yawiki.org entry for BIOS interrupt call

	Navigation Home Recent Most Active Popular Blog Credits RSS
	Interaction Register Statistics
	Help Suggestions Contact Us How to Edit Help

[Edit]

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.

top

Interrupt Table

top

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

top

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.

top

INT 13h AH00h: Reset Disk Drives
Parameters:

top

INT 13h AH01h: Check Drive Status
Parameters:

Results:

top

INT 13h AH02h: Read Sectors From Drive
Parameters:

Results:

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.

top

INT 13h AH08h: 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.

top

INT 13h AH0Ah: 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.

top

INT 13h AH41h: 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)

top

INT 13h AH42h: 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

top

INT 13h AH48h: 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.

top

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.

top

	Source Privacy License Download Contact Us Atlas Scientus.org Dictionary (Yet Another Wiki) RC : 1.39 MIT OpenCourseWare This article is licensed under the GNU Free Documentation License [copyleft]. It uses material from the Wikipedia article "BIOS interrupt call". link