BIOS interrupt call

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.

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

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