0x7C00. Because boot programs are always loaded at this fixed address, there is no need or motivation for a boot program to be relocatable. DL contains the drive number, as used with INT 13h, of the boot device, unless the BIOS is one that does not set the drive number in DL – and then DL is undefined. SS:SP points to a valid stack that is presumably large enough to support hardware interrupts, but otherwise SS and SP are undefined. (A stack must be already set up in order for interrupts to be serviced, and interrupts must be enabled in order for the system timer-tick interrupt, which BIOS always uses at least to maintain the time-of-day count and which it initializes during POST, to be active and for the keyboard to work. The keyboard works even if the BIOS keyboard service is not called; keystrokes are received and placed in the 15-character type-ahead buffer maintained by BIOS.) The boot program must set up its own stack (or at least MS-DOS 6 acts like it must), because the size of the stack set up by BIOS is unknown and its location is likewise variable; although the boot program can investigate the default stack by examining SS:SP, it is easier and shorter to just unconditionally set up a new stack. At boot time, all BIOS services are available, and the memory below address 0x00400 contains the interrupt vector table. BIOS POST has initialized the system timers (8253 or 8254 IC), interrupt controller(s), DMA controller(s), and other motherboard/chipset hardware as necessary to bring all BIOS services to ready status. DRAM refresh for all system DRAM in conventional memory and extended memory, but not necessarily expanded memory, has been set up and is running. The interrupt vectors corresponding to the BIOS interrupts have been set to point at the appropriate entry points in the BIOS, hardware interrupt vectors for devices initialized by the BIOS have been set to point to the BIOS-provided ISRs, and some other interrupts, including ones that BIOS generates for programs to hook, have been set to a default dummy ISR that immediately returns. The BIOS maintains a reserved block of system RAM at addresses 0x00400–0x004FF with various parameters initialized during the POST. All memory at and above address 0x00500 can be used by the boot program; it may even overwrite itself. Extensions (Option ROMs).
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.)
There are many methods and utilities for examining the contents of various motherboard BIOS and expansion ROMs, such as Microsoft DEBUG or the Unix dd.
Boot Procedure.
If an 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) in a cooperative way, 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 specification can be downloaded from the ACPICA website. The official title is BIOS Boot Specification (Version 1.01, 11 January 1996). Also, if an expansion ROM wishes to change the way the system boots unilaterally, it can simply hook INT 19h or other interrupts normally called from interrupt 19h, such as INT 13h, the BIOS disk service, to intercept the BIOS boot process. Then it can replace the BIOS boot process with one of its own, or it can merely modify the boot sequence by inserting its own boot actions into it, by preventing the BIOS from detecting certain devices as bootable, or both. Before the BIOS Boot Specification was promulgated, this was the only way for expansion ROMs to implement boot capability for devices not supported for booting by the native BIOS of the motherboard.
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 check sum of the specified number of 512-byte blocks is calculated, and if the ROM has a valid check sum, 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. While the actions mentioned are typical behaviors of BIOS entension ROMs, each option ROM receives total control of the computer and may do anything at all, as noted with more detail in the Extensions section below; it is possible that an option ROM will not returning 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.
Physical Placement.
Option ROMs normally reside on adapter cards. However, the original PC, and perhaps also the PC XT, have a spare ROM socket on the motherboard (the "system board" in IBM's terms) into which an option ROM can be inserted, and the four ROMs that contain the BASIC interpreter can also be removed and replaced with custom ROMs which can be option ROMs. The IBM PCjr is unique among PCs in having two ROM cartridge slots on the front. Cartridges in these slots map into the same region of the upper memory area used for option ROMs, and the cartridges can contain option ROM modules that the BIOS would recognize. The cartridges can also contain other types of ROM modules, such as BASIC programs, that are handled differently. One PCjr cartridge can contain several ROM modules of different types, possibly stored together in one ROM chip.
Operating System Services.
