MS-DOS 2.0

System Calls Reference

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| |
| C A V E A T P R O G R A M M E R |
| |
| Certain structures, constants and system calls below |
| are private to the DOS and are extremely |
| version-dependent. They may change at any time at the |
| implementors' whim. As a result, they must not be |
| documented to the general public. If an extreme case |
| arises, they must be documented with this warning. |
| |
| Those structures and constants that are subject to the |
| above will be marked and bracketed with the flag: |
| |
| C A V E A T P R O G R A M M E R |
| |
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Section 1

Extensions to existing call structure

Name: * Alloc - allocate memory

Assembler usage:
MOV BX,size
MOV AH,Alloc
INT 21h
; AX:0 is pointer to allocated memory
; if alloc fails, BX is the largest block available

Description:
Alloc returns a pointer to a free block of memory
that has the requested size in paragraphs.

Error return:
AX = error_not_enough_memory
The largest available free block is smaller
than that requested or there is no free block.
= error_arena_trashed
The internal consistency of the memory arena
has been destroyed. This is due to a user
program changing memory that does not belong
to it.

Name: * CharOper - change incompatible configuration
parameters

Assembler usage:
MOV AH, CharOper
MOV AL, func
MOV DL, data
INT 21h
; on read functions, data is returned in DL

Description:
CharOper allows a program to change system
parameters to allow for switch indicators and whether
devices are available at every level of the directory
tree.

A function code is passed in AL:

AL Function
-- --------
0 DL, on return, will contain the DOS switch
character. On most systems this will default to
'-'.
1 Set the switch character to the character in DL.
2 Read the device availability byte into DL. If
this byte is 0, then devices must be accessed in
file I/O calls by /dev/device. If this byte is
non-zero, then the devices are available at every
node of the directory tree (i.e. CON is the
console device not the file CON). This byte is
generally 0.
3 Set the device availability byte to the value in
DL.

Error returns:
AL = FF
The function code specified in AL is not in
the range 0:3

Name: * CurrentDir - return text of current directory

Assembler usage:
MOV AH,CurrentDir
LDS SI,area
MOV DL,drive
INT 21h
; DS:SI is a pointer to 64 byte area that contains
; drive current directory.

Description:
CurrentDir returns the current directory for a
particular drive. The directory is root-relative and
does not contain the drive specifier. The drive code
passed in DL is 0=default, 1=A, 2=B, etc.

Error returns:
AX = error_invalid_drive
The drive specified in DL was invalid.

Name: * Dealloc - free allocated memory

Assembler usage:
MOV ES,block
MOV AH,dealloc
INT 21h

Description:
Dealloc returns a piece of memory to the system
pool that was allocated by alloc.

Name: * FileTimes - get/set the write times of a
handle

Assembler usage:
MOV AH, FileTimes
MOV AL, func
MOV BX, handle
; if AL = 1 then then next two are mandatory
MOV CX, time
MOV DX, date
INT 21h
; if AL = 0 then CX/DX has the last write time/date
; for the handle.

Description:
FileTimes returns or sets the last-write time for
a handle. These times are not recorded until the file
is closed.

A function code is passed in AL:

AL Function
-- --------
0 Return the time/date of the handle in CX/DX
1 Set the time/date of the handle to CX/DX

Error returns:
AX = error_invalid_function
The function passed in AL was not in the range
0:1.
= error_invalid_handle
The handle passed in BX was not currently
open.

Name: * FindFirst - find matching file

Assembler usage:
MOV AH, FindFirst
LDS DX, pathname
MOV CX, attr
INT 21h
; DMA address has datablock

Description:
FindFirst takes a pathname with wildcards in the
last component (passed in DS:DX), a set of attributes
(passed in CX) and attempts to find all files that
match the pathname and have a subset of the required
attributes. A datablock at the current DMA is written
that contains information in the following form:

find_buf STRUC
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| C A V E A T P R O G R A M M E R |
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find_buf_sattr DB ? ; attribute of search
find_buf_drive DB ? ; drive of search
find_buf_name DB 11 DUP (?); search name
find_buf_LastEnt DW ? ; LastEnt
find_buf_ThisDPB DD ? ; This DPB
find_buf_DirStart DW ? ; DirStart
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| C A V E A T P R O G R A M M E R |
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find_buf_attr DB ? ; attribute found
find_buf_time DW ? ; time
find_buf_date DW ? ; date
find_buf_size_l DW ? ; low(size)
find_buf_size_h DW ? ; high(size)
find_buf_pname DB 13 DUP (?) ; packed name
find_buf ENDS

To obtain the subsequent matches of the pathname,
see the description of FindNext

Error Returns:
AX = error_file_not_found
The path specified in DS:DX was an invalid
path.
= error_no_more_files
There were no files matching this
specification.

