite system to a host machine (which is essentially the same technical way it was used, if it was used, in the manufacturing environment).
Boot process.
After the option ROM scan is completed and all detected ROM modules with valid check sums have been called, or immediately after POST in a BIOS version that does not scan for option ROM's, the BIOS calls INT 19h to start boot processing. Post-boot, Programs loaded can also call INT 19h to reboot the system, but they must be careful to disable interrupts and other asynchronous hardware processes that may interfere with the BIOS rebooting process, or else the system may hang or crash while it is rebooting. Unique to the original IBM BIOS was that it would attempt to load a maintenance program through the keyboard port before performing any other elements of the boot process, such as before scanning for option ROM's or executing a boot loader.
When INT 19h is called, the BIOS attempts to locate boot loader software held on a storage device designated as a "boot device", such as a hard disk, a floppy disk, CD, or DVD. It loads and executes the first boot software it finds, giving it control of the PC. This is the process that is known as booting (sometimes informally called "booting up"), which is short for "bootstrapping". The BIOS selects candidate boot devices using information collected by POST and configuration information from EEPROM, CMOS RAM or, in the earliest PCs, DIP switches. Following the boot priority sequence in effect, BIOS checks each device in order to see if it is bootable. For a disk drive or a device that logically emulates a disk drive, such as a USB Flash drive or perhaps a tape drive, to perform this check the BIOS attempts to load the first sector (boot sector) from the disk into RAM at memory address
0x0000:0x7C00. If the sector cannot be read (due to a missing or unformatted disk, or due to a hardware failure), the BIOS considers the device unbootable and proceeds to check the next device. If the sector is read successfully, some BIOSes will also check for the boot sector signature 0x55 0xAA in the last two bytes of the (512 byte long) sector, before accepting a boot sector and considering the device bootable. The BIOS proceeds to test each device sequentially until a bootable device is found, at which time the BIOS transfers control to the loaded sector with a jump instruction to its first byte at address 0x0000:0x7C00 (exactly 1 KiB below the 32 KiB mark); see MBR invocation and VBR invocation. (This location is one reason that an IBM PC requires at least 32 KiB of RAM in order to be equipped with a disk system; with 31 KiB or less, it would be impossible to boot from any disk, removable or fixed, using the BIOS boot protocol.) Most, but not all, BIOSes load the drive number (as used by INT 13h) of the boot drive into CPU register DL before jumping to the first byte of the loaded boot sector.
Note well that the BIOS does not interpret or process the contents of the boot sector other than to possibly check for the boot sector signature in the last two bytes; all interpretation of data structures like MBR partition tables and so-called BIOS Parameter Blocks is done by the boot program in the boot sector itself or by other programs loaded through the boot process and is beyond the scope of BIOS. Nothing about BIOS predicates these data structures or impedes their replacement or improvement.
A non-disk device such as a network adapter attempts booting by a procedure that is defined by its option ROM or the equivalent integrated into the motherboard BIOS ROM. As such, option ROMs may also influence or supplant the boot process defined by the motherboard BIOS ROM.
Boot Priority.
The user can control the boot process, to cause one medium to be booted instead of another when two or more bootable media are present, by taking advantage of the boot priority implemented by the BIOS. For example, most computers have a hard disk that is bootable, but usually there is a removable-media drive that has higher boot priority, so the user can cause a removable disk to be booted, simply by inserting it, without removing the hard disk drive or altering its contents to make it unbootable. In most modern BIOSes, the boot priority order of all potentially bootable devices can be freely configured by the user through the BIOS configuration utility. In older BIOSes, limited boot priority options are selectable; in the earliest BIOSes, a fixed priority scheme was implemented, with floppy disk drives first, fixed disks (i.e. hard disks) second, and typically no other boot devices supported, subject to modification of these rules by installed option ROMs. The BIOS in an early PC also usually would only boot from the first floppy disk drive or the first hard disk drive, even if there were two drives of either type installed. All more advanced boot priority sequences evolved as incremental improvements on this basic system. Historically the BIOS would try to boot from a floppy drive first and a hard disk second. The default for CD or DVD booting is an extension of this. With the El Torito optical media boot standard, the optical drive actually emulates a 3.5" high-density floppy disk to the BIOS for boot purposes. Optical disks are a special case, because their lowest level of data organization is typically a fairly high-level file system (e.g. ISO 9660 for CD-ROM). eading the "first sector" of a CD-ROM or DVD-ROM is not a simply defined operation like it is on a floppy disk or a hard disk. Furthermore, the complexity of the medium makes it difficult to write a useful boot program in one sector, even though optical media sectors are typically 2048 bytes each, four times the standard 512-byte size of floppy and legacy hard disk sectors. Therefore, optical media booting uses the El Torito standard, which specifies a way for an optical disk to contain an image of a high-density (1.44 MB) floppy disk and for the drive to provide access to this disk image in a simple manner that emulates floppy disk drive operations. Therefore, CD-ROM drives boot as emulated floppy disk drives; the bootable virtual floppy disk can contain software that provides access to the optical medium in its native format.
Boot Failure.
The behavior if the BIOS does not find a bootable device has varied as personal computers developed. The original IBM PC and XT had Microsoft Cassette BASIC in ROM, and if no bootable device was found, ROM BASIC was started by calling INT 18h. Therefore, barring a hardware failure, an original IBM PC or XT would never fail to boot, either into BASIC or from disk (or through an option ROM). One model of the original IBM PC was available with no disk drive; a cassette recorder could be attached via the cassette port on the rear, for loading and saving BASIC programs to tape. Since few programs used BASIC in ROM, clone PC makers left it out; then a computer that failed to boot from a disk would display "No ROM BASIC" and halt (in response to INT 18h). Later computers would display a message like "No bootable disk found"; some would prompt for a disk to be inserted and a key to be pressed, and when a key was pressed they would restart the boot process. Modern BIOSes may display nothing or may automatically enter the BIOS configuration utility when the boot process fails. Unlike earlier BIOSes, modern BIOSes are often written with the assumption that if the computer cannot be booted from a hard disk, the user will not have software that they want to boot from removable media instead. (Lately, typically it will only be a specialist computer technician who does that, only to get the computer back into a condition where it can be booted from the hard disk.)