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Encyclopedia > BIOS interrupt call

BIOS Interrupt Calls are a facility that MS-DOS programs, and some other software such as boot loaders, use to invoke the BIOS's facilities. Some operating systems also use the BIOS to probe and initialise hardware resources during their early stages of booting. Microsofts disk operating system, MS-DOS, was Microsofts implementation of DOS, which was the first popular operating system for the IBM PC, and until recently, was widely used on the PC compatible platform. ... In computing, booting is a bootstrapping process that starts operating systems when the user turns on a computer system. ... This article is about the software. ... In computing, an operating system (OS) is the system software responsible for the direct control and management of hardware and basic system operations. ...

Contents


Interrupt Table

Interrupt Description
INT 00h CPU: Division By Zero
INT 01h CPU: Single Step for debugging
INT 02h CPU: NMI, used e.g. by POST for memory errors
INT 03h CPU: Breakpoint for debugging
INT 04h CPU: Numeric Overflow
INT 05h CPU: Print Screen
INT 08h hardware interrupt request IRQ0: called by system timer 18.2 times per second
INT 09h IRQ1: called by keyboard
INT 0Bh IRQ3: called by 2nd serial port COM2
INT 0Ch IRQ4: called by 1st serial port COM1
INT 0Dh IRQ5: called by hard disk controller (PC/XT) or 2nd parallel port LPT2 (AT)
INT 0Eh IRQ6: called by floppy disk controller
INT 0Fh IRQ7: called by 1st parallel port LPT1 (printer)
INT 10h Video Services
AH=00h Set Video Mode
AH=01h Set Cursor Shape
AH=02h Position Cursor
AH=03h Get Cursor Position And Shape
AH=04h Get Light Pen Position
AH=05h Set Display Page
AH=06h Clear/Scroll Up Screen
AH=07h Clear/Scroll Down Screen
AH=08h Read Character From Screen
AH=09h Write Character(s) To Screen
AH=0Ah Write Character To Screen With Existing Attribute
AH=0Bh Set Border Color
AH=0Eh Write Character To Screen With Existing Attribute And Reposition
AH=0Fh Get Video Mode
INT 11h Equipment Installed
INT 12h Memory Available
INT 13h Low Level Disk Services
AH=00h Reset Disk Drives
AH=01h Check Drive Status
AH=02h Read Sectors From Drive
AH=03h Write Sectors To Drive
AH=04h Verifies Sectors On Drive
AH=0Ch Moves Disk Head To Specified Track
AH=0Dh Resets Hard Disk Controller
INT 14h Serial I/O
AH=00h Serial Port Initialization
AH=01h Transmit Character
AH=02h Receive Character
AH=03h Status
INT 15h Miscellaneous
INT 16h Keyboard
AH=00h Input Key From Keyboard
AH=01h Check For Input Availability
AH=02h Check Keyboard Shift Status
INT 17h Print Services
AH=00h Print Character To Printer
AH=01h Initialize Printer
AH=02h Check Printer Status
INT 18h This interrupt will be called if INT 19h fails, in early BIOS version then ROM Basic was loaded
INT 19h After POST this interrupt is used by BIOS to load the operating system
INT 1Ah Real Time Clock Services
AH=00h Read RTC
AH=01h Set RTC
INT 1Bh Called by break key
INT 1Ch Software Timer Interrupt, called by INT 08h
INT 1Dh address pointer: VPT = Video Parameter Table
INT 1Eh address pointer: DPT = Diskette Parameter Table
INT 1Fh address pointer: VGCT = Video Graphics Character Table
INT 21h MS-DOS Functions
AH=00h Exit To Shell
AH=01h Read Input From Keyboard
AH=02h Write Character To Output
INT 41h address pointer: FDPT = Fixed Disk Paramter Table (1st hard drive)
INT 46h address pointer: FDPT = Fixed Disk Paramter Table (2nd hard drive)

A Non-Maskable Interrupt (NMI) is a special type of interrupt that can not be ignored by standard interrupt masking techniques. ... Look up post in Wiktionary, the free dictionary. ... Microsofts disk operating system, MS-DOS, was Microsofts implementation of DOS, which was the first popular operating system for the IBM PC, and until recently, was widely used on the PC compatible platform. ...