Name: * FindNext - step through a directory matching
files

Assembler usage:
; DMA points at area returned by find_first
MOV AH, findnext
INT 21h
; next entry is at dma

Description:
FindNext finds the next matching entry in a
directory. The current DMA address must point at a
block returned by FindFirst (see FindFirst).

Error Returns:
AX = error_no_more_files
There are no more files matching this pattern.

Name: * GetDMA - get current DMA transfer address

Assembler usage:
MOV AH,GetDMA
INT 21h
; ES:BX has current DMA transfer address

Description:
Return DMA transfer address.

Error returns:
None.

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Name: * GetDSKPT(DL) - get pointer to drive parameter
block

Assembler usage:
MOV AH,GetDSKPT
INT 21h
; DS:BX has address of drive parameter block

Description:
Return pointer to default drive parameter block.

Error returns:
None.

Assembler usage:
MOV DL,DrvNUM
MOV AH,GetDSKPTDL
INT 21h
; DS:BX has address of drive parameter block

Description:
Return pointer to drive parameter block for drive
designated in DL (0=Default, A=1, B=2 ...)

Error returns:
AL = FF
The drive given in DL is invalid.
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Name: * GetFreespace - get Disk free space

Assembler usage:
MOV AH,GetFreespace
MOV DL,Drive ;0 = default, A = 1
INT 21h
; BX = Number of free allocation units on drive
; DX = Total number of allocation units on drive
; CX = Bytes per sector
; AX = Sectors per allocation unit

Description:
Return Free space on disk along with additional
information about the disk.

Error returns:
AX = FFFF
The drive number given in DL was invalid.

NOTE: This call returns the same information in the same
registers (except for the FAT pointer) as the get FAT
pointer calls did in previous versions of the DOS.

Name: * GetInDOSF - get DOS critical-section flag

Assembler usage:
MOV AH,GetInDOSF
INT 21h
; ES:BX has location of the flag
MOV CritSEG, ES
MOV CritOFF, BX
...
IntVec:
MOV AX, DWORD PTR Crit
CMP AX,0
JZ DoFunc
IRET
DoFunc: ...

Description:
Return location of indos flag. On return ES:BX is
the address of a byte memory cell inside the DOS. If
used in an interrupt service routine, it indicates
whether or not the DOS was interrupted in a critical
section. If the cell was zero, then the DOS was not
in a critical section and thus can be called by the
interrupt routine. If the cell was non-zero, the DOS
should be considered to be in an uninterruptable state
and for reliability, no DOS calls should be given.

Error returns:
None.

Name: * GetVector - get interrupt vector

Assembler usage:
MOV AH,GetVector
MOV AL,interrupt
INT 21h
; ES:BX now has long pointer to interrupt routine

Description:
Return interrupt vector associated with an
interrupt.

Error returns:
None.

Name: * GetVerifyFlag - return current setting of the
verify after write flag.

Assembler usage:
MOV AH,GetVerifyFlag
INT 21h
; AL is the current verify flag value

Description:
The current value of the verify flag is returned
in AL.

Error returns:
None.

Name: * GetVersion - get DOS version number

Assembler usage:
MOV AH,GetVersion
INT 21h
; AL is the major version number
; AH is the minor version number
; BH is the OEM number
; BL:CX is the (24 bit) user number

Description:
Return MS-DOS version number. On return AL.AH
will be the two part version designation, ie. for
MS-DOS 1.28 AL would be 1 and AH would be 28. For pre
1.28 DOS AL = 0. Note that version 1.1 is the same as
1.10, not the same as 1.01.

Error returns:
None.

Name: * International - return country dependent
information

Assembler usage:
LDS DX, blk
MOV AH, International
MOV AL, func
INT 21h

Description:
This call returns in the block of memory pointed
to by DS:DX, the following information pertinent to
international applications:

+---------------------------+
| WORD Date/time format |
+---------------------------+
| BYTE ASCIZ string |
| currency symbol |
+---------------------------+
| BYTE ASCIZ string |
| thousands separator |
+---------------------------+
| BYTE ASCIZ string decimal |
| separator |
+---------------------------+

The date/time format has the following values and
meanings:

0 - USA standard h:m:s m/d/y
1 - Europe standard h:m:s d/m/y
2 - Japan standard y/m/d h:m:s

The value passed in AL is either 0 (for current
country) or a country code (to be defined later.
Currently the country code must be zero).

Error returns:
AX = error_invalid_function
The function passed in AL was not 0
(currently).

