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INT 13H
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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 vector passed
to the instruction (interrupts start at zero, and are labeled with hexadecimal values).
[hide] |
[edit]Overview
See also: EnhancedBIOS
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, giving roughly 7.875 GB of addressing (1024 * 256 * 63 * 512).
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.[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 BIOS maps the drives to have at most 255 heads. Also some cache
drivers will flush their buffers when detecting that MS-DOS is bypassed by applications that directly issues 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]
[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 | 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 |
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 Drive
Parameters:AH | 00h |
DL | Drive |
CF | Set on error |
[edit]INT
13h AH=01h: Get Status of Last Drive Operation
Parameters:AH | 01h |
DL | Drive$ |
Results:
AL | Return Code
| ||||||||||||||||||||||||||||||||||||||||||||||||||||
CF | Set On Error, Clear If No Error |
[edit]INT
13h AH=02h: Read Sectors From Drive
Parameters:AH | 02h |
AL | Sectors To Read Count |
CX | Track / Sector |
DH | Head |
DL | Drive |
ES:BX | Buffer Address Pointer |
CF | Set On Error, Clear If No Error |
AH | Return Code |
AL | Actual Sectors Read Count |
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.
[edit]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 |
CF | Set On Error, Clear If No Error |
AH | Return Code |
AL | Actual Sectors Written Count |
[edit]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 |
CF | Set On Error, Clear If No Error |
AH | Return Code |
AL | Actual Sectors Verified Count |
[edit]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$ |
Byte | Meaning | Allowable Values |
1 | Track | |
2 | Head | |
3 | Sector | |
4 | Bytes/Sector | 0=128, 1-256, 2-512, 3-1024 |
CF | Set On Error, Clear If No Error |
AH | Return Code |
[edit]INT
13h AH=06h: Format Track Set Bad Sector Flags
Parameters:AH | 06h |
AL | Interleave |
CH | Track |
CL | Sector |
DH | Head |
DL | Drive |
CF | Set On Error, Clear If No Error |
AH | Return Code |
[edit]INT
13h AH=07h: Format Track Set Bad Sector Flags
Parameters:AH | 07h |
AL | Interleave |
CH | Track |
CL | Sector |
DH | Head |
DL | Drive |
CF | Set On Error, Clear If No Error |
AH | Return Code |
[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) |
ES:DI[4] | set to 0000h:0000h to work around some buggy BIOS |
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 startswith 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) |
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=09h: Init Drive Pair Characteristics
Parameters:AH | 09h |
DL | Drive |
CF | Set On Error, Clear If No Error |
AH | Return Code |
[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[5] |
DL | drive index (e.g. 1st HDD = 80h) |
BX | 55AAh |
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..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) |
CF | Set On Error, Clear If No Error |
AH | Return Code |
[edit]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 |
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) |
CF | Set On Error, Clear If No Error |
AH | Return Code |
[edit]See
also
INT 10HBIOS interrupt call
Cylinder-head-sector
INT (x86 instruction)
DPMI (DOS Protected Mode Interface)
[edit]References
^ [1]^ Disk
size limitations
^ ctyme.com
- DISK - READ SECTOR(S) INTO MEMORY
^ a b c d e f ctyme.com
- DISK - GET DRIVE PARAMETERS (PC,XT286,CONV,PS,ESDI,SCSI)
^ ctyme.com
- IBM/MS INT 13 Extensions - INSTALLATION CHECK
[edit]External
links
BIOS Interrupt 13h ExtensionsRalf Brown's comprehensive Interrupt List
Norton Guide about int 13h, ah = 00h .. 1ah
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Categories:
IBM PC compatibles
BIOS
Interrupts
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