The BIOS ROM is customized to the particular manufacturer's hardware, allowing low-level services (such as reading a keystroke or writing a sector of data to diskette) to be provided in a standardized way to programs, including operating systems. For example, an IBM PC might have either a monochrome or a color display adapter (using different display memory addresses and hardware), but a single, standard, BIOS system call may be invoked to display a character at a specified position on the screen in text mode or graphics mode. The BIOS provides a small library of basic input/output functions to operate peripherals (such as the keyboard, rudimentary text and graphics display functions and so forth). When using MS-DOS, BIOS services could be accessed by an application program (or by MS-DOS) by executing an INT 13h interrupt instruction to access disk functions, or by executing one of a number of other documented BIOS interrupt calls to access video display, keyboard, cassette, and other device functions. Operating systems and executive software that are designed to supersede this basic firmware functionality provide replacement software interfaces to application software. Applications can also provide these services to themselves. This began even in the 1980s under MS-DOS, when programmers observed that using the BIOS video services for graphics display was very slow. To increase the speed of screen output, many programs bypassed the BIOS and programmed the video display hardware directly. Other graphics programmers, particularly but not exclusively in the demo scene, observed that there were technical capabilities of the PC display adapters that were not supported by the IBM BIOS and could not be taken advantage of without circumventing it. Since the AT-compatible BIOS ran in Intel real mode, operating systems that ran in protected mode on 286 and later processors required hardware device drivers compatible with protected mode operation to replace BIOS services. In modern personal computers running modern operating systems the BIOS is used only during booting and initial loading of system software. Before the operating system's first graphical screen is displayed, input and output are typically handled through BIOS. A boot menu such as the textual menu of Windows, which allows users to choose an operating system to boot, to boot into the safe mode, or to use the last known good configuration, is displayed through BIOS and receives keyboard input through BIOS.
However, it is also important to note that modern PCs can still boot and run legacy operating systems such as MS-DOS or DR-DOS that rely heavily on BIOS for their console and disk I/O. Thus, while not as central as they once were, the BIOS services are still important.
Processor Microcode Updates.
Intel processors have re programmable microcode since the P6 micro architecture. The BIOS may contain patches to the processor microcode that fix errors in the initial processor microcode; reprogramming is not persistent, thus loading of microcode updates is performed each time the system is powered up. Without re programmable microcode, an expensive processor swap would be required: for example, the Pentium FDIV bug became an expensive fiasco for Intel as it required a product recall because the original Pentium processor's defective microcode could not be reprogrammed.
Identification.
Some BIOS contain a "SLIC" (software licensing description table), a digital signature placed inside the BIOS by the manufacturer, for example Dell. (It is often casually called a BIOS tattoo or a tattooed BIOS.) This SLIC is inserted in the ACPI table and contains no active code.
Computer manufacturers that distribute OEM versions of Microsoft Windows and Microsoft application software can use the SLIC to authenticate licensing to the OEM Windows Installation disk and system recovery disc containing Windows software. Systems having a SLIC can be pre activated with an OEM product key, and they verify an XML formatted OEM certificate against the SLIC in the BIOS as a means of self-activating (see System Locked Pre installation). If a user performs a fresh install of Windows, they will need to have possession of both the OEM key and the digital certificate for their SLIC in order to bypass activation; in practice this is extremely unlikely and hence the only real way this can be achieved is if the user performs a restore using a pre-customised image provided by the OEM. Cracks for non-genuine Windows distributions usually edit the SLIC or emulate it in order to bypass Windows activation.
Overclocking.
Some BIOS implementations allow overclocking, an action in which the CPU is adjusted to a higher clock rate than its manufacturer rating for guaranteed capability. Overclocking may, however, seriously compromise system reliability in insufficiently cooled computers and generally shorten component lifespan. Overclocking, when incorrectly performed, may also cause components to overheat so quickly that they mechanically destroy themselves.
Modern Use.
Some operating systems, for example MS-DOS, rely on the BIOS to carry out most input/output tasks within the PC. Because the BIOS still runs in 16-bit real mode, calling BIOS services directly is inefficient for protected-mode operating systems. BIOS services are not used by modern multi tasking GUI operating systems after they initially load, so the importance of the primary part of BIOS is greatly reduced from what it was initially. Later BIOS implementations took on more complex functions, by including interfaces such as Advanced Configuration and Power Interface (ACPI); these functions include power management, hot swapping, and thermal management. At the same time, since 2010 BIOS technology is in a transitional process toward the UEFI.