Name: * KeepProcess - terminate process and remain
resident

Assembler usage:
MOV AL, exitcode
MOV DX, parasize
MOV AH, KeepProcess
INT 21h

Description:
This call terminates the current process and
attempts to set the initial allocation block to a
specific size in paragraphs. It will not free up any
other allocation blocks belonging to that process.
The exit code passed in AX is retrievable by the
parent via Wait.

Error Returns:
None.

Name: * Rename - move a directory entry

Assembler usage:
LDS DX, source
LES DI, dest
MOV AH, Rename
INT 21h

Description:
Rename will attempt to rename a file into another
path. The paths must be on the same device.

Error returns:
AX = error_file_not_found
The file name specifed by DS:DX was not found.
= error_not_same_device
The source and destination are on different
drives.
= error_access_denied
The path specified in DS:DX was a directory or
the file specified by ES:DI exists or the
destination directory entry could not be
created.

Name: * SetBlock - modify allocated blocks

Assembler usage:
MOV ES,block
MOV BX,newsize
MOV AH,setblock
INT 21h
; if setblock fails for growing, BX will have the
; maximum size possible

Description:
Setblock will attempt to grow/shrink an allocated
block of memory.

Error return:
AX = error_invalid_block
The block passed in ES is not one allocated
via Alloc.
= error_arena_trashed
The internal consistency of the memory arena
has been destroyed. This is due to a user
program changing memory that does not belong
to it.
= error_not_enough_memory
There was not enough free memory after the
specified block to satisfy the grow request.

Name: * SetCtrlCTrapping - turn on/off broad ^C
checking

Assembler usage:
MOV DL,val
MOV AH,SetCtrlCTrapping
MOV AL,func
INT 21h
; If AL was 0, then DL has the current value of the
; ^C check

Description:
MSDOS ordinarily checks for a ^C on the
controlling device only when doing a function 1-12
operation to that device. SetCtrlCTrapping allows the
user to expand this checking to include any system
call. For example, with the ^C trapping off, all disk
I/O will proceed without interruption while with ^C
trapping on, the ^C interrupt is given at the system
call that initiates the disk operation.

Error return:
AL = FF
The function passed in AL was not in the range
0:1.

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Name: * Set_OEM_Handler - set handler for OEM
specific INT 21H calls.

Assembler usage:
LDS DX,handler_address
MOV AH,Set_OEM_Handler
INT 21H

Description:
Set handler address for 0F9H-0FFH INT 21H system
calls to DS:DX. To return the 0F9H-0FFH calls to
the uninitialized state, give DS=DX=-1.

Error returns:
None.

Handler entry:
All registers as user set them when INT 21H
issued (including SS:SP). INT 21 return is on
stack, so the correct method for the OEM handler
to return to the user is to give an IRET. The
OEM handler is free to make any INT 21H system
call (including the 0F9H- 0FFH group if the OEM
handler is re-entrant).

The AH INT 21H function codes 0F8H through 0FFH are
reserved for OEM extensions to the INT 21H calling
convention. These calls have two states, initialized
and uninitialized. There will be one handler for all 7
(0F9-0FFH) functions. When the DOS is first
initialized, these calls are uninitialized. The AH=0F8H
call is the call which will set the handler address for
the 0F9-0FFH calls. If the 0F9-0FFH calls are
uninitialized, an attempt to call them results in the
normal invalid system call number return.
OEMs should NOT document the 0F8 call.
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Section 2

XENIX-compatible system calls

Previous to version 2.0, MSDOS had a simple single
directory structure that sufficed for small (160k to 320K)
diskettes. As the need for hard disk support grows, and
as MSDOS 2.0 will support a wide variety of hard disks,
the need for better disk organization also grows. Merely
expanding the directory is not an effective solution;
doing a 'DIR' on a directory with 1000 files is not a
user-friendly characteristic.

People, by nature, think in hierarchical terms:
organization charts and family trees, for example. It
would be nice to allow users to organize their files on
disk in a similar manner. Consider the following:

In a particular business, both sales and accounting
share a computer with a large disk and the individual
employees use it for preparation of reports and
maintaining accounting information. One would naturally
view the organization of files on the disk in this
fashion:

+-disk-+
/ \
/ \
/ \
sales accounting
/ | | \
/ | | \
/ | | \
John Mary Steve Sue
/ | (A) | | | \
/ | | | | \
/ | | | | \
report accts. report accts. report report
receiv. receiv

In MSDOS 2.0 the user can arrange his files in such a
manner that files that are not part of his current task do
not interfere with that task. Pre-2.0 versions of MSDOS
has a single directory that contains files. MSDOS extends
this concept to allow a directory to contain both files
and directories and to introduce the notion of the
'current' directory.

To specify a filename, the user could use one of two
methods, either specify a path from the root node to the
file, or specify a path from the current node to the file.
A path is a series of directory names separated by '/' and
ending with a filename. A path that starts at the root
begins with a '/'.

