INT 21 - DOS 2+ - "OPEN" - OPEN EXISTING FILE
	AH = 3Dh
	AL = access and sharing modes (see #01402)
	DS:DX -> ASCIZA NUL-terminated ASCII string.	The ASCIZ string "ABC" consists of the four bytes 41h, 42h, 43h, and 00h.  Unless otherwise specified, maximum lengths given in the interrupt list do not include the terminating NUL. filename
	CL = attribute mask of files to look for (server call only)
Return: CF clear if successful
	    AX = file handle
	CF set on error
	    AX = error code (01h,02h,03h,04h,05h,0Ch,56h) (see #01680 at AH=59h)
Notes:	file pointer is set to start of file
	if SHARE or a network is loaded, the file open may fail if the file
	  is already open, depending on the combination of sharing modes
	  (see #01403,#01404)
	file handles which are inherited from a parent also inherit sharing
	  and access restrictions
	files may be opened even if given the hidden or system attributes
	under the FlashTek X-32 DOS extender, the pointer is in DS:EDX
	DR DOS checks the system password or explicitly supplied password at
	  the end of the filename (following a semicolon) against the reserved
	  field in the directory entry before allowing access
	sharing modes are only effective on local drives if SHARE is loaded
BUG:	Novell DOS 7 SHARE v1.00 would refuse file access in the cases in
	  #01403 marked with [1] (read-only open of a read-only file
	  which had previously been opened in compatibility mode); this was
	  fixed in SHARE v1.01 of 09/29/94
SeeAlso: AH=0Fh,AH=3Ch,AX=4301h,AX=5D00h,INT 2F/AX=1116h,INT 2F/AX=1226h


Bitfields for access and sharing modes:
Bit(s)	Description	(Table 01402)
 2-0	access mode
	000 read only
	001 write only
	010 read/write
	011 (DOS 5+ internal) passed to redirector on EXEC to allow
		case-sensitive filenames
 3	reserved (0)
 6-4	sharing mode (DOS 3.0+) (see #01403)
	000 compatibility mode
	001 "DENYALL" prohibit both read and write access by others
	010 "DENYWRITE" prohibit write access by others
	011 "DENYREAD" prohibit read access by others
	100 "DENYNONE" allow full access by others
	111 network FCBsee File Control Block (only available during server call)
 7	inheritance
	if set, file is private to current process and will not be inherited
	  by child processes
SeeAlso: #01782,#01403


(Table 01403)
Values of DOS 2-6.22 file sharing behavior:
	  |	Second and subsequent Opens
 First	  |Compat  Deny	  Deny	 Deny	Deny
 Open	  |	   All	  Write	 Read	None
	  |R W RW R W RW R W RW R W RW R W RW
 - - - - -| - - - - - - - - - - - - - - - - -
 Compat R |Y Y Y  N N N	 1 N N	N N N  1 N N
	W |Y Y Y  N N N	 N N N	N N N  N N N
	RW|Y Y Y  N N N