INT 13H

Not to be confused with Mode 13h.

INT 13h is shorthand for BIOS interrupt call 13hex, the 20th interrupt vector in an x86-based computer system. 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.

INT is an x86 instruction that triggers a software interrupt, and 13hex is the interrupt number (as a hexadecimal value) being called.

Overview

Under real mode operating systems, such as MS-DOS, calling INT 13h would jump into the computer's ROM-BIOS code for low-level disk services, which would carry out physical sector-based disk read or write operations for the program. In MS-DOS, it serves as the low-level interface for the built-in block device drivers for hard disks and floppy disks. This allows INT 25h and INT 26h to provide absolute disk read/write functions for logical sectors to the FAT file system driver in the DOS kernel, which handles file-related requests through MS-DOS API (INT 21h) functions.

Under protected mode operating systems, such as Microsoft Windows NT derivatives (e.g. NT4, 2000, XP, and Server 2003) and Linux with dosemu, the OS 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 13h 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 IBM PC/XT disk hardware. The BIOS used the cylinder-head-sector (CHS) address given in the INT 13h call, and transferred 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 13h interface, and the goal of maintaining backward compatibility. There were originally a number of BIOSes that offered incompatible versions of this interface, but eventually the computer industry standardized on the interface developed in the Microid Research ("MR BIOS") in 1989. This limit uses 1024 cylinders, 256 heads, 63 sectors, and 512 byte blocks, allowing exactly 7.875 GiB of addressing (1024 * 256 * 63 * 512 bytes).

To support even larger addressing modes, an interface known as INT 13h Extensions was introduced by Western Digital and Phoenix Technologies as part of BIOS Enhanced Disk Drive Services (EDD).[1] It uses 64-bit logical block addressing (LBA) which allows addressing up to 8 ZiB (the drive can also support 28-bit or 48-bit LBA which allows up to 128 GiB or 128 PiB respectively, assuming a 512-byte sector/block size). 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 13h interface. This packet is a data structure that contains an interface version, data size, and LBAs.

All versions of MS-DOS, including MS-DOS 7, and Windows 95 contain a bug that prevents booting hard disks with 256 heads (register value 0xFF),[2] so many modern BIOSes map the drives to have at most 255 heads, reducing the total addressable space to exactly 8032.5 MiB (approx 7.844 GiB). Also some cache drivers will flush their buffers when detecting that MS-DOS is bypassed by applications that directly issue INT 13h requests. AMI BIOSes from around 1990-1991 trash word unaligned buffers. Some MS-DOS and TSR programs clobber interrupt enabling and registers so PC DOS and MS-DOS install their own filters to prevent this.[3]

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   Get Status of Last Drive Operation
AH = 02h   Read Sectors From Drive
AH = 03h   Write Sectors To Drive
AH = 04h   Verify Sectors
AH = 05h   Format Track
AH = 06h   Format Track Set Bad Sector Flags
AH = 07h   Format Drive starting at 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 = 19h   Park Heads
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

If the second column is empty then the function may be used both for floppy and hard disk.

INT 13h AH=00h: Reset Disk Drive

Parameters:

AH 00h
DL Drive

Results:

CF Set on error

INT 13h AH=01h: Get Status of Last Drive Operation

Parameters:

AH 01h
DL Drive

Bit 7=0 for floppy drive, bit 7=1 for fixed drive

Results:

AH 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 timed out, assumed not ready
AAh Drive not ready
BBh Undefined error
CCh Write fault
E0h Status error
FFh Sense operation failed
CF Set On Error, Clear If No Error

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

Parameters:

AH 02h
AL Sectors To Read Count
CH Cylinder
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). Cylinder and Sector bits are numbered below:

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

Examples of translation:

CX := ( ( cylinder and 255 ) shl 8 ) or ( ( cylinder and 768 ) shr 2 ) or sector;
cylinder := ( (CX and 0xFF00) shr 8 ) or ( (CX and 0xC0) 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. Utilizing memory segmentation, 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=03h: Write Sectors To Drive

Parameters:

AH 03h
AL Sectors To Write 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 Written Count

INT 13h AH=04h: Verify Sectors From Drive

Parameters:

AH 04h
AL Sectors To Verify 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 Verified Count

INT 13h AH=05h: Format Track

Parameters:

AH 05h
AL Sectors To Format Count
CH Track
CL Sector
DH Head
DL Drive
ES:BX Buffer Address Pointer

4-byte address field (applies to PC/XT 286,AT, PS/1 and PS/2)

Byte Meaning Allowable Values
1 Track
2 Head
3 Sector
4 Bytes/Sector 0=128, 1-256, 2-512, 3-1024

Results:

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

INT 13h AH=06h: Format Track Set Bad Sector Flags

Parameters:

AH 06h
AL Interleave
CH Track
CL Sector
DH Head
DL Drive

Results:

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

INT 13h AH=07h: Format Drive Starting at Track

Parameters:

AH 07h
AL Interleave
CH Track
CL Sector
DH Head
DL Drive

Results:

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

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)
ES:DI[4] set to 0000h:0000h to work around some buggy BIOS


Results:

CF Set On Error, Clear If No Error
AH Return Code
DL number of hard disk drives
DH[4] logical last index of heads = number_of - 1 (because index starts with 0)
CX [7:6] [15:8][4] logical last index of cylinders = number_of - 1 (because index starts with 0)

[5:0][4] logical last index of sectors per track = number_of (because index starts with 1)

BL[4] drive type (only AT/PS2 floppies)
ES:DI[4] pointer to drive parameter table (only for floppies)

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=09h: Init Drive Pair Characteristics

Parameters:

AH 09h
DL Drive

Results:

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

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[5]
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..03h 2 bytes number of sectors to be read, (some Phoenix BIOSes are limited to a maximum of 127 sectors)
04h..07h 4 bytes segment:offset pointer to the memory buffer to which sectors will be transferred (note that x86 is little-endian: if declaring the segment and offset separately, the offset must be declared before the segment)
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=43h: Extended Write Sectors to Drive

Parameters:

Registers
AH 43h = function number for extended write
AL bit 0 = 0: close write check,

bit 0 = 1: open write check,

bit 1-7:reserved, set to 0

DL drive index (e.g. 1st HDD = 80h)
DS:SI segment:offset pointer to the DAP

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

References

  1. ↑
  2. ↑ Disk size limitations
  3. ↑ ctyme.com - DISK - READ SECTOR(S) INTO MEMORY
  4. 1 2 3 4 5 6 ctyme.com - DISK - GET DRIVE PARAMETERS (PC,XT286,CONV,PS,ESDI,SCSI)
  5. ↑ ctyme.com - IBM/MS INT 13 Extensions - INSTALLATION CHECK

External links

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