There is a special directory entry in each directory,
denoted by '..' that is the parent of the directory. The
root directory's parent is itself (who created God?).

Using a directory structure like the hierarchy above,
and assuming that the current directory is at point (D),
to reference the report under John, the following are all
equivalent:

report
/sales/John/report
../John/report

To refer to the report under Mary, the following are
all equivalent:

../Mary/report
/sales/Mary/report

To refer to the report under Sue, the following are
all equivalent.

../../accounting/Sue/report
/accounting/Sue/report

There is no restriction in MSDOS 2.0 on the depth of a
tree (the length of the longest path from root to leaf)
except in the number of allocation units available. The
root directory will have a fixed number of entries, 64 for
the single sided diskettes to XXX for a large hard disk.
For non-root directories, there is no limit to the number
of files per directory excepting in the number of
allocation units available.

Old (pre-2.0) disks will appear to MSDOS 2.0 as having
only a root directory with files in it and no
subdirectories whatever.

Implementation of the tree-structure is simple. The
root directory is the pre-2.0 directory. Subdirectories
of the root have a special attribute set indicating that
they are directories. The subdirectories themselves are
files, linked through the FAT as usual. Their contents
are identical in character to the contents of the root
directory.

Pre-2.0 programs that use system calls not described
below will not be able to make use of files in other
directories. They will only be able to access files in
the current directory. This is no great loss of
functionality as users will aggregate their files into
sub-directories on basis of functionality; the files that
are being used will be found in the current directory.
Those that are not necessary for the current task will be
placed in other directories. Out of sight, out of mind.

There are also new attributes in 2.0. These and the
old attributes apply to the tree structured directories in
the following manner:

Attribute Meaning/Function Meaning/Function
for files for directories

volume_id Present at the root. Meaningless.
Only one file may have
this set.

directory Meaningless. Indicates that the
directory entry is a
directory. Cannot be
changed with ChMod.

read_only Old fcb-create, new Meaningless.
Creat, new open (for
write or read/write)
will fail.

archive Set when file is Meaningless.
written. Set/reset via
ChMod.

hidden/ Prevents file from Prevents directory
system being found in search entry from being
first/search next. found. ChDir to
New open will fail. directory will still
work.

Name: * ChDir - Change the current directory

Assembler usage:
LDS DX, name
MOV AH, ChDir
INT 21h

Description:
ChDir is given the ASCIZ name of the directory
which is to become the current directory. If any
member of the specified pathname does not exist, then
the current directory is unchanged. Otherwise, the
current directory is set to the string.

Error returns:
AX = error_path_not_found
The path specified in DS:DX either indicated a
file or the path was invalid.

Name: * ChMod - change write protection

Assembler usage:
LDS DX, name
MOV CX, attribute
MOV AL, func
MOV AH, ChMod
INT 21h

Description:
Given an ASCIZ name, ChMod will set/get the
attributes of the file to those given in CX.

A function code is passed in AL:

AL Function
-- --------
0 Return the attributes of the file in CX
1 Set the attributes of the file to those in CX

Error returns:
AX = error_path_not_found
The path specified was invalid.
= error_access_denied
The attributes specified in CX contained one
that could not be changed (directory, volume
ID).
= error_invalid_function
The function passed in AL was not in the range
0:1.

Name: * Close - close a file handle

Assembler usage:
MOV BX, handle
MOV AH, Close
INT 21h

Description:
In BX is passed a file handle (like that returned
by Open, Creat or Dup); the Close call will close the
associated file. Internal buffers are flushed.

Error return:
AX = error_invalid_handle
The handle passed in BX was not currently
open.

Name: * Creat - create a file

Assembler usage:
LDS DX, name
MOV AH, Creat
MOV CX, attribute
INT 21h
; AX now has the handle

Description:
Creat creates a new file or truncates an old file
to zero length in preparation for writing. If the
file did not exist, then the file is created in the
appropriate directory and the file is given the
read/write protection code of access.

CX contains the default attributes to be set for
the file. Currently, the read-only bit must be off.

Error returns:
AX = error_access_denied
The attributes specified in CX contained one
that could not be created (directory, volume
ID), a file already existed with a more
inclusive set of attributes, or a directory
existed with the same name.
= error_path_not_found
The path specified was invalid.
= error_too_many_open_files
The file was created with the specified
attributes, but there were no free handles
available for the process or that the internal
system tables were full.

Name: * Dup - duplicate a file handle

Assembler usage:
MOV BX, fh
MOV AH, Dup
INT 21h
; AX has the returned handle

Description:
Dup takes an already opened file handle and
returns a new handle that refers to the same file at
the same position.

