BIOS interrupt call

 

BIOS Interrupt Calls are a facility that 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.

Interrupt Table

Interrupt	Description
INT 00h	CPU: Executed after an attempt to divide by zero or when the quotient does not fit in the destination
INT 01h	CPU: Executed after every instruction while the trace flag is set
INT 02h	CPU: NMI, used e.g. by POST for memory errors
INT 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
INT 04h	CPU: Numeric Overflow. Usually caused by the INTO instruction when the overflow flag is set.
INT 05h	Executed when Shift-PrintScreen is pressed, as well as when the BOUND instruction detects a bound failure.
INT 06h	CPU: Called when the Undefined Opcode (invalid instruction) exception occurs. Usually installed by the operating system.
INT 07h	CPU: Called when an attempt was made to execute a floating-point instruction and no numeric coprocessor was available.
INT 08h	IRQ0: Implemented by the system timing component; called 18.2 times per second (once every 55 ms) by the PIC
INT 09h	IRQ1: Called after every key press and release (as well as during the time when a key is being held)
INT 0Bh	IRQ3: Called by serial ports 2 and 4 (COM2/4) when in need of attention
INT 0Ch	IRQ4: Called by serial ports 1 and 3 (COM1/3) when in need of attention
INT 0Dh	IRQ5: Called by hard disk controller (PC/XT) or 2nd parallel port LPT2 (AT) when in need of attention
INT 0Eh	IRQ6: Called by floppy disk controller when in need of attention
INT 0Fh	IRQ7: Called by 1st parallel port LPT1 (printer) when in need of attention
INT 10h	Video Services - installed by the BIOS or operating system; called by software programs AH=00h Set Video Mode AH=01h Set Cursor Shape AH=02h Set Cursor Position AH=03h Get Cursor Position And Shape AH=04h Get Light Pen Position AH=05h Set Display Page AH=06h Clear/Scroll Screen Up AH=07h Clear/Scroll Screen Down AH=08h Read Character and Attribute at Cursor AH=09h Write Character and Attribute at Cursor AH=0Ah Write Character at Cursor AH=0Bh Set Border Color AH=0Eh Write Character in TTY Mode AH=0Fh Get Video Mode AH=13h Write String
INT 11h	Installed by the BIOS; returns equipment list
INT 12h	Installed by the BIOS or operating system; returns Conventional Memory Size
INT 13h	Low Level Disk Services; installed by the BIOS or operating system; called by software programs 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=05h Format Track On Drive AH=08h Get Drive Parameters AH=09h Init Fixed Drive Parameters AH=0Ch Seek To Specified Track AH=0Dh Reset Fixed Disk Controller AH=15h Get Drive Type AH=16h Get Floppy Drive Media Change Status
INT 14h	Routines for communicating via the serial port. Used by software programs. AH=00h Serial Port Initialization AH=01h Transmit Character AH=02h Receive Character AH=03h Status
INT 15h	Miscellaneous AH=4FH Keyboard Intercept AH=83H Event Wait AH=84H Read Joystick AH=85H Sysreq Key Callout AH=86H Wait AH=87H Move Block AH=88H Get Extended Memory Size AH=C0H Get System Parameters AH=C1H Get Extended BIOS Data Area Segment AH=C2H Pointing Device Functions
INT 16h	Implemented by the BIOS or operating system. Provides routines to be called by software programs which communicate with the keyboard. AH=00h Read Character AH=01h Read Input Status AH=02h Read Keyboard Shift Status AH=10h Read Character Extended AH=11h Read Input Status Extended AH=12h Read Keyboard Shift Status Extended
INT 17h	Print Services - used by software programs to communicate with the printer AH=00h Print Character to Printer AH=01h Initialize Printer AH=02h Check Printer Status
INT 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)
INT 19h	After POST this interrupt is used by BIOS to load the operating system.
INT 1Ah	Real Time Clock Services - called by software programs to communicate with the RTC AH=00h Read RTC AH=01h Set RTC AH=02h Read RTC Time AH=03h Set RTC Time AH=04h Read RTC Date AH=05h Set RTC Date AH=06h Set RTC Alarm AH=07h Reset RTC Alarm
INT 1Bh	Installed by the operating system; automatically called by INT 9 when Ctrl-Break has been pressed
INT 1Ch	Called automatically by INT 08; available for use by software programs when a routine needs to be executed regularly
INT 1Dh	Not to be called; simply a pointer to the VPT (Video Parameter Table), which contains data on video modes
INT 1Eh	Not to be called; simply a pointer to the DPT (Diskette Parameter Table), containing a variety of information concerning the diskette drives
INT 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
INT 41h	Address pointer: FDPT = Fixed Disk Paramter Table (1st hard drive)
INT 46h	Address pointer: FDPT = Fixed Disk Paramter Table (2nd hard drive)
INT 4Ah	Called by RTC for alarm
INT 70h	IRQ8: Called by RTC
INT 74h	IRQ12: Called by mouse
INT 75h	IRQ13: Called by math coprocessor
INT 76h	IRQ14: Called by primary IDE controller
INT 77h	IRQ15: Called by secondary IDE controller

INT 13h: Low Level Disk Services

Drive Table

DL = 00h	1st floppy disk ( "drive A:" )
DL = 01h	2nd floppy disk ( "drive B:" )
DL = 80h	1st hard disk
DL = 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=41h: Check Extensions Present

Parameters:

Registers
AH	41h = function number for extensions check
DL	drive index (e.g. 1st HDD = 80h)
BX	55AAh

Results:

CF	Set On Not Present, Clear If Present
AH	Error Code or Major Version Number
BX	AA55h
CX	Interface support bitmask: 1 - Device Access using the packet structure 2 - Drive Locking and Ejecting 4 - Enhanced Disk Drive Support (EDD)

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 read, 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 transferred
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. (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 self-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

• Interrupt
• INT 13
• Input/Output Base Address

External links

• Embedded BIOS User’s Manual
• http://www.missl.cs.umd.edu/winint/index1.html
• 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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