INT 13h: Low Level Disk Services

Drive Table

DH = 00h 1st floppy disk ( "drive A:" )
DH = 01h 2nd floppy disk ( "drive B:" )
DH = 80h 1st hard disk
DH = 81h 2nd hard disk

Function Table

AH = 00h   Reset Disk Drives
AH = 01h   Check Drive Status
AH = 02h   Read Sectors From Drive
AH = 03h   Write Sectors To Drive
AH = 04h   Verify Sectors
AH = 05h   Format Track
AH = 08h   Read Drive Parameters
AH = 09h HD Initialize Disk Controller
AH = 0Ah HD Read Long Sectors From Drive
AH = 0Bh HD Write Long Sectors To Drive
AH = 0Ch HD Move Drive Head To Cylinder
AH = 0Dh HD Reset Disk Drives
AH = 0Eh PS/2 Controller Read Test
AH = 0Fh PS/2 Controller Write Test
AH = 10h HD Test Whether Drive Is Ready
AH = 11h HD Recalibrate Drive
AH = 12h PS/2 Controller RAM Test
AH = 13h PS/2 Drive Test
AH = 14h HD Controller Diagnostic
AH = 15h   Read Drive Type
AH = 16h FD Detect Media Change
AH = 17h FD Set Media Type For Format ( used by DOS versions <= 3.1 )
AH = 18h FD Set Media Type For Format ( used by DOS versions >= 3.2 )
AH = 41h EXT Test Whether Extensions Are Available
AH = 42h EXT Read Sectors From Drive
AH = 43h EXT Write Sectors To Drive
AH = 44h EXT Verify Sectors
AH = 45h EXT Lock/Unlock Drive
AH = 46h EXT Eject Drive
AH = 47h EXT Move Drive Head To Sector
AH = 48h EXT Read Drive Parameters
AH = 49h EXT Detect Media Change

Second column is empty == function may be used both for floppy and hard disk.
"FD" == for floppy disk only.
"HD" == for hard disk only.
"PS/2" == for hard disk on PS/2 system only.
"EXT" == part of the Int 13h Extensions which were written in the 1990s to support hard drives with more than 8 GBytes.


INT 13h AH=00h: Reset Disk Drives

Parameters:

AH 00h
DL Drive Index

INT 13h AH=01h: Check Drive Status

Parameters:

AH 01h

Results:

AL Return Code
00h Success
01h Invalid Command
02h Cannot Find Address Mark
03h Attempted Write On Write Protected Disk
04h Sector Not Found
05h Reset Failed

INT 13h AH=02h: Read Sectors From Drive

Parameters:

AH 02h
AL Sectors To Read Count
CX Track + Sector / See remark
DH Head
DL Drive
ES:BX Buffer Address Pointer

Results:

CF Set On Error, Clear If No Error
AH Return Code
AL Actual Sectors Read Count

Remarks:
Register CX contains both the cylinder number (10 bits, possible values are 0 to 1023) and the sector number (6 bits, possible values are 1 to 63):

 CX = ---CH--- ---CL--- cylinder : 76543210 98 sector : 543210

Examples of translation:

 Turbo Pascal: CX := ( ( cylinder and 255 ) shl 8 ) or ( ( cylinder and 768 ) shr 2 ) or sector; cylinder := hi ( CX ) or ( ( lo ( CX ) and 192 ) shl 2 ); sector := CX and 63;

Addressing of Buffer should guarantee that the complete buffer is inside the given segment, i.e. ( BX + size_of_buffer ) <= 10000h. Otherwise the interrupt may fail with some BIOS or hardware versions.
Example: Assume you want to read 16 sectors (= 2000h Bytes) and your buffer starts at memory address 4FF00h. There are different ways to calculate the register values, e.g.:

 ES = segment = 4F00h BX = offset = 0F00h sum = memory address = 4FF00h would be a good choice because 0F00h + 2000h = 2F00h <= 10000h ES = segment = 4000h BX = offset = FF00h sum = memory address = 4FF00h would be no good choice because FF00h + 2000h = 11F00h > 10000h

Function 02h of interrupt 13h may only read sectors of the first 16,450,560 sectors of your hard drive, to read sectors beyond the 8 GByte limit you should use function 42h of Int 13h Extensions. Another alternate may be DOS interrupt 25h which reads sectors within a partition.


