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CHAP5
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Chapter 5
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
Interrupts 22h Through 86h
Interrupt 22h Terminate Address
(0:0088h)
This interrupt transfers control to the far (dword) address at this interrupt
location when an application program terminates. The default address for this
interrupt is 0:0088h through 0:008Bh. This address is copied into the program's
Program Segment Prefix at bytes 0Ah through 0Dh at the time the segment is
created and is restored from the PSP when the program terminates. The calling
program is normally COMMAND.COM or an application. Do not issue this interrupt
directly, as the EXEC function call does this for you. If an application
spawns a child process, it must set the Terminate Address prior to issuing the
EXEC function call, otherwise when the second program terminated it would
return to the calling program's Terminate Address rather than its own. This
address may be set with int 21, function 25h.
Interrupt 23h Ctrl-Break Exit Address
(0:008Ch)
If the user enters a Ctrl-Break during STDIN, STDOUT, STDPRN, or STDAUX, int
23h is executed. If BREAK is on, int 23h is checked on MOST function calls
(notably 06h). If the user written Ctrl-Break routine saves all registers, it
may end with a return-from-interrupt instruction (IRET) to continue program
execution. If the user-written interrupt program returns with a long return, the
carry flag is used to determine whether the program will be aborted. If the
carry flag is set, the program is aborted, otherwise execution continues (as
with a return by IRET). If the user-written Ctrl-Break interrupt uses function
calls 09h or 0Ah, then ctrl-C/CR/LF are output. If execution is continued with
an IRET, I/O continues from the start of the line. When the interrupt occurs,
all registers are set to the value they had when the original function call to
DOS was made. There are no restrictions on what the Ctrl-Break handler is
allowed to do, including DOS function calls, as long as the registers are
unchanged if an IRET is used. If the program creates a new segment and loads a
second program which itself changes the Ctrl-Break address, the termination of
the second program and return to the first causes the Ctrl-Break address to
be restored from the PSP to the value it had before execution of the second
program.
Interrupt 24h Critical Error Handler
(0:0090h)
When a critical error occurs within DOS, control is transferred to an error
handler with an int 24h. This may be the standard DOS error handler (ABORT,
RETRY, IGNORE) or a user-written routine.
On entry to the error handler, AH will have its bit 7=0 (high order bit)
if the error was a disk error (probably the most common error), bit 7=1 if
not.
BP:SI contains the address of a Device Header Control Block from which
additional information can be retrieved (see below).
The register is set up for a retry operation and an error code is in the
lower half of the DI register with the upper half undefined. These are the
error codes:
The user stack is in effect and contains the following from top to bottom:
IP DOS registers from issuing int 24h
CS int 24h
flags
AX user registers at time of signal
BX int 21h request
CX
DX
SI
DI
BP
DS
ES
IP from original int 21h
CS
flags
To reroute the critical error handler to a user-writen critical error handler,
the following should be done:
Before an int 24h occurs:
1) The user application initialization code should save the int 24h vector and
replace the vector with one pointing to the user error routine.
When the int 24h occurs:
2) When the user error routine received control it should push the flag
registers onto the stack and execute a far call to the original int 24h
vector saved in step 1.
3) DOS gives the appropriate prompt, and waits for user input (Abort, Retry,
Ignore, Fail). After the user input, DOS returns control to the user error
routine instruction following the far call.
4) The user error routine can now do any tasks nescessary. To return to the
original application at the point the error occurred, the error routine needs
to execute an IRET instruction. Otherwise, the user error routine should
remove the IP, CS, and flag registers from the stack. Control can then be
passed to the desired point.
Int 24h provides the following values in registers on entry to interrupt
handler:
entry AH status byte
bit 7 0 disk I/O hard error
1 other error - if block device, bad FAT
- if char device, code in DI
6 unused
5 0 if IGNORE is not allowed
1 if IGNORE is allowed
4 0 if RETRY is not allowed
1 if RETRY is allowed
3 0 if FAIL is not allowed
1 if FAIL is allowed
2 \ disk area of error 00 = DOS area 01 = FAT
1 / 10 = root dir 11 = data area
0 0 if read operation
1 if write operation
AL drive number if AH bit 7 = 1, otherwise undefined
If it is as hard error on disk (AH bit 7=0), register AL
contains the failing drive number (0=A:, 1=B:, etc.).
BP:SI address of a Device Header Control Block for which error
occurred block device if high bit of BP:SI+4 = 1
low byte of DI: error code (note: high byte is undefined)
error code description
00h attempt to write on write-protected diskette
01h unknown unit
02h drive not ready
03h unknown command
04h data error (bad CRC)
05h bad request structure length
06h seek error
07h unknown media type
08h sector not found
09h printer out of paper
0Ah write fault
0Bh read fault
0Ch general failure
0Fh invalid disk change (DOS 3.x)
handler must return
The registers are set such that if an IRET is executed, DOS responds according
to (AL) as follows:
AL 00h ignore the error
01h retry the operation
02h terminate via int 22h
03h fail the system call that is in progress (DOS 3.2+)
note 1) Be careful when choosing to ignore a response because this causes DOS to
beleive that an operation has completed successfully when it may not
have.
2) If the error was a character device, the contents of AL are invalid.
