BIOS interrupt call

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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 on IBM PC compatible computers. Some operating systems also use the BIOS to probe and initialize hardware resources during their early stages of booting.

In all computers, software instructions control the physical hardware (screen, disk, keyboard, etc.) from the moment the power switch is pressed.

The BIOS, preloaded in the mainboard, takes control as the machine is turned on, initializes the hardware, loads and runs the OS loader, and provides basic hardware control to the operating system installed on the machine.

Operating systems communicate with the BIOS software, in order to control the installed hardware. This system of communication is called an interrupt.

The basic idea is, BIOS and the operating system share an agreed-upon area of the computer's memory, to pass messages. The messages are broken into categories, each with its own "interrupt number", and contains sub-categories ("function numbers"). In practice, the operating system software "calls" to BIOS software by specifying an interrupt and function number, followed with whatever data needs to go in or out of the attached hardware.

The BIOS software "responds" with an error code if not successful, or with whatever data was requested if successful. The data itself can be as small as one bit or as large as an entire raw disk sector, being read or written (thousands of bytes).

Many modern operating systems (such as newer versions of Windows and Linux) bypass the built-in BIOS interrupt communication system altogether, preferring to use their own software to control the attached hardware directly.

Invoking an interrupt[edit]

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[edit]

A list of common BIOS interrupt classes can be found below. Note that some BIOSes (particularly old ones) do not implement all of these interrupt classes.

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 programmable interval timer
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
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
10h Set Palette Registers (EGA, VGA, SVGA)
11h Character Generator (EGA, VGA, SVGA)
12h Alternate Select Functions (EGA, VGA, SVGA)
13h Write String
1Ah Get or Set Display Combination Code (VGA, SVGA)
1Bh Get Functionality Information (VGA, SVGA)
1Ch Save or Restore Video State (VGA, SVGA)
4Fh VESA BIOS Extension Functions (SVGA)
11h Returns equipment list
12h Return conventional memory size
13h Low Level Disk Services
AH Description
00h Reset Disk Drives
01h Check Drive Status
02h Read Sectors
03h Write Sectors
04h Verify Sectors
05h Format Track
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
17h Set Disk Type
18h Set Floppy Drive Media Type
41h Extended Disk Drive (EDD) Installation Check
42h Extended Read Sectors
43h Extended Write Sectors
44h Extended Verify Sectors
45h Lock/Unlock Drive
46h Eject Media
47h Extended Seek
48h Extended Get Drive Parameters
49h Extended Get Media Change Status
4Eh Extended Set Hardware Configuration
14h Serial port services
AH Description
00h Serial Port Initialization
01h Transmit Character
02h Receive Character
03h Status
15h Miscellaneous system services
AH AL 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
89h Switch to Protected Mode
C0h Get System Parameters
C1h Get Extended BIOS Data Area Segment
C2h Pointing Device Functions
C3h Watchdog Timer Functions - PS/2 systems only
C4h Programmable Option Select - MCA bus PS/2 systems only
D8h EISA System Functions - EISA bus systems only
E8h 01h Get Extended Memory Size (Newer function, since 1994). Gives results for memory size above 64 Mb.
E8h 20h Query System Address Map. The information returned from E820 supersedes what is returned from the older AX=E801h and AH=88h interfaces.
16h Keyboard services
AH Description
00h Read Character
01h Read Input Status
02h Read Keyboard Shift Status
05h Store Keystroke in Keyboard Buffer
10h Read Character Extended
11h Read Input Status Extended
12h Read Keyboard Shift Status Extended
17h Printer services
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
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
1Ah PCI Services - implemented by BIOSes supporting PCI 2.0 or later
AX Description
B101h PCI Installation Check
B102h Find PCI Device
B103h Find PCI Class Code
B106h PCI Bus-Specific Operations
B108h Read Configuration Byte
B109h Read Configuration Word
B10Ah Read Configuration Dword
B10Bh Write Configuration Byte
B10Ch Write Configuration Word
B10Dh Write Configuration Dword
B10Eh Get IRQ Routine Information
B10Fh Set PCI IRQ
1Bh Ctrl-Break handler - called by INT 09 when Ctrl-Break has been pressed
1Ch Timer tick handler - called by INT 08
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[edit]

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.

INT 18h: execute BASIC[edit]

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[edit]

References[edit]