The function fails if:Given an ASCIIZ pathname, opens, creates or replaces a file in the designated or default directory on the designated or default disk drive. Returns a handle that can be used by the program for subsequent access to the file.
Call with:
AH = 6CH
AL = 00H
BX = open mode
Bit(s) Significance
0—2 access type
000 = read-only
001 = write-only
010 = read/write
3 reserved (0)
4—6 sharing mode
000 = compatibility
001 = deny read/write (deny all)
010 = deny write
011 = deny read
100 = deny none
7 inheritance
0 = child process inherits handle
1 = child does not inherit handle
8—12 reserved (0)
13 critical error handling
0 = execute Int 24H
1 = return error to process
14 write-through
0 = writes may be buffered and deferred
1 = physical write at request time
15 reserved (0)
CX = file attribute (bits may be combined; ignored if open)
Bit(s) Significance (if set)
0 read-only
1 hidden
2 system
3 volume label
4 reserved (0)
5 archive
6—15 reserved (0)
DX = open flag
Bits Significance
0—3 action if file exists
0000 = fail
0001 = open file
0010 = replace file
4—7 action if file doesn't exist
0000 = fail
0001 = create file
8—15 reserved (0)
DS:SI = segment:offset of ASCIIZ pathname
Returns:
If function successful
Carry flag = clear
AX = handle
CX = action taken
1 = file existed and was opened
2 = file did not exist and was created
3 = file existed and was replaced
If function failed
Carry flag = set
AX = error code
Notes:
The function fails if:
any element of the pathname does not exist.
the file is being created in the root directory and the root directory is full.
the file is being created and a file with the same name and the read-only attribute already exists in the specified directory.
the program is running on a network and the user running the program has insufficient access rights.
A file is usually given a normal (0) attribute when it is created. The file's attribute can subsequently be modified with Int 21H Function 43H.
This function combines the capabilities of Int 21H Functions 3CH, 3DH, and 5BH. It was added to MS-DOS for compatibility with the DosOpen function of OS/2.
Example:
Create the file MYFILE.DAT, if it does not already exist, in directory \MYDIR on drive C, and save the handle for subsequent access to the file.
fname db 'C:\MYDIR\MYFILE.DAT',0
fhandle dw ?
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mov ax,6c00h ; function number
mov bx,4042h ; read/write, deny none,
; write-through mode
xor cx,cx ; normal attribute
mov dx,0010h ; create if doesn't exist,
; fail if exists
mov si,seg fname ; address of pathname
mov ds,si
mov si,offset fname
int 21h ; transfer to MS-DOS
jc error ; jump if open failed
mov fhandle,ax ; save file handle
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