OTHER ERRORS
If AH bit 7=1, the error occurred on a character device, or was the result of
a bad memory image of the FAT. The device header passed in BP:SI can be examined
to determine which case exists. If the attribute byte high-order bit indicates
a block device, then the error was a bad FAT. Otherwise, the error is on a
character device.
If a character device is involved, the contents of AL are unpredictable, the
error code is in DI as above.
Notes:
1. Before giving this routine control for disk errors, DOS performs several
retries. The number of retries varies according to the DOS version.
2. For disk errors, this exit is taken only for errors occurring during an
int 21h function call. It is not used for errors during an int 25h or 26h.
3. This routine is entered in a disabled state.
4. All registers must be preserved.
5. This interrupt handler should refrain from using DOS function calls. If
necessary, it may use calls 01h through 12h. Use of any other call destroys
the DOS stack and leaves DOS in an unpredictable state.
6. The interrupt handler must not change the contents of the device header.
7. If the interrupt handler handles errors itself rather than returning to DOS,
it should restore the application program's registers from the stack,
remove all but the last three words on the stack, then issue an IRET. This
will return to the program immediately after the int 21h that experienced
the error. Note that if this is done DOS will be in an unstable state until
a function call higher than 12h is issued, therefore not recommended.
8. For DOS 3.x, IGNORE requests (AL=0) are converted to FAIL for critical
errors that occur on FAT or DIR sectors.
9. For DOS 3.10 up, IGNORE requests (AL=0) are converted to FAIL requests
for network critical errors (50-79).
The device header pointed to by BP:SI is as follows:
DWORD Pointer to next device (0FFFFh if last device)
WORD Attributes:
Bit 15 1 if character device.
If bit 15 is 1:
Bit 0 = 1 if current standard input
Bit 1 = 1 if current standard output
Bit 2 = 1 if current NULL device
Bit 3 = 1 if current CLOCK device
0 if block device
Bit 14 is the IOCTL bit
WORD pointer to device driver strategy entry point
WORD pointer to device driver interrupt entry point
8-BYTE character device named field for block devices. The first byte is the
number of units.
To tell if the error occurred on a block or character device, look at bit 15
in the attribute field (WORD at BP:SI+4).
If the name of the character device is desired, look at the eight bytes
starting at BP:SI+10.
HANDLING OF INVALID RESPONSES (DOS 3.x)
A) If IGNORE (AL=0) is specified by the user and IGNORE is not allowed
(bit 5=0), make the response FAIL (AL=3).
B) If RETRY (AL=1) is specified by the user and RETRY is not allowed
(bit 4=0), make the response FAIL (AL=3).
C) If FAIL (AL=3) is specified by the user and FAIL is not allowed (bit
3=0), make the response ABORT. (AL=2)
Interrupt 25h Absolute Disk Read
Interrupt 26h Absolute Disk Write
(0:0094h, 0:0098h)
These transfer control directly to the device driver. On return, the original
flags are still on the stack (put there by the INT instruction). This is
necessary because return information is passed back in the current flags.
The number of sectors specified is transferred between the given drive and the
transfer address. Logical sector numbers are obtained by numbering each sector
sequentially starting from track 0, head 0, sector 1 (logical sector 0) and
continuing along the same head, then to the next head until the last sector on
the last head of the track is counted. Thus, logical sector 1 is track 0, head
0, sector 2; logical sector 2 is track 0, head 0, sector 3; and so on. Numbering
then continues wih sector 1 on head 0 of the next track. Note that although the
sectors are sequentially numbered (for example, sectors 2 and 3 on track 0 in
the example above), they may not be physically adjacent on disk, due to
interleaving. Note that the mapping is different from that used by DOS 1.10 for
dual-sided diskettes.
The request is as follows:
int 25 for Absolute Disk Read,
int 26 for Absolute Disk Write
entry AL drive number (0=A:, 1=B:, etc)
CX number of sectors to read
DS:BX disk transfer address (buffer)
DX first relative sector to read - beginning logical sector number
return CF set if error
AL error code issued to int 24h in low half of DI
AH 01h bad command
02h bad address mark
03h write-protected disk
04h requested sector not found
08h DMA failure
10h data error (bad CRC)
20h controller failed
40h seek operation failed
80h attachment failed to respond
note 1) Original flags on stack! Be sure to pop the stack to prevent
uncontrolled growth
2) Ints 25 and 26 will try rereading a disk if they get an error the first
time.
3) All registers except the segment registers are destroyed by these calls
Interrupt 27h Terminate And Stay Resident
(0:009Ch) (obsolete)
This vector is used by programs that are to remain resident when COMMAND.COM
regains control.
After initializing itself, the program must set DX to its last address plus
one relative to the program's initial DS or ES value (the offset at which other
programs can be loaded), then execute interrupt 27h. DOS then considers the
program as an extension of itself, so the program is not overlaid when other
programs are executed. This is useful for loading programs such as utilities
and interrupt handlers that must remain resident.
entry CS current program segment
DX last program byte + 1
return none
note 1) This interrupt must not be used by .EXE programs that are loaded into
the high end of memory.
2) This interrupt restores the interrupt 22h, 23h, and 24h vectors in the
same manner as interrupt 20h. Therefore, it cannot be used to install
permanently resident Ctrl-Break or critical error handler routines.
3) The maximum size of memory that can be made resident by this method is
64K.
4) Memory can be more efficiently used if the block containing a copy of
the environment is deallocated before terminating. This can be done by
loading ES with the segment contained in 2Ch of the PSP, and issuing
function call 49h (Free Allocated Memory).