Error returns:
AX = error_invalid_handle
The handle passed in BX was not currently
open.
= error_too_many_open_files
There were no free handles available in the
current process or the internal system tables
were full.

Name: * Dup2 - force a duplicate of a handle

Assembler usage:
MOV BX, fh
MOV CX, newfh
MOV AH, Dup2
INT 21h

Description:
Dup2 will cause newfh to refer to the same stream
as fh. If there was an open file on newfh, then it is
closed first.

Error returns:
AX = error_invalid_handle
The handle passed in BX was not currently
open.

Name: * Exec - load / execute a program

Assembler usage:
LDS DX, name
LES BX, blk
MOV AH, Exec
MOV AL, func
INT 21h

Description:
This call allows a program to load another program
into memory and (default) begin execution of it.
DS:DX points to the ASCIZ name of the file to be
loaded. ES:BX points to a parameter block for the
load.

A function code is passed in AL:

AL Function
-- --------
0 Load and execute the program. A program header is
established for the program and the terminate and
^C addresses are set to the instruction after the
EXEC system call.

NOTE: When control is returned, via a ^C or
terminate, from the program being EXECed ALL
registers are altered including the stack.
This is because control is returned from the
EXECed program, not the system. To regain
your stack, store an SS:SP value in a data
location reachable from your CS.

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| C A V E A T P R O G R A M M E R |
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1 Load, create the program header but do not begin
execution. The CS:IP/SS:SP of the program are
returned in the area provided by the user.
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| C A V E A T P R O G R A M M E R |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

3 Load, do not create the program header, and do not
begin execution. This is useful in loading
program overlays.

For each value of AL, the block has the following
format:

AL = 0 -> load/execute program

+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| C A V E A T P R O G R A M M E R |
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AL = 1 -> load program

AL = 3 -> load overlay

+---------------------------+
| WORD segment address where|
| file will be loaded. |
+---------------------------+
| WORD relocation factor to |
| be applied to the image. |
+---------------------------+

Note that all open files of a process are
duplicated in the child process after an Exec. This
is extremely powerful; the parent process has control
over the meanings of stdin, stdout, stderr, stdaux and
stdprn. The parent could, for example, write a series
of records to a file, open the file as standard input,
open a listing file as standard output and then Exec a
sort program that takes its input from stdin and
writes to stdout.

Also inherited (or passed from the parent) is an
'environment'. This is a block of text strings (less
than 32K bytes total) that convey various
configurations parameters. The format of the
environment is as follows:

(paragraph boundary)
+---------------------------+
| BYTE asciz string 1 |
+---------------------------+
| BYTE asciz string 2 |
+---------------------------+
| ... |
+---------------------------+
| BYTE asciz string n |
+---------------------------+
| BYTE of zero |
+---------------------------+

Typically the environment strings have the form:

parameter=value

for example, COMMAND.COM always passes its execution
search path as:

PATH=A:/BIN;B:/BASIC/LIB

A zero value of the environment address will cause the
child process to inherit the parent's environment
unchanged.

Note that on a successful return from EXEC, all
registers, except for CS:IP, are changed.

Error return:
AX = error_invalid_function
The function passed in AL was not 0, 1 or 3.
= error_bad_environment
The environment was larger than 32Kb.
= error_bad_format
The file pointed to by DS:DX was an EXE format
file and contained information that was
internally inconsistent.
= error_not_enough_memory
There was not enough memory for the process to
be created.
= error_file_not_found
The path specified was invalid or not found.

Name: * Exit - terminate a process

Assembler usage:
MOV AL, code
MOV AH, Exit
INT 21h

Description:
Exit will terminate the current process,
transferring control to the invoking process. In
addition, a return code may be sent. All files open
at the time are closed.

Error returns:
None.

Name: * Ioctl - I/O control for devices

Assembler usage:
MOV BX, Handle

(or MOV BL, drive for calls AL=4,5
0=default,A=1...)

MOV DX, Data

(or LDS DX, buf and
MOV CX, count for calls AL=2,3,4,5)

MOV AH, Ioctl
MOV AL, func
INT 21h
; For calls AL=2,3,4,5 AX is the number of bytes
; transferred (same as READ and WRITE).
; For calls AL=6,7 AL is status returned, AL=0 if
; status is not ready, AL=0FFH otherwise.

Description:
Set or Get device information associated with open
Handle, or send/receive control string to device
Handle or device.

The following values are allowed for func:

Request Function
------ --------
0 Get device information (returned in DX)
1 Set device information (as determined by DX)
2 Read CX number of bytes into DS:DX from device
control channel.
3 Write CX number of bytes from DS:DX to device
control channel.
4 Same as 2 only drive number in BL
0=default,A=1,B=2,...
5 Same as 3 only drive number in BL
0=default,A=1,B=2,...
6 Get input status
7 Get output status

Ioctl can be used to get information about device
channels. It is ok to make Ioctl calls on regular
files but only calls 0,6 and 7 are defined in that
case (AL=0,6,7), all other calls return an
error_invalid_function error.

