BIOS interrupt call


BIOS interrupt calls are a facility that operating systems and application programs use to invoke the facilities of the Basic Input/Output System software on IBM PC compatible computers. Traditionally, BIOS calls are mainly used by DOS programs and some other software such as boot loaders. BIOS only runs in the real address mode of the x86 CPU, so programs that call BIOS either must also run in real mode or must switch from protected mode to real mode before calling BIOS and then switching back again. For this reason, modern operating systems that use the CPU in Protected Mode generally do not use the BIOS to support system functions, although some of them use the BIOS to probe and initialize hardware resources during their early stages of booting.
In all computers, software instructions control the physical hardware from the moment the power is switched on. In a PC, the BIOS, pre-loaded in ROM on the main board, takes control immediately after the processor is reset, including during power-up, when a hardware reset button is pressed, or when a critical software failure causes the mainboard circuitry to automatically trigger a hardware reset. The BIOS tests the hardware and initializes its state; finds, loads, and runs the boot program ; and provides basic hardware control to the software running on the machine, which is usually an operating system but may be a directly booting single software application.
For IBM's part, they provided all the information needed to use their BIOS fully or to directly utilize the hardware and avoid BIOS completely, when programming the early IBM PC models. From the beginning, programmers had the choice of using BIOS or not, on a per-hardware-peripheral basis. IBM did strongly encourage the authorship of "well-behaved" programs that accessed hardware only through BIOS INT calls, to support compatibility of software with current and future PC models having dissimilar peripheral hardware, but IBM understood that for some software developers and hardware customers, a capability for user software to directly control the hardware was a requirement. In part, this was because a significant subset of all the hardware features and functions was not exposed by the BIOS services. For two examples, the MDA and CGA adapters are capable of hardware scrolling, and the PC serial adapter is capable of interrupt-driven data transfer, but the IBM BIOS supports neither of these useful technical features.
Today, the BIOS in a new PC still supports most, if not all, of the BIOS interrupt function calls defined by IBM for the IBM AT, along with many more newer ones, plus extensions to some of the originals promulgated by various other organizations and collaborative industry groups. This, combined with a similar degree of hardware compatibility, means that most programs written for an IBM AT can still run correctly on a new PC today, assuming that the faster speed of execution is acceptable. Despite the considerable limitations of the services accessed through the BIOS interrupts, they have proven extremely useful and durable to technological change.

Purpose of BIOS calls

BIOS interrupt calls perform hardware control or I/O functions requested by a program, return system information to the program, or do both. A key element of the purpose of BIOS calls is abstraction - the BIOS calls perform generally defined functions, and the specific details of how those functions are executed on the particular hardware of the system are encapsulated in the BIOS and hidden from the program. So, for example, a program that wants to read from a hard disk does not need to know whether the hard disk is an ATA, SCSI, or SATA drive. The program only needs to identify the BIOS-defined number of the drive it wishes to access and the address of the sector it needs to read or write, and the BIOS will take care of translating this general request into the specific sequence of elementary operations required to complete the task through the particular disk controller hardware that is connected to that drive. The program is freed from needing to know how to control at a low level every type of hard disk that it may need to access. This both makes programming operating systems and applications easier and makes the programs smaller, reducing the duplication of program code, as the functionality that is included in the BIOS does not need to be included in every program that needs it; relatively short calls to the BIOS are included in the programs instead.
The BIOS also frees computer hardware designers from being constrained to maintain exact hardware compatibility with old systems when designing new systems, in order to maintain compatibility with existing software. For example, the keyboard hardware on the IBM PCjr works very differently than the keyboard hardware on earlier IBM PC models, but to programs that use the keyboard only through the BIOS, this difference is nearly invisible.
In addition to giving access to hardware facilities, BIOS provides added facilities that are implemented in the BIOS software. For example, the BIOS maintains separate cursor positions for up to eight text display pages and provides for TTY-like output with automatic line wrap and interpretation of basic control characters such as carriage return and line feed, whereas the CGA-compatible text display hardware has only one global display cursor and cannot automatically advance the cursor, use the cursor position to address the display memory, or interpret control characters. For another example, the BIOS keyboard interface interprets many keystrokes and key combinations to keep track of the various shift states, to call the print-screen service when Shift+PrtScrn is pressed, to reboot the system when Ctrl+Alt+Del is pressed, to keep track of the lock states and, in AT-class machines, control the corresponding lock-state indicator lights on the keyboard, and to perform other similar interpretive and management functions for the keyboard. In contrast, the ordinary capabilities of the standard PC and PC-AT keyboard hardware are limited to reporting to the system each primitive event of an individual key being pressed or released, performing a commanded reset and self-test of the keyboard unit, and, for AT-class keyboards, executing a command from the host system to set the absolute states of the lock-state indicators.