5) DOS function call 4Ch allows a program to pass a completion code to DOS,
which can be interpreted with processing (see function call 31h).
6) Terminate and stay resident programs do not close files.
7) Int 21, function 31h is the preferred method to cause a program to
remain resident because this allows return information to be passed and
allows a program larger than 64K to remain resident.
Interrupt 28h (not documented by Microsoft)
* DOS Idle Interrupt
This interrupt is continuously called by DOS itself whenever it is in a wait
state (i.e., when it is waiting for keyboard input) during a function call of
01h through 0Ch. DOS uses 3 separate internal stacks: one for calls 01h through
0Ch; another for calls 0Dh and above; and a third for calls 01h through 0Ch when
a Critical Error is in progress. When int 28h is called, any calls above 0Ch can
be executed without destroying the internal stack used by DOS at the time.
It is used primarily by the PRINT.COM routines, but any number of other
routines can be chained to it by saving the original vector and calling it with
a FAR call (or just JMPing to it) at the end of the new routine.
Int 28h is being issued it is usually safe to do DOS calls. You won't get int
28hs if a program is running that doesn't do its keyboard input through DOS. You
should rely on the timer interrupt for these.
Int 28h is not called at all when any non-trivial foreground task is running.
As soon as a foreground program has a file open, INT28 no longer gets called.
Could make a good driver for for abackground program that really works as long
as there is nothing else going on in the machine.
entry no parameters availible
return none
note 1) The int 28h handler may invoke any int 21h function except functions
00h through 0Ch (and 50h/51h under DOS 2.x).
2) Apparently int 28h is also called during screen writes
3) Until some program installs its own routine, this interrupt vector
simply points to an IRET opcode.
4) Supported in OS/2 1.0's DOS Compatibility Box
Interrupt 29h (not documented by Microsoft)
* Internal - Quick Screen Output
This method is extremely fast (much faster than DOS 21h subfunctions 2 and 9,
for example), and it is portable, even to "non-compatible" MS-DOS computers.
entry AL character to output to screen
return unknown
note 1) Documented by Digital Research's DOS Reference as provided with the
DEC Rainbow
2) If ANSI.SYS is installed, character output is filtered through it.
3) Works on the IBM PC and compatibles, Wang PC, HP-150 and Vectra, DEC
Rainbow, NEC APC, Texas Instruments PC and others
4) This interrupt is called from the DOS's output routines if output is
going to a device rather than a file, and the device driver's attribute
word has bit 3 (04h) set to "1".
5) This call has been tested with MSDOS 2.11, PCDOS 2.1, PCDOS 3.1, PCDOS
3.2, and PCDOS 3.3.
6) Used in IBMBIO.COM as a vector to int 10, function 0Eh (write TTY)
followed by an IRET.
Interrupt 2Ah Microsoft Networks - Session Layer Interrupt
* (not documented by Microsoft)
entry AH 00h check to see if network BIOS installed
return: AH <> 0 if installed
01h execute NETBIOS request
02h set net printer mode
03h get shared-device status (check direct I/O)
AL 00h
DS:SI pointer to ASCIIZ disk device name
return CF 0 if allowed
04h execute NETBIOS
AL 0 for error retry
1 for no retry
ES:BX pointer to ncb
return AX 0 for no error
AH 1 if error
AL error code
05h get network resource information
AL 00h
return AX reserved
BX number of network names
CX number of commands
DX number of sessions
82h unknown
return ??
note called by the int 21h function dispatcher in DOS 3.10
Interrupt 2Bh (not documented by Microsoft)
* Unknown - Internal Routine for DOS (IRET)
Interrupt 2Ch (not documented by Microsoft)
* Unknown - Internal Routine for DOS (IRET)
Interrupt 2Dh (not documented by Microsoft)
* Unknown - Internal Routine for DOS (IRET)
Interrupt 2Eh (undocumented by Microsoft)
* Internal Routine for DOS (Alternate EXEC)
This interrupt passes a command line addressed by DS:SI to COMMAND.COM. The
command line must be formatted just like the unformatted parameter area of a
Program Segment Prefix. That is, the first byte must be a count of characters,
and the second and subsequent bytes must be a command line with parameters,
terminated by a carriage return character.
When executed, int 2Eh will reload the transient part of the command
interpreter if it is not currently in memory. If called from a program that
was called from a batch file, it will abort the batch file. If executed from a
program which has been spawned by the EXEC function, it will abort the whole
chain and probably lock up the computer. Int 2Eh also destroys all registers
including the stack pointer.
Int 2Eh is called from the transient portion of the program to reset the DOS
PSP pointers using the above Functions #81 & #80, and then reenters the
resident program.
When called with a valid command line, the command will be carried out by
COMMAND.COM just as though you had typed it in at the DOS prompt. Note that the
count does not include the carriage return. This is an elegant way to perform a
SET from an application program against the master environment block for
example.
entry DS:SI pointer to an ASCIIZ command line in the form:
count byte
ASCII string
carriage return
null byte
note 1) Destroys all registers including stack pointer
2) Seems to work OK in both DOS 2.x and 3.x
3) It is reportedly not used by DOS.
4) As far as known, int 2Eh is not used by DOS 3.1, although it was called
by COMMAND.COM of PCDOS 3.0, so it appears to be in 3.1 only for the
sake of compatibility.