CALLS AL=0 and AL=1

The bits of DX are defined as follows for calls
AL=0 and AL=1. Note that the upper byte MUST be zero
on a set call.

|
15 14 13 12 11 10 9 8|7 6 5 4 3 2 1 0
+--+--+--+--+--+--+-+-+-+-+-+-+-+-+-+-+
| R| C| |I|E|R|S|I|I|I|I|
| e| T| |S|O|A|P|S|S|S|S|
| s| R| Reserved |D|F|W|E|C|N|C|C|
| | L| |E| | |C|L|U|O|I|
| | | |V| | |L|K|L|T|N|
+--+--+--+--+--+--+-+-+-+-+-+-+-+-+-+-+
|

ISDEV = 1 if this channel is a device
= 0 if this channel is a disk file (Bits 8-15 =
0 in this case)

If ISDEV = 1

EOF = 0 if End Of File on input
RAW = 1 if this device is in Raw mode
= 0 if this device is cooked
ISCLK = 1 if this device is the clock device
ISNUL = 1 if this device is the null device
ISCOT = 1 if this device is the console output
ISCIN = 1 if this device is the console input
SPECL = 1 if this device is special

CTRL = 0 if this device can NOT do control strings
via calls AL=2 and AL=3.
CTRL = 1 if this device can process control
strings via calls AL=2 and AL=3.
NOTE that this bit cannot be set.

If ISDEV = 0
EOF = 0 if channel has been written
Bits 0-5 are the block device number for the
channel (0 = A, 1 = B, ...)

Bits 15,8-13,4 are reserved and should not be altered.

Calls 2..5:
These four calls allow arbitrary control strings to be
sent or received from a device. The Call syntax is
the same as the READ and WRITE calls, except for 4 and
5 which take a drive number in BL instead of a handle
in BX.

An error_invalid_function error is returned if the
CTRL bit (see above) is 0.

An error_access_denied is returned by calls AL=4,5 if
the drive number is invalid.

Calls 6,7:
These two calls allow the user to check if a file
handle is ready for input or output. Status of
handles open to a device is the intended use of these
calls, but status of a handle open to a disk file is
OK and is defined as follows:

Input:
Always ready (AL=FF) until EOF reached, then
always not ready (AL=0) unless current
position changed via LSEEK.
Output:
Always ready (even if disk full).

IMPORTANT NOTE:
The status is defined at the time the system is
CALLED. On future versions, by the time control is
returned to the user from the system, the status
returned may NOT correctly reflect the true current
state of the device or file.

Error returns:
AX = error_invalid_handle
The handle passed in BX was not currently
open.
= error_invalid_function
The function passed in AL was not in the range
0:7.
= error_invalid_data
= error_access_denied (calls AL=4..7)

Name: * LSeek - move file read/write pointer

Assembler usage:
MOV DX, offsetlow
MOV CX, offsethigh
MOV AL, method
MOV BX, handle
MOV AH, LSeek
INT 21h
; DX:AX has the new location of the pointer

Description:
LSeek moves the read/write pointer according to
method:

Method Function
------ --------
0 The pointer is moved to offset bytes from the
beginning of the file.
1 The pointer is moved to the current location
plus offset.
2 The pointer is moved to the end of file plus
offset.

Offset should be regarded as a 32-bit integer with
CX occupying the most significant 16 bits.

Error returns:
AX = error_invalid_handle
The handle passed in BX was not currently
open.
= error_invalid_function
The function passed in AL was not in the range
0:2.

Name: * MkDir - Create a directory entry

Assembler usage:
LDS DX, name
MOV AH, MkDir
INT 21h

Description:
Given a pointer to an ASCIZ name, create a new
directory entry at the end.

Error returns:
AX = error_path_not_found
The path specified was invalid or not found.
= error_access_denied
The directory could not be created (no room in
parent directory), the directory/file already
existed or a device name was specified.

Name: * Open - access a file

Assembler usage:
LDS DX, name
MOV AH, Open
MOV AL, access
INT 21h
; AX has error or file handle
; If successful open

Description:
Open associates a 16-bit file handle with a file.

The following values are allowed for access:

ACCESS Function
------ --------
0 file is opened for reading
1 file is opened for writing
2 file is opened for both reading and writing.

DS:DX point to an ASCIZ name of the file to be
opened.

The read/write pointer is set at the first byte of
the file and the record size of the file is 1 byte.
The returned file handle must be used for subsequent
I/O to the file.

