INT 13

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INT 13 is shorthand for interrupt 0x13, the 20th interrupt vector in an x86 based computer system (interrupts start at zero, and are labeled with hexadecimal values). INT is an x86 instruction that triggers a software interrupt, and 0x13 is the vector passed to the instruction. The BIOS typically sets up a real mode interrupt handler at this vector that provides sector based hard disk and floppy disk read and write services using Cylinder Head Sector (CHS) addressing.

Contents 1 Overview 2 List of INT 13h services 2.1 Drive Table 2.2 Function Table 2.3 INT 13h AH=00h: Reset Disk Drives 2.4 INT 13h AH=01h: Check Drive Status 2.5 INT 13h AH=02h: Read Sectors From Drive 2.6 INT 13h AH=08h: Read Drive Parameters 2.7 INT 13h AH=0Ah: Read Long Sectors From Drive 2.8 INT 13h AH=41h: Check Extensions Present 2.9 INT 13h AH=42h: Extended Read Sectors From Drive 2.10 INT 13h AH=48h: Extended Read Drive Parameters 3 See also 4 External links

Overview

Under real mode operating systems, such as MS-DOS, calling INT 0x13 would jump into the computer's BIOS code for Low Level Disk Services, which will carry out sector-based disk read or write for the program. In MS-DOS it serves as a foundation for higher-level MS-DOS API (INT 21h) functions which deal with file system access.

Under protected mode operating systems, such as Microsoft Windows NT derivatives (e.g. NT4, 2000, XP, and 2003 Server) or Linux with dosemu, the O/S intercepts the call and passes it to the operating system's native disk I/O mechanism. Windows 9x and Windows for Workgroups 3.11 also bypass BIOS routines when using 32-bit File Access.

The original BIOS real-mode INT 0x13 interface supports drives of sizes up to about 504 MB using what is commonly referred to as Physical CHS addressing. This limit originates from the hardware interface of the x86 disk hardware. The BIOS used the cylinder-head-sector (CHS) address given in the INT 0x13 call, and transmitted it directly to the hardware interface.

This interface was later extended to support addressing of up to exactly 8064 MB using what is commonly referred to as Logical CHS addressing. This limit originates from a combination of the register value based calling convention used in the INT 0x13 interface, and the goal of maintaining backward compatibility. There were originally a number of BIOS that offered incompatible versions of this interface, but eventually the computer industry standardized on the interface developed in the //strike//Award BIOS//replace// Microid Research ("MR BIOS") in 1989. This limit uses 1024 cylinders, 256 heads, 63 sectors, and 512 byte blocks, giving roughly 7.875 GB of addressing (1024 * 256 * 63 * 512).

To support even larger addressing modes, an interface known as INT13h Extensions was introduced by Western Digital and Phoenix Technologies as part of BIOS Enhanced Disk Drive Services (EDD) standard. It uses 64-bit logical block addressing (LBA) which allows addressing up to 8 ZB (the drive can also support 32-bit or 48-bit LBA which allows up to 2 TB or 128 PB respectively). This is a packet interface, because it uses a pointer to a packet of information rather than the register based calling convention of the original INT 13 interface. This packet is a data structure that contains an interface version, data size, and LBAs.

List of INT 13h 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 GB.

INT 13h AH=00h: Reset Disk Drives

Parameters:

AH	00h
DL	Drive Index (e.g. 1st HDD = 80h)

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 06h Disk change line 'active' 07h Drive parameter activity failed 08h DMA overrun 09h Attempt to DMA over 64kb boundary 0Ah Bad sector detected 0Bh Bad cylinder (track) detected 0Ch Media type not found 0Dh Invalid number of sectors 0Eh Control data address mark detected 0Fh DMA out of range 10h CRC/ECC data error 11h ECC corrected data error 20h Controller failure 40h Seek failure 80h Drive not ready BBh Undefined error CCh Write fault E0h Status error FFh Sense operation failed

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

Parameters:

AH	02h
AL	Sectors To Read Count
CH	Track
CL	Sector
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 not be a 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 GB 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.

See also

• INT 10
• BIOS interrupt call
• Cylinder-head-sector
• INT (x86 instruction)
• DPMI (DOS Protected Mode Interface)

External links

• BIOS Interrupt 13h Extensions
• BIOS Interrupt 13h Extensions
• History of BIOS and IDE limits
• Ralf Brown's comprehensive Interrupt List
• Norton Guide about int 13h, ah = 00h .. 1ah