BIOS interrupt call - Wikipedia, the free encyclopedia

# BIOS interrupt call

From Wikipedia, the free encyclopedia

BIOS interrupt calls are a facility that DOS programs and some other software, such as boot loaders, use to invoke the facilities of the Basic Input/Output System. Some operating systems also use the BIOS to probe and initialize hardware resources during their early stages of booting.

## 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 would be executed:

    mov ah, 0x0e
    mov al, '!'
    int 0x10

## Interrupt table

A list of common BIOS interrupts can be found below. Note that some BIOSes (particularly old ones) will not support all of these interrupts.

Interrupt vector | Description
--- | ---
`.00h.` | CPU: Executed after an attempt to divide by zero or when the quotient does not fit in the destination
`.01h.` | CPU: Executed after every instruction while the trace flag is set
`.02h.` | CPU: NMI, used e.g. by POST for memory errors
`.03h.` | CPU: The lowest non-reserved interrupt, it is used exclusively for debugging, and the `.INT 03.` handler is always implemented by a debugging program
`.04h.` | CPU: Numeric Overflow. Usually caused by the `.INTO.` instruction when the overflow flag is set.
`.05h.` | Executed when Shift-Print screen is pressed, as well as when the `.BOUND.` instruction detects a bound failure.
`.06h.` | CPU: Called when the Undefined Opcode (invalid instruction) exception occurs. Usually installed by the operating system.
`.07h.` | CPU: Called when an attempt was made to execute a floating-point instruction and no numeric coprocessor was available.
`.08h.` | IRQ0: Implemented by the system timing component; called 18.2 times per second (once every 55 ms) by the PIC
`.09h.` | IRQ1: Called after every key press and release (as well as during the time when a key is being held)
`.0Bh.` | IRQ3: Called by serial ports 2 and 4 (COM2/4) when in need of attention
`.0Ch.` | IRQ4: Called by serial ports 1 and 3 (COM1/3) when in need of attention
`.0Dh.` | IRQ5: Called by hard disk controller (PC/XT) or 2nd parallel port LPT2 (AT) when in need of attention
`.0Eh.` | IRQ6: Called by floppy disk controller when in need of attention
`.0Fh.` | IRQ7: Called by 1st parallel port LPT1 (printer) when in need of attention

`.10h.`

Video Services - installed by the BIOS or operating system; called by software programs

`.AH.` | Description
--- | ---
`.00h.` | Set Video Mode
`.01h.` | Set Cursor Shape
`.02h.` | Set Cursor Position
`.03h.` | Get Cursor Position And Shape
`.04h.` | Get Light Pen Position
`.05h.` | Set Display Page
`.06h.` | Clear/Scroll Screen Up
`.07h.` | Clear/Scroll Screen Down
`.08h.` | Read Character and Attribute at Cursor
`.09h.` | Write Character and Attribute at Cursor
`.0Ah.` | Write Character at Cursor
`.0Bh.` | Set Border Color
`.0Ch.` | Write Graphics Pixel
`.0Dh.` | Read Graphics Pixel
`.0Eh.` | Write Character in TTY Mode
`.0Fh.` | Get Video Mode
`.13h.` | Write String

`.11h.` | Installed by the BIOS; returns equipment list
`.12h.` | Installed by the BIOS or operating system; returns Conventional Memory Size

`.13h.`

Low Level Disk Services; installed by the BIOS or operating system; called by software programs

`.AH.` | Description
--- | ---
`.00h.` | Reset Disk Drives
`.01h.` | Check Drive Status
`.02h.` | Read Sectors From Drive
`.03h.` | Write Sectors To Drive
`.04h.` | Verify Sectors On Drive
`.05h.` | Format Track On Drive
`.08h.` | Get Drive Parameters
`.09h.` | Init Fixed Drive Parameters
`.0Ch.` | Seek To Specified Track
`.0Dh.` | Reset Fixed Disk Controller
`.15h.` | Get Drive Type
`.16h.` | Get Floppy Drive Media Change Status

`.14h.`

Routines for communicating via the serial port. Used by software programs.