Interrupt 2Fh Multiplex Interrupt
Interrupt 2Fh is the multiplex interrupt. A general interface is defined
between two processes. It is up to the specific application using interrupt
2Fh to define specific functions and parameters.
Every multiplex interrupt handler is assigned a specific multiplex number.
The multiplex number is specified in the AH register; the AH value tells which
program your request is directed toward. The specific function that the handler
is to perform is placed in the AL register. Other parameters are places in the
other registers as needed. The handlers are chained into the 2Fh interrupt
vector and the multiplex number is checked to see if any other application is
using the same multiplex number. There is no predefined method for assigning a
multiplex number to a handler. You must just pick one. To avoid a conflict if
two applications choose the same multiplex number, the multiplex numbers used by
an application should be patchable. In order to check for a previous
installation of the current application, you can search memory for a unique
string included in your program. If the value you wanted in AH is taken but
you don't find the string, then another application has grabbed that location.
Int 2Fh was not documented under DOS 2.x. There is no reason not to use int 2Fh
as the multiplex interrupt in DOS 2.x. The only problem is that DOS does not
initialize the int 2Fh vector, so when you try to chain to it like you are
supposed to, it will crash. But if your program checks the vector for being zero
and doesn't chain in that case, it will work for you in 2.x just the same as
3.x.
Int 2Fh doesn't require any support from DOS itself for it to be used in
application programs. It's not handled by DOS, but by the programs themselves.
The only support DOS has to provide is to initialize the vector to an IRET. DOS
3.2 does itself contain some int 2Fh handlers - it uses values of 08h, 13h, and
0F8h. There may be more.
entry AH 01h PRINT.COM
AL 00h PRINT Get Installed State
This call must be defined by all int 2Fh handlers. It
is used by the caller of the handler to determine if
the handler is present. On entry, AL=0. On return, AL
contains the installed state as follows:
return AL 0FFh installed
01h not installed, not OK to install
00h not installed, OK to install
01h PRINT Submit File
DS:DX pointer to submit packet
format BYTE level
DWORD pointer to ASCIIZ filename
return CF set if error
AX error code
note 1) A submit packet contains the level (BYTE) and a pointer
to the ASCIIZ string (DWORD in offset:segment form).
The ASCIIZ string must contain the drive, path, and
filename of the file you want to print. The filename
cannot contain global filename characters.
return CF set if error
AX error code
02h PRINT Cancel File
On entry, AL=2 and DS:DX points to the ASCIIZ string for
the print file you want to cancel. Global filename
characters are allowed in the filename.
DS:DX pointer to ASCIIZ file name to cancel (wildcards OK)
return CF set if error
AX error code
03h PRINT remove all files
return CF set if error
AX error code
04h PRINT hold queue/get status
This call holds the jobs in the print queue so that you
can scan the queue. Issuing any other code releases the
jobs. On entry, AL=4. On return, DX contains the error
count. DS:SI points to the print queue. The print queue
consists of a series of filename entries. Each entry is
64 bytes long. The first entry in the queue is the file
currently being printed. The end of the queue is marked
by the entry having a null as the first character.
return DX error count
DS:SI pointer to print queue (null-string terminated
list of 64-byte ASCIIZ filenames)
CF set if error
AX error code
01h function invalid
02h file not found
03h path not found
04h too many open files
05h access denied
08h queue full
09h spooler busy
0Ch name too long
0Fh drive invalid
05h PRINT restart queue
return CF set if error
AX error code
AH 05h DOS 3.x critical error handler
AL 00h installation check
return AL 00h not installed, OK to install
01h not installed, can't install
0FFh installed
note This set of functions allows a user program to
partially or completely override the default
critical error handler in COMMAND.COM
AL 01h handle error - nonzero error code in AL
return CF clear
ES:DI pointer to ASCIIZ error message
CF set use default error handler
AL (?)
AH 06h ASSIGN
00h installation check
return AH <> 0 if installed
01h get memory segment
return ES segment of ASSIGN work area
AH 10h SHARE
00h installation check
return AL 00h not installed, OK to install
01h not installed, not OK to install
0FFh installed
AH 11h multiplex - network redirection
00h installation check
return AL 00h not installed, OK to install
01h not installed, not OK to install
0FFh installed
01h unknown
02h unknown
03h unknown
04h unknown
05h unknown
06h close remote file
07h unknown
08h unknown
09h unknown
0Ah unknown
STACK: WORD (?)
return CF set on error
0Bh unknown
STACK: WORD (?)
return CF set on error(?)
0Ch unknown
0Dh unknown
0Eh unknown
STACK: WORD (?)
return (?)
0Fh unknown
11h unknown
13h unknown
16h unknown
17h unknown
STACK: WORD (?)
return (?)
18h unknown
STACK: WORD (?)
return (?)
19h unknown
1Bh unknown
1Ch unknown
1Dh unknown
1Eh do redirection
STACK: WORD function to execute
return CF set on error
1Fh printer setup
STACK: WORD function(?)
return CF set on error(?)
20h unknown
21h unknown
22h unknown
23h unknown
24h unknown
25h unknown
STACK: WORD (?)