INT 13h AH=08h: Read Drive Parameters

Parameters:

Registers
AH 08h = function number for read_drive_parameters
DL drive index (e.g. 1st HDD = 80h)


Results:

CF Set On Error, Clear If No Error
AH Return Code
DL number of hard disk drives
DH logical last index of heads = number_of - 1 (because index starts with 0)
CX logical last index of cylinders = number_of - 1 (because index starts with 0)
logical last index of sectors per track = number_of (because index starts with 1)

Remarks:
Logical values of function 08h may/should differ from physical CHS values of function 48h.
Result register CX contains both cylinders and sector/track values, see remark of function 02h.


INT 13h AH=0Ah: Read Long Sectors From Drive

The only difference between this function and function 02h (see above) is that function 0Ah reads 516 bytes per sector instead of only 512. The last 4 bytes contains the Error Correction Code ECC, a checksum of sector data.


INT 13h AH=42h: Extended Read Sectors From Drive

Parameters:

Registers
AH 42h = function number for extended read
DL drive index (e.g. 1st HDD = 80h)
DS:SI segment:offset pointer to the DAP, see below
DAP : Disk Address Packet
offset range size description
00h 1 byte size of DAP = 16 = 10h
01h 1 byte unused, should be zero
02h 1 byte number of sectors to be readed, 0..127 (= 7Fh)
03h 1 byte unused, should be zero
04h..07h 4 bytes segment:offset pointer to the memory buffer to which sectors will be transfered
08h..0Fh 8 bytes absolute number of the start of the sectors to be read (1st sector of drive has number 0)

Results:

CF Set On Error, Clear If No Error
AH Return Code

INT 13h AH=48h: Extended Read Drive Parameters

Parameters:

Registers
AH 48h = function number for extended_read_drive_parameters
DL drive index (e.g. 1st HDD = 80h)
DS:SI segment:offset pointer to Result Buffer, see below
Result Buffer
offset range size description
00h..01h 2 bytes size of Result Buffer = 30 = 1Eh
02h..03h 2 bytes information flags
04h..07h 4 bytes physical number of cylinders = last index + 1 (because index starts with 0)
08h..0Bh 4 bytes physical number of heads = last index + 1 (because index starts with 0)
0Ch..0Fh 4 bytes physical number of sectors per track = last index (because index starts with 1)
10h..17h 8 bytes absolute number of sectors = last index + 1 (because index starts with 0)
18h..19h 2 bytes bytes per sector
1Ah..1Dh 4 bytes optional pointer to Enhanced Disk Drive (EDD) configuration parameters which may be used for subsequent interrupt 13h Extension calls (if supported)


Results:

CF Set On Error, Clear If No Error
AH Return Code

Remark: Physical CHS values of function 48h may/should differ from logical values of function 08h.


INT 18h: Execute BASIC

Description:


This interrupt 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. As time went on and BASIC was no longer shipped on all PCs, this interrupt would simply prompt the user to insert a bootable volume and press a key. BASIC (Beginners All-purpose Symbolic Instruction Code) is a family of high-level programming languages. ...


INT 21h: MS-DOS Functions

Microsofts disk operating system, MS-DOS, was Microsofts implementation of DOS, which was the first popular operating system for the IBM PC, and until recently, was widely used on the PC compatible platform. ...

INT 21h AH=00h: Exit To Shell

Parameters:

AH 00h

INT 21h AH=01h: Read Input From Keyboard

Parameters:

AH 01h

Results:

AL ASCII Value Of Character Read

INT 21h AH=02h: Write Character To Output

Parameters:

AH 02h
DL Character To Output

See also

In computer science, an interrupt is an asynchronous signal from hardware or software indicating the need for attention. ... Base address is also called I/O port, I/O address, I/O port address and base port. ...

External links


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