[edit]Overview
Under real modeoperating 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 INT 21functions which deal with file systemaccess.
Newer protected modeoperating systems such as Microsoft Windows NTderivatives (e.g. NT4, 2000 XP and 2003 Server) or Linuxwith dosemu, will intercept the call and pass it to the operating system's native disk I/O mechanism. Windows 9xand Windows for Workgroups 3.11could 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 x86disk hardware. The BIOS used the 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 Award BIOS. 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 Extensionswas introduced by Western Digitaland Phoenix Technologiesas 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 TBor 128 PBrespectively). This is a packet interface, because it uses a pointer to a packetof 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.
[edit]List of INT 13h 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 GB.
[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 CXcontains 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 nogood 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 withina 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
