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 BIOS's facilities. Some operating systems also use the BIOS to probe and initialise hardware resources during their early stages of booting.

Contents

[edit] 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 Print Screen
INT 08h 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 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 Equipment Installed
INT 12h Get Conventional Memory Size
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=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 Serial I/O
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 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
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 IBM 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
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 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 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

[edit] INT 13h: Low Level Disk Services

[edit] 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

[edit] 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.

[edit] INT 13h AH=00h: Reset Disk Drives

Parameters:

AH 00h
DL Drive Index

[edit] 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

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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)

[edit] 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

[edit] 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.

[edit] 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.

[edit] See also

[edit] External links

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