`.AH.` | Description
--- | ---
`.00h.` | Serial Port Initialization
`.01h.` | Transmit Character
`.02h.` | Receive Character
`.03h.` | Status

`.15h.`

Miscellaneous (System services support routines)

`.AH.` | `.AL.` | `.AX.` | Description
--- | --- | --- | ---
`.00h.` |  |  | Turn on cassette drive motor
`.01h.` |  |  | Turn off cassette drive motor
`.02h.` |  |  | Read data blocks from cassette
`.03h.` |  |  | Write data blocks to cassette
`.4Fh.` |  |  | Keyboard Intercept
`.83h.` |  |  | Event Wait
`.84h.` |  |  | Read Joystick
`.85h.` |  |  | Sysreq Key Callout
`.86h.` |  |  | Wait
`.87h.` |  |  | Move Block
`.88h.` |  |  | Get Extended Memory Size
`.C0h.` |  |  | Get System Parameters
`.C1h.` |  |  | Get Extended BIOS Data Area Segment
`.C2h.` |  |  | Pointing Device Functions
`.E8h.` | `.01h.` | `.E801h.` | Get Extended Memory Size (Newer function, since 1994). Gives results for memory size above 64 Mb.
`.E8h.` | `.20h.` | `.E820h.` | Query System Address Map. The information returned from e820 supersedes what is returned from the older `AX=E801h` and `AH=88h` interfaces.

`.16h.`

Implemented by the BIOS or operating system. Provides routines to be called by software programs which communicate with the keyboard.

`.AH.` | Description
--- | ---
`.00h.` | Read Character
`.01h.` | Read Input Status
`.02h.` | Read Keyboard Shift Status
`.10h.` | Read Character Extended
`.11h.` | Read Input Status Extended
`.12h.` | Read Keyboard Shift Status Extended

`.17h.`

Print Services - used by software programs to communicate with the printer

`.AH.` | Description
--- | ---
`.00h.` | Print Character to Printer
`.01h.` | Initialize Printer
`.02h.` | Check Printer Status

`.18h.` | Execute Cassette BASIC: True IBM computers contain BASIC in the ROM to be interpreted and executed by this routine in the event of a boot failure (called by the BIOS)
`.19h.` | After POST this interrupt is used by BIOS to load the operating system.

`.1Ah.`

Real Time Clock Services - called by software programs to communicate with the RTC

`.AH.` | Description
--- | ---
`.00h.` | Read RTC
`.01h.` | Set RTC
`.02h.` | Read RTC Time
`.03h.` | Set RTC Time
`.04h.` | Read RTC Date
`.05h.` | Set RTC Date
`.06h.` | Set RTC Alarm
`.07h.` | Reset RTC Alarm

`.1Bh.` | Installed by the operating system; automatically called by `.INT 9.` when Ctrl-Break has been pressed
`.1Ch.` | Called automatically by `.INT 08.`; available for use by software programs when a routine needs to be executed regularly
`.1Dh.` | Not to be called; simply a pointer to the VPT (Video Parameter Table), which contains data on video modes
`.1Eh.` | Not to be called; simply a pointer to the DPT (Diskette Parameter Table), containing a variety of information concerning the diskette drives
`.1Fh.` | Not to be called; simply a pointer to the VGCT (Video Graphics Character Table), which contains the data for ASCII characters `.80h.` to `.FFh.`
`.41h.` | Address pointer: FDPT = Fixed Disk Parameter Table (1st hard drive)
`.46h.` | Address pointer: FDPT = Fixed Disk Parameter Table (2nd hard drive)
`.4Ah.` | Called by RTC for alarm
`.70h.` | IRQ8: Called by RTC
`.74h.` | IRQ12: Called by mouse
`.75h.` | IRQ13: Called by math coprocessor
`.76h.` | IRQ14: Called by primary IDE controller
`.77h.` | IRQ15: Called by secondary IDE controller

## DOS hooks

On MS-DOS systems IO.SYS hooks INT 13 for floppy disc change detection, tracking formatting calls, correcting DMA boundary errors, working around problems in IBM's ROM BIOS "01/10/84" with model code 0xFC before the first call. The interrupt vector 0x13 may point to a software hook rather than the BIOS routine, which some Bulgarian viruses are known to use against virus monitoring software. [1]

## `.INT 18h.`: execute BASIC

`.INT 18h.` traditionally jumped to an implementation of BASIC stored in ROM. This call would typically be invoked if the BIOS was unable to identify any bootable volumes on startup. (At the time the original IBM PC was released in 1981, the BASIC in ROM was a key feature.) 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 (famously, "No ROM BASIC", or more explanatory messages in later BIOS versions); in other BIOS versions it would prompt the user to insert a bootable volume and press a key, and then after the user did so it would loop back to the bootstrap loader to try booting again.

## See also

- DOS interrupt call
- Interrupt descriptor table
- Input/Output Base Address
- Ralf Brown's Interrupt List

## References

1. ^ "empty". http://www2.informatik.uni-halle.de/lehre/asm_pc/interrupt.d.txt. 090912 www2.informatik.uni-halle.de

## External links

- The x86 Interrupt List (a.k.a. RBIL, Ralf Brown's Interrupt List)
- Embedded BIOS User’s Manual
- http://hdebruijn.soo.dto.tudelft.nl/newpage/interupt/out-0100.htm
- Turbo Pascal examples for reading sectors
- HTML version of Ralf Brown Interrupt List

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