Calling BIOS: BIOS software interrupts

Operating systems and other software communicate with the BIOS software, in order to control the installed hardware, via software interrupts. A software interrupt is a specific variety of the general concept of an interrupt. An interrupt is a mechanism by which the CPU can be
directed to stop executing the main-line program and immediately execute a special program, called an Interrupt Service Routine, instead. Once the ISR finishes, the CPU continues with the main program. On x86 CPUs, when an interrupt occurs, the ISR to call is found by looking it up in a table of ISR starting-point addresses in memory: the Interrupt Vector Table. An interrupt is invoked by its type number, from 0 to 255, and the type number is used as an index into the Interrupt Vector Table, and at that index in the table is found the address of the ISR that will be run in response to the interrupt. A software interrupt is simply an interrupt that is triggered by a software command; therefore, software interrupts function like subroutines, with the main difference that the program that makes a software interrupt call does not need to know the address of the ISR, only its interrupt number. This has advantages for modularity, compatibility, and flexibility in system configuration.
BIOS interrupt calls can be thought of as a mechanism for passing messages between BIOS and BIOS client software such as an operating system. The messages request data or action from BIOS and return the requested data, status information, and/or the product of the requested action to the caller. The messages are broken into categories, each with its own interrupt number, and most categories contain sub-categories, called "functions" and identified by "function numbers". A BIOS client passes most information to BIOS in CPU registers, and receives most information back the same way, but data too large to fit in registers, such as tables of control parameters or disk sector data for disk transfers, is passed by allocating a buffer in memory and passing the address of the buffer in registers. The interrupt number is specified as the parameter of the software interrupt instruction, and the function number is specified in the AH register; that is, the caller sets the AH register to the number of the desired function. In general, the BIOS services corresponding to each interrupt number operate independently of each other, but the functions within one interrupt service are handled by the same BIOS program and are not independent.
The BIOS software usually returns to the caller with an error code if not successful, or with a status code and/or requested data if successful. The data itself can be as small as one bit or as large as 65,536 bytes of whole raw disk sectors. BIOS has been expanded and enhanced over the years many times by many different corporate entities, and unfortunately the result of this evolution is that not all the BIOS functions that can be called use consistent conventions for formatting and communicating data or for reporting results. Some BIOS functions report detailed status information, while others may not even report success or failure but just return silently, leaving the caller to assume success. Sometimes it can also be difficult to determine whether or not a certain BIOS function call is supported by the BIOS on a certain computer, or what the limits of a call's parameters are on that computer.
Because BIOS interrupt calls use CPU register-based parameter passing, the calls are oriented to being made from assembly language and cannot be directly made from most high-level languages. However, a high level language may provide a library of wrapper routines which translate parameters from the form used by the high-level language to the register-based form required by BIOS, then back to the HLL calling convention after the BIOS returns. In some variants of C, BIOS calls can be made using inline assembly language within a C module.

Invoking an interrupt

Invoking an interrupt can be done using the INT x86 assembly language instruction. For example, to print a character to the screen using BIOS interrupt 0x10, the following x86 assembly language instructions could be executed:

mov ah, 0x0e ; function number = 0Eh : Display Character
mov al, '!' ; AL = code of character to display
int 0x10 ; call INT 10h, BIOS video service

Interrupt table

A list of common BIOS interrupt classes can be found below. Note that some BIOSes do not implement all of these interrupt classes.
The BIOS also uses some interrupts to relay hardware event interrupts to programs which choose to receive them or to route messages for its own use. The table below includes only those BIOS interrupts which are intended to be called by programs to request services or information.
Interrupt vectorDescription
Executed when Shift-Print screen is pressed, as well as when the instruction detects a bound failure.
INT 10H|Video Services

: execute BASIC

traditionally jumped to an implementation of Cassette BASIC stored in Option ROMs. This call would typically be invoked if the BIOS was unable to identify any bootable disk volumes on startup.
At the time the original IBM PC was released in 1981, the BASIC in ROM was a key feature. Contemporary popular personal computers such as the Commodore 64 and the Apple II line also had Microsoft Cassette BASIC in ROM, so in a substantial portion of its intended market, the IBM PC needed BASIC to compete. As on those other systems, the IBM PC's ROM BASIC served as a primitive diskless operating system, allowing the user to load, save, and run programs, as well as to write and refine them.
As time went on and BASIC was no longer shipped on all PCs, this interrupt would simply display an error message indicating that no bootable volume was found ; in other BIOS versions it would prompt the user to insert a bootable volume and press a key, and then after the user pressed a key it would loop back to the bootstrap loader to try booting again.
Digital's Rainbow 100B used to call its BIOS, which was incompatible with the IBM BIOS. Turbo Pascal, Turbo C and Turbo C++ repurposed INT 18 for memory allocation and paging. Other programs also reused this vector for their own purposes.

BIOS hooks

DOS

On DOS systems, IO.SYS or IBMBIO.COM hooks INT 13 for floppy disk change detection, tracking formatting calls, correcting DMA boundary errors, and working around problems in IBM's ROM BIOS "01/10/84" with model code 0xFC before the first call.

Bypassing BIOS

Many modern operating systems bypass the built-in BIOS interrupt communication system altogether, preferring to use their own software to control the attached hardware directly. The original reason for this was primarily that these operating systems run the processor in protected mode, whereas calling BIOS requires switching to real mode and back again, and switching to real mode is slow. However, there are also serious security reasons not to switch to real mode, and the BIOS code has limitations both in functionality and speed that motivate operating system designers to find a replacement for it. In fact, the speed limitations of the BIOS made it common even in the DOS era for programs to circumvent it in order to avoid its performance limitations, especially for video graphics display and fast serial communication. The problems with BIOS functionality include limitations in the range of functions defined, inconsistency in the subsets of those functions supported on different computers, and variations in the quality of BIOSes. By taking matters into their own hands and avoiding reliance on BIOS, operating system developers can eliminate some of the risks and complications they face in writing and supporting system software. On the other hand, by doing so those developers become responsible for providing "bare-metal" driver software for every different system or peripheral device they intend for their operating system to work with. Thus it should be apparent that compact operating systems developed on small budgets would tend to use BIOS heavily, while large operating systems built by huge groups of software engineers with large budgets would more often opt to write their own drivers instead of using BIOS—that is, even without considering the compatibility problems of BIOS and protected mode.
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