26h unknown
AH 12h multiplex, DOS 3.x internal services
00h installation check
return AL 0FFh for compatibility with other
int 2Fh functions
01h close file (?)
stack word value - unknown
return BX unknown
CX unknown
ES:DI pointer to unknown value
note Can be called only from within DOS
02h get interrupt address
stack: word vector number
return ES:BX pointer to interrupt vector
Stack unchanged
03h get DOS data segment
return DS segment of IBMDOS
04h normalize path separator
stack: word character to normalize
return AL normalized character (forward
slash turned to backslash)
Stack unchanged
05h output character
stack: word character to output
return Stack unchanged
note Can be called only from within DOS
06h invoke critical error
return AL 0-3 for Abort, Retry, Ignore,
Fail
note Can be called only from within DOS
07h move disk buffer (?)
DS:DI pointer to disk buffer
return buffer moved to end of buffer list
note Can be called only from within DOS
08h decrement word
ES:DI pointer to word to decrement
return AX new value of word
note Word pointed to by ES:DI decremented,
skipping zero
09h unknown
DS:DI pointer to disk buffer(?)
return (?)
note Can be called only from within DOS
0Ah unknown
note Can be called only from within DOS
0Bh unknown
ES:DI pointer to system file table entry(?)
return AX (?)
note Can be called only from within DOS
0Ch unknown
note Can be called only from within DOS
0Dh get date and time
return AX current date in packed format
DX current time in packed format
note Can be called only from within DOS
0Eh do something to all disk buffers (?)
return DS:DI pointer to first disk buffer
note can be called only from within DOS
0Fh unknown
DS:DI pointer to (?)
return DS:DI pointer to (?)
note 1) Can be called only from within DOS
2) Calls on function 1207h
10h find dirty/clean(?) buffer
DS:DI pointer to first disk buffer
return DS:DI pointer to first disk buffer
which has (?) flag clear
ZF clear if found
set if not found
11h normalize ASCIIZ filename
DS:SI pointer to ASCIZ filename to normalize
ES:DI ptr to buffer for normalized filename
return destination buffer filled with upper-
case filename, with slashes turned to
backslashes
12h get length of ASCIIZ string
ES:DI pointer to ASCIZ string
return CX length of string
13h uppercase character
stack: word character to convert to uppercase
return AL uppercase character
Stack unchanged
14h compare far pointers
DS:SI first pointer
ES:DI second pointer
return ZF set if pointers are equal
ZF clear if not equal
15h unknown
DS:DI pointer to disk buffer
stack: word (?)
return Stack unchanged
note Can be called only from within DOS
16h get address of system FCB
BX system file table entry number
return ES:DI pointer to system file table entry
17h set default drive (?)
stack: word drive (0 = A:, 1 = B:, etc)
return DS:SI pointer to drive data block for
specified drive
Stack unchanged
note Can be called only from within DOS
18h get something (?)
return DS:SI pointer to (?)
19h unknown
stack: word drive (0 = default, 1 = A:, etc)
return (?)
Stack unchanged
note 1) Can be called only from within DOS
2) Calls function 1217h
1Ah get file's drive
DS:SI pointer to filename
return AL drive
(0=default, 1=A:, etc, 0FFh=invalid)
1Bh set something (?)
CL unknown
return AL (?)
note Can be called only from within DOS
1Ch checksum memory
DS:SI pointer to start of memory to checksum
CX number of bytes
DX initial checksum
return DX checksum
note Can be called only from within DOS
1Dh unknown
DS:SI pointer to (?)
CX (?)
DX (?)
return AX (?)
CX (?)
DX = (?)
1Eh compare filenames
DS:SI pointer to first ASCIIZ filename
ES:DI pointer to second ASCIIZ filename
return ZF set if filenames equivalent
clear if not
1Fh build drive info block
stack: word drive letter
return ES:DI pointer to drive info block
(will be overwritten by next call)
Stack unchanged
note Can be called only from within DOS
20h get system file table number
BX file handle
return CF set on error
AL 6 (invalid file handle)
CF clear if successful
byte ES:[DI] = system file table entry
number for file handle
21h unknown
DS:SI pointer to (?)
return (?)
note Can be called only from within DOS
22h unknown
SS:SI pointer to (?)
return nothing(?)
note Can be called only from within DOS
23h check if character device (?)
return DS:SI pointer to device driver with
same name as (?)
note Can be called only from within DOS
24h delay
return after delay of (?) ms
note Can be called only from within DOS
25h get length of ASCIIZ string
DS:SI pointer to ASCIIZ string
return CX length of string
AH 43h Microsoft Extended Memory Specification (XMS)
AH 64h SCRNSAV2.COM
AL 00h installation check
return AL 00h not installed
0FFh installed
note SCRNSAV2.COM is a screen saver for PS/2's with
VGA by Alan Ballard
AH 7Ah Novell NetWare
AL 00h installation check
note Returns address of entry point for IPX and SPX
AH 0AAh VIDCLOCK.COM
AL 00h installation check
return AL 00h not installed
0FFh installed
note VIDCLOCK.COM is a memory-resident clock by
Thomas G. Hanlin III
AH 0B7h APPEND
AL 00h APPEND installation check
return AH <> 0 if installed
01h APPEND - unknown
02h APPEND - version check
AH 0B8h Microsoft Networks
AL 00h network program installation check
return AH <> 0 if installed
BX installed component flags (test in this
order!)
bit 6 server
bit 2 messenger
bit 7 receiver
bit 3 redirector
01h unknown
02h unknown
03h get current POST address
return ES:BX POST address
04h set new POST address
ES:BX new POST address
09h version check
AH 0BBh Network functions
AL 00h net command installation check
03h get server POST address
04h get server POST address
AH 0F7h AUTOPARK.COM (PD TSR hard disk parking utility)
AL 00h installation check
return AL 00h not installed
0FFh installed
note AUTOPARK is a TSR HD parker by Alan D. Jones
01h set parking delay
BX:CX 32 bit count of 55ms timer ticks
return AX Error
Codes Description
01h invalid function number
02h file not found
03h path not found
04h too many open files
05h access denied
06h invalid handle
08h queue full
09h busy
0Ch name too long
0Fh invalid drive was specified
CF clear (0) if OK
set (1) if error - error returned in AX
note 1) The multiplex numbers AH=0h through AH=7Fh are reserved for DOS.
Applications should use multiplex numbers 80h through 0FFh.
2) When in the chain for int 2Fh, if your code calls DOS or if you execute
with interrupts enabled, your code must be reentrant/recursive.
Interrupt 30h (not a vector!) far jump instruction for CP/M-style calls
Interrupt 31h Unknown
note The CALL 5 entry point does a FAR jump to here
Interrupt 32h Unknown
Interrupt 33h Used by Microsoft Mouse Driver
Function Calls
00h Reset Driver and Read Status
entry AH 00h
return AH status
0 hardware/driver not installed
-1 hardware/driver installed
BX number of buttons
-1 two buttons
0 other than two
3 Mouse Systems mouse
01h Show Mouse Cursor
entry AH 01h
return unknown
02h Hide Mouse Cursor
entry AH 02h
return unknown
note multiple calls to hide the cursor will require multiple calls
to function 01h to unhide it.
03h Return Position and Button Status
entry AH 03h
return BX button status
bit 0 left button pressed if 1
bit 1 right button pressed if 1
bit 2 middle button pressed if 1 (Mouse Systems mouse)
CX column
DX row
04h Position Mouse Cursor
entry AH 04h
CX column
DX row
return unknown
05h Return Button Press Data
entry AH 05h
BX button
0 left
1 right
2 middle (Mouse Systems mouse)
return AH button states
bit 0 left button pressed if 1
bit 1 right button pressed if 1
bit 2 middle button pressed if 1 (Mouse Systems mouse)
BX no. of times specified button pressed since last call
CX column at time specified button was last pressed
DX row at time specified button was last pressed
06h Return Button Release Data
entry AH 06h
BX button
0 left
1 right
2 middle (Mouse Systems mouse)
return AH button states
bit 0 left button pressed if 1
bit 1 right button pressed if 1
bit 2 middle button pressed if 1 (Mouse Systems mouse)
BX no. of times specified button released since last call
CX column at time specified button was last released
DX row at time specified button was last released
07h Define Horizontal Curos Range
entry AH 0007h
CX minimum column
DX maximum column
return unknown
08h Define Vertical Cursor Range
entry AH 08h
CX minimum row
DX maximum row
return unknown
09h Define Graphics Cursor
entry AH 09h
BX column of cursor hot spot in bitmap (-16 to 16)
CX row of cursor hot spot (-16 to 16)
ES:DX pointer to bitmap
16 words screen mask
16 words cursor mask
return unknown
note Each word defines the sixteen pixels of a row, low bit
rightmost
0Ah Define Text Cursor
entry AH 0Ah
BX hardware/software text cursor
00h software
CX screen mask
DX cursor mask
01h hardware
CX start scan line
DX end scan line
return unknown
note When the software cursor is selected, the char/attribute data
at the current screen position is ANDed with the screen mask
and the with the cursor mask
0BH Read Motion Counters
entry AH 0Bh
return CX number of mickeys mouse moved horiz. since last call
DX number of mickeys mouse moved vertically
note 1) A mickey is the smallest increment the mouse can sense.
Positive values indicate up/right
0Ch Define Interrupt Subroutine Parameters
entry AH 0Ch
CX call mask bit
bit 0 call if mouse moves
bit 1 call if left button pressed
bit 2 call if left button released
bit 3 call if right button pressed
bit 4 call if right button released
bit 5 call if middle button pressed (Mouse Systems)
bit 6 call if middle button released (Mouse Systems)
ES:DX address of FAR routine
return unknown
note when the subroutine is called, it is passed these values:
AH condition mask (same bit assignments as call mask)
BX button state
CX cursor column
DX cursor row
DI horizontal mickey count
SI vertical mickey count
0Dh Light Pen Emulation On
entry AH 0Dh
return unknown
0Eh Light Pen Emulation Off
entry AH 0Eh
return unknown
0Fh Define Mickey/Pixel Ratio
entry AH 0Fh
CX number of mickeys per 8 pixels horizontally
DX number of mickeys per 8 pixels vertically
return unknown
10h Define Screen Region for Updating
entry AH 10h
CX,DX X,Y coordinates of upper left corner
SI,DI X,Y coordinates of lower right corner
return unknown
note Mouse cursor is hidden during updating, and needs to be
explicitly turned on again
11h not documented by Microsoft
12h Set Large Graphics Cursor Block
AH 12h
BH cursor width in words
CH rows in cursor
BL horizontal hot spot (-16 to 16)
CL vertical hot spot (-16 to 16)
ES:DX pointer to bit map of screen and cursor maps
return AH -1 if successful
note PC Mouse. Not dodcumented by Microsoft
13h Define Double-Speed Threshold
entry AH 13h
DX threshold speed in mickeys/second,
0 = default of 64/second
return unknown
note If speed exceeds threshold, the cursor's on-screen motion
is doubled
14h Exchange Interrupt Subroutines
entry AH 14h
return unknown
15h Return Drive Storage Requirements
entry AH 15h
return BX size of buffer needed to store driver state
16h Save Driver State
entry AH 16h
ES:DX pointer to buffer
return unknown
17h Restore Driver State
entry AH 17h
ES:DX pointer to buffer containing saved state
return unknown
18h not documented by Microsoft
19h not documented by Microsoft
1Ah not documented by Microsoft
1Bh not documented by Microsoft
1Ch not documented by Microsoft
1Dh Define Display Page Number
entry AH 1Dh
1Eh Return Display Page Number
entry AH 1Eh
return unknown
42h PCMouse - Get MSmouse Storage Requirements
AH 42h
return AX 0FFFFh successful
BX buffer size in bytes for functions 50h and 52h
00h MSmouse not installed
42h functions 42h, 50h, and 52h not supported
52h PCMouse - Save MSmouse State
entry AH 50h
BX buffer size
ES:DX pointer to buffer
return AX 0FFFFh if successful
52h PCMouse - restore MSmouse state
entry AH 52h
BX buffer size
ES:DX pointer to buffer
return AX 0FFFFh if successful
Int 33: In addition, the following functions are appended to BIOS int 10h and
implemented as the EGA Register Interface Library:
0F0h read one register
0F1h write one register
0F2h read consecutive register range
0F3h write consecutive register range
0F4h read non-consecutive register set
0F5h write non-consecutive register set
0F6h revert to default register values
0F7h define default register values
0FAh get driver status
Interrupt 34h Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0D8h
Interrupt 35h Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0D9h
Interrupt 36h Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0DAh
Interrupt 37h Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0DBh
Interrupt 38h Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0DCh
Interrupt 39h Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0DDh
Interrupt 3Ah Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0DEh
Interrupt 3Bh Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0DFh
Interrupt 3Ch Turbo C/Microsoft languages - Floating Point emulation
This int emulates instructions with an ES segment override
Interrupt 3Dh Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates a standalone FWAIT instruction
Interrupt 3Eh Turbo C/Microsoft languages - Floating Point emulation
Interrupt 3Fh Overlay manager interrupt (Microsoft LINK.EXE)
Interrupt 40h Hard Disk BIOS
Pointer to disk BIOS entry when a hard disk controller is
installed. The BIOS routines use int 30h to revector the
diskette handler (original int 13h) here so int 40 may be used
for hard disk control
Interrupt 41h Hard Disk Parameters (XT,AT,XT2,XT286,PS except ESDI disks)
Pointer to first Hard Disk Parameter Block, normally located
in the controller card's ROM. This table may be copied to RAM
and changed, and this pointer revectored to the new table.
note 1) format of parameter table is:
dw cylinders
db heads
dw starting reduced write current cylinder (XT only, 0 for others)
db maximum ECC burst length
db control byte
bits 0-2 drive option (XT only, 0 for others)
bit 3 set if more than 8 heads
bit 4 always 0
bit 5 set if manufacturer's defect map on max cylinder+1
bit 6 disable ECC retries
bit 7 disable access retries
db standard timeout (XT only, 0 for others)
db formatting timeout (XT only, 0 for others)
db timeout for checking drive (XT only, 0 for others)
dw landing zone (AT, PS/2)
db sectors/track (AT, PS/2)
db 0
2) normally vectored to ROM table when system is initialized.
Interrupt 42h Pointer to screen BIOS entry (EGA, VGA, PS/2)
Relocated (by EGA, etc.) video handler (original int 10h).
Revectors int 10 calls to EGA BIOS.
Interrupt 43h Pointer to EGA initialization parameter table. The POST
initializes this vector pointing to the default table located
in the EGA ROM BIOS. (PC-2 and up). Not initialized if EGA not
present.
Interrupt 44h Pointer to EGA graphics character table (also PCjr). This
(0:0110h) table contains the dot patterns for the first 128 characters
in video modes 4,5, and 6, and all 256 characters in all
additional graphics modes. Not initialized if EGA not present.
2) EGA/VGA/CONV/PS - EGA/PCjr fonts, characters 00h to 7Fh
3) Novell NetWare - High-Level Language API
Interrupt 45h Reserved by IBM (not initialized)
Interrupt 46h Pointer to second hard disk, parameter block (AT, XT/286, PS/2)
(see int 41h) (except ESDI hard disks) (not initialized unless
specific user software calls for it)
Interrupt 47h Reserved by IBM (not initialized)
Interrupt 48h Cordless Keyboard Translation (PCjr, XT [never delivered])
(0:0120h) This vector points to code to translate the cordless keyboard
scancodes into normal 83-key values. The translated scancodes
are then passed to int 9. (not initialized on PC or AT)
Interrupt 49h Non-keyboard Scan Code Translation Table Address (PCjr)
(0:0124h) This interrupt has the address of a table used to translate
non-keyboard scancodes (greater than 85 excepting 255). This
interrupt can be revectored by a user application. IBM
recommends that the default table be stored at the beginning
of an application that required revectoring this interrupt,
and that the default table be restored when the application
terminates. (not initialized on PC or AT)
Interrupt 4Ah Real-Time Clock Alarm (Convertible, PS/2)
(not initialized on PC or AT)
Invoked by BIOS when real-time clock alarm occurs
Interrupt 4Bh Reserved by IBM (not initialized)
Interrupt 4Ch Reserved by IBM (not initialized)
Interrupt 4Dh Reserved by IBM (not initialized)
Interrupt 4Eh Reserved by IBM (not initialized)
Interrupt 4Fh Reserved by IBM (not initialized)
Interrupt 50-57 IRQ0-IRQ7 relocated by DesQview
(normally not initialized)
Interrupt 58h Reserved by IBM (not initialized)
Interrupt 59h Reserved by IBM (not initialized)
GSS Computer Graphics Interface (GSS*CGI)
DS:DX Pointer to block of 5 array pointers
return CF 0
AX return code
CF 1
AX error code
note 1) Int 59 is the means by which GSS*CGI language bindings
communicate with GSS*CGI device drivers and the GSS*CGI
device driver controller.
2) Also used by the IBM Graphic Development Toolkit
Interrupt 5Ah Reserved by IBM (not initialized)
Interrupt 5Bh Reserved by IBM (not initialized)
Interrupt 5Ah Cluster Adapter BIOS entry address
(normally not initialized)
Interrupt 5Bh Reserved by IBM (not initialized) (cluster adapter?)
Interrupt 5Ch NETBIOS interface entry port
ES:BX pointer to network control block
note 1) When the NETBIOS is installed, interrupts 13 and 17 are interrupted by
the NETBIOS; interrupt 18 is moved to int 86 and one of int 2 or 3 is
used by NETBIOS. Also, NETBIOS extends the int 15 function 90 and 91h
functions (scheduler functions)
2) Normally not initialized.
3) TOPS network card uses DMA 1, 3 or none.
Interrupt 5Dh Reserved by IBM (not initialized)
Interrupt 5Eh Reserved by IBM (not initialized)
Interrupt 5Fh Reserved by IBM (not initialized)
Interrupt 60h-67h User Program Interrupts (availible for general use)
Interrupt 67h Used by Lotus-Intel-Microsoft Expanded Memory Specification
user and Ashton-Tate/Quadram/AST Enhanced Expanded Memory
specification (See Chapter 10)
Interrupt 68h Not Used (not initialized)
Interrupt 69h Not Used (not initialized)
Interrupt 6Ah Not Used (not initialized)
Interrupt 6Bh Not Used (not initialized)
Interrupt 6Ch System Resume Vector (Convertible) (not initialized on PC)
Interrupt 6Dh Not Used (not initialized)
Interrupt 6Fh Not Used (not initialized)
Interrupt 70h IRQ 8, Real Time Clock Interrupt (AT, XT/286, PS/2)
Interrupt 71h IRQ 9, Redirected to IRQ 8 (AT, XT/286, PS/2)
LAN Adapter 1 (rerouted to int 0Ah by BIOS)
Interrupt 72h IRQ 10 (AT, XT/286, PS/2) Reserved
Interrupt 73h IRQ 11 (AT, XT/286, PS/2) Reserved
Interrupt 74h IRQ 12 Mouse Interrupt (AT, XT/286, PS/2)
Interrupt 75h IRQ 13, Coprocessor Error, BIOS Redirect to int 2 (NMI) (AT)
Interrupt 76h IRQ 14, Hard Disk Controller (AT, XT/286, PS/2)
Interrupt 77h IRQ 15 (AT, XT/286, PS/2) Reserved
Interrupt 78h Not Used
Interrupt 79h Not Used
Interrupt 7Ah Novell NetWare - LOW-LEVEL API
Interrupt 7Bh-7Fh Not Used
Interrupt 80h-85h Reserved by BASIC
note interrupts 80h through ECh are apparently unused and not initialized.
Interrupt 86h Relocated by NETBIOS int 18
Interrupt 86h-0F0h Used by BASIC when BASIC interpreter is running
Intrerrupt 0E0h CP/M-86 function calls
Interrupt 0E4h Logitech Modula-2 v2.0 MONITOR
entry AX 05h monitor entry
06h monitor exit
BX priority
Interrupts 0F1h-0FFh (absolute addresses 3C4-3FF)
Location of Interprocess Communications Area
Interrupt 0F8h Set Shell Interrupt (OEM)
Set OEM handler for int 21h calls from 0F9h through 0FFh
entry AH 0F8h
DS:DX pointer to handler for Functions 0F9h thru 0FFh
note 1) To reset these calls, pass DS and DX with 0FFFFh. DOS is set up to
allow ONE handler for all 7 of these calls. Any call to these handlers
will result in the carry bit being set and AX will contain 1 if they are
not initialized. The handling routine is passed all registers just as
the user set them. The OEM handler routine should be exited through an
IRET.
2) 10 ms interval timer (Tandy?)
Interrupt 0F9h First of 8 SHELL service codes, reserved for OEM shell (WINDOW);
use like HP Vectra user interface?
Interrupt 0FAh USART ready (RS-232C)
Interrupt 0FBh USART RS ready (keyboard)
Interrupt 0FCh Unknown
Interrupt 0FDh reserved for user interrupt
Interrupt 0FEh AT/XT286/PS50+ - destroyed by return from protected mode
Interrupt 0FFh AT/XT286/PS50+ - destroyed by return from protected mode