The DOS, on initialization, will have a maximum
number of files. See the configuration file document
for information on changing this default.

Error returns:
AX = error_invalid_access
The access specified in AL was not in the
range 0:2.
= error_file_not_found
The path specified was invalid or not found.
= error_access_denied
The user attempted to open a directory or
volume-id, or open a read-only file for
writing.
= error_too_many_open_files
There were no free handles available in the
current process or the internal system tables
were full.

Name: * Read - Do file/device I/O

Assembler usage:
LDS DX, buf
MOV CX, count
MOV BX, handle
MOV AH, Read
INT 21h
; AX has number of bytes read

Description:
Read transfers count bytes from a file into a
buffer location. It is not guaranteed that all count
bytes will be read; for example, reading from the
keyboard will read at most one line of text. If the
returned value is zero, then the program has tried to
read from the end of file.

All I/O is done using normalized pointers; no
segment wraparound will occur.

Error returns:
AX = error_invalid_handle
The handle passed in BX was not currently
open.
= error_access_denied
The handle passed in BX was opened in a mode
that did not allow reading.

Name: * RmDir - Remove a directory entry

Assembler usage:
LDS DX, name
MOV AH, RmDir
INT 21h

Description:
RmDir is given an asciz name of a directory. That
directory is removed from its parent

Error returns:
AX = error_path_not_found
The path specified was invalid or not found.
= error_access_denied
The path specified was not empty, not a
directory, the root directory or contained
invalid information.
= error_current_directory
The path specified was the current directory
on a drive.

Name: * Unlink - delete a directory entry

Assembler usage:
LDS DX, name
MOV AH, Unlink
INT 21h

Description:
Unlink removes a directory entry associated with a
filename. If the file is currently open on another
handle, then no removal will take place.

Error returns:
AX = error_file_not_found
The path specified was invalid or not found.
= error_access_denied
The path specified was a directory or
read-only.

Name: * Wait - retrieve the return code of a child

Assembler usage:
MOV AH, Wait
INT 21h
; AX has the exit code

Description:
Wait will return the Exit code specified by a
child process. It will return this Exit code only
once. The low byte of this code is that sent by the
Exit routine. The high byte is one of the following:

0 - terminate/abort
1 - ^C
2 - Hard error
3 - Terminate and stay resident

Error returns:
None.

Name: * Write - write to a file

Assembler usage:
LDS DX, buf
MOV CX, count
MOV BX, handle
MOV AH, Write
INT 21h
; AX has number of bytes written

Description:
Write transfers count bytes from a buffer into
a file. It should be regarded as an error if the
number of bytes written is not the same as the number
requested.

It is important to note that the write system
call with a count of zero (CX = 0) will truncate
the file at the current position.

All I/O is done using normalized pointers; no
segment wraparound will occur.

Error Returns:
AX = error_invalid_handle
The handle passed in BX was not currently
open.
= error_access_denied
The handle was not opened in a mode that
allowed writing.

The following XENIX convention is followed for the new 2.0
system calls:

o If no error occurred, then the carry flag will be
reset and register AX will contain the appropriate
information.

o If an error occurred, then the carry flag will be
set and register AX will contain the error code.

The following code sample illustrates the recommended method
of detecting these errors:

...
MOV errno,0
INT 21h
JNC continue
MOV errno,AX
continue:
...

The word variable errno will now have the correct error code
for that system call.

The current equates for the error codes are:

no_error_occurred EQU 0

error_invalid_function EQU 1
error_file_not_found EQU 2
error_path_not_found EQU 3
error_too_many_open_files EQU 4
error_access_denied EQU 5
error_invalid_handle EQU 6
error_arena_trashed EQU 7
error_not_enough_memory EQU 8
error_invalid_block EQU 9
error_bad_environment EQU 10
error_bad_format EQU 11
error_invalid_access EQU 12
error_invalid_data EQU 13
error_invalid_drive EQU 15
error_current_directory EQU 16
error_not_same_device EQU 17
error_no_more_files EQU 18

System call assignments:

ABORT EQU 0 ; 0 0
STD_CON_INPUT EQU 1 ; 1 1
STD_CON_OUTPUT EQU 2 ; 2 2
STD_AUX_INPUT EQU 3 ; 3 3
STD_AUX_OUTPUT EQU 4 ; 4 4
STD_PRINTER_OUTPUT EQU 5 ; 5 5
RAW_CON_IO EQU 6 ; 6 6
RAW_CON_INPUT EQU 7 ; 7 7
STD_CON_INPUT_NO_ECHO EQU 8 ; 8 8
STD_CON_STRING_OUTPUT EQU 9 ; 9 9
STD_CON_STRING_INPUT EQU 10 ; 10 A
STD_CON_INPUT_STATUS EQU 11 ; 11 B
STD_CON_INPUT_FLUSH EQU 12 ; 12 C
DISK_RESET EQU 13 ; 13 D
SET_DEFAULT_DRIVE EQU 14 ; 14 E
FCB_OPEN EQU 15 ; 15 F
FCB_CLOSE EQU 16 ; 16 10
DIR_SEARCH_FIRST EQU 17 ; 17 11
DIR_SEARCH_NEXT EQU 18 ; 18 12
FCB_DELETE EQU 19 ; 19 13
FCB_SEQ_READ EQU 20 ; 20 14
FCB_SEQ_WRITE EQU 21 ; 21 15
FCB_CREATE EQU 22 ; 22 16
FCB_RENAME EQU 23 ; 23 17
GET_DEFAULT_DRIVE EQU 25 ; 25 19
SET_DMA EQU 26 ; 26 1A
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| C A V E A T P R O G R A M M E R |
| |
GET_DEFAULT_DPB EQU 31 ; 31 1F
| |
| C A V E A T P R O G R A M M E R |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
FCB_RANDOM_READ EQU 33 ; 33 21
FCB_RANDOM_WRITE EQU 34 ; 34 22
GET_FCB_FILE_LENGTH EQU 35 ; 35 23
GET_FCB_POSITION EQU 36 ; 36 24
SET_INTERRUPT_VECTOR EQU 37 ; 37 25
CREATE_PROCESS_DATA_BLOCK EQU 38 ; 38 26
FCB_RANDOM_READ_BLOCK EQU 39 ; 39 27
FCB_RANDOM_WRITE_BLOCK EQU 40 ; 40 28
PARSE_FILE_DESCRIPTOR EQU 41 ; 41 29
GET_DATE EQU 42 ; 42 2A
SET_DATE EQU 43 ; 43 2B
GET_TIME EQU 44 ; 44 2C
SET_TIME EQU 45 ; 45 2D
SET_VERIFY_ON_WRITE EQU 46 ; 46 2E
; Extended functionality group
GET_DMA EQU 47 ; 47 2F
GET_VERSION EQU 48 ; 48 30
KEEP_PROCESS EQU 49 ; 49 31
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| C A V E A T P R O G R A M M E R |
| |
GET_DPB EQU 50 ; 50 32
| |
| C A V E A T P R O G R A M M E R |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
SET_CTRL_C_TRAPPING EQU 51 ; 51 33
GET_INDOS_FLAG EQU 52 ; 52 34
GET_INTERRUPT_VECTOR EQU 53 ; 53 35
GET_DRIVE_FREESPACE EQU 54 ; 54 36
CHAR_OPER EQU 55 ; 55 37
INTERNATIONAL EQU 56 ; 56 38
; XENIX CALLS
; Directory Group
MKDIR EQU 57 ; 57 39
RMDIR EQU 58 ; 58 3A
CHDIR EQU 59 ; 59 3B
; File Group
CREAT EQU 60 ; 60 3C
OPEN EQU 61 ; 61 3D
CLOSE EQU 62 ; 62 3E
READ EQU 63 ; 63 3F
WRITE EQU 64 ; 64 40
UNLINK EQU 65 ; 65 41
LSEEK EQU 66 ; 66 42
CHMOD EQU 67 ; 67 43
IOCTL EQU 68 ; 68 44
XDUP EQU 69 ; 69 45
XDUP2 EQU 70 ; 70 46
CURRENT_DIR EQU 71 ; 71 47
; Memory Group
ALLOC EQU 72 ; 72 48
DEALLOC EQU 73 ; 73 49
SETBLOCK EQU 74 ; 74 4A
; Process Group
EXEC EQU 75 ; 75 4B
EXIT EQU 76 ; 76 4C
WAIT EQU 77 ; 77 4D
FIND_FIRST EQU 78 ; 78 4E
; Special Group
FIND_NEXT EQU 79 ; 79 4F
; SPECIAL SYSTEM GROUP
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| C A V E A T P R O G R A M M E R |
| |
SET_CURRENT_PDB EQU 80 ; 80 50
GET_CURRENT_PDB EQU 81 ; 81 51
GET_IN_VARS EQU 82 ; 82 52
SETDPB EQU 83 ; 83 53
| |
| C A V E A T P R O G R A M M E R |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
GET_VERIFY_ON_WRITE EQU 84 ; 84 54
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| C A V E A T P R O G R A M M E R |
| |
DUP_PDB EQU 85 ; 85 55
| |
| C A V E A T P R O G R A M M E R |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
RENAME EQU 86 ; 86 56
FILE_TIMES EQU 87 ; 87 57
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