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.
This interrupt is becoming more commonly used as the availible Int\21
functions are getting to be in short supply. 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.
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 2.x does not
initialize the int 2Fh vector, so when you try to chain to it like you are
supposed to, it will crash. If your program checks the vector for being zero
and initializes it itself or doesn't chain in that case, it will work for you
n 2.x just the same as 3.x.
Function 01h PRINT.COM
Register AH identifies which program is to handle the interrupt.
Values 00h-7Fh are reserved for DOS, not that anyone cares much.
Values 0C0h-0FFh are reserved for applications.
Register AL contains the function code if used
entry AH 01h
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
Function 05h DOS 3.x critical error handler
entry AH 05h
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 (?)
Function 06h ASSIGN
entry AH 06h
00h installation check
return AH 0 if installed
01h get memory segment
return ES segment of ASSIGN work area
Function 10h SHARE
entry AH 10h
AL 00h installation check
return AL 00h not installed, OK to install
01h not installed, not OK to install
0FFh installed
Function 11h Multiplex - network redirection
entry AH 11h
AL 00h installation check
return AL 00h not installed, OK to install
01h not installed, not OK to install
0FFh installed
01h-05h unknown
06h close remote file
07h-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-16h unknown
17h unknown
stack word (?)
return (?)
18h unknown
stack word (?)
return (?)
19h-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-25h unknown
stack word (?)
26h unknown
Function 12h multiplex, DOS 3.x internal services
entry AH 12h
AL 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.COM file
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 ASCIIZ filename to normalize
ES:DI pointer to buffer for normalized filename
return destination buffer filled with uppercase
filename, with slashes turned to backslashes
12h get length of ASCIIZ string
ES:DI pointer to ASCIIZ 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, error code in AL
AL 06h (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\drivers 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
Function 43h Microsoft Extended Memory Specification (XMS)
Function 5453h TesSeRact Standard for Ram-Resident Program Communication
Function 64h SCRNSAV2.COM
entry AH 64h
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
Function 7Ah Novell NetWare
entry AH 7Ah
AL 00h installation check
note 1) Returns address of entry point for IPX and SPX
2) Parameters are listed under Int\21
Function 087h APPEND
entry AH 087h
AL 00h APPEND installation check
return AH 0 if installed
01h APPEND - unknown
02h APPEND - version check
Function 088h Microsoft Networks
entry AH 088h
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 network version check
Function 0AAh VIDCLOCK.COM
entry AH 0AAh
AL 00h installation check
return AL 00h not installed
0FFh installed
note VIDCLOCK.COM is a memory-resident clock by Thomas G. Hanlin III
Function 0B8h Network
Function 0BBh Network Functions
entry AH 0BBh
AL 00h net command installation check
01h, 02h unknown
03h get server POST address
04h get server POST address
Function 0F7h AUTOPARK.COM (PD TSR hard disk parking utility)
entry AH 0F7h
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
MSDOS 2Fh functions 01h (PRINT), 02h (ASSIGN), 10h (SHARE):
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.
3) Important! In versions of DOS prior to 3.0, the int 2Fh vector was
initialized to zero rather than being pointed into the DOS service area.
You must initialize this vector manually under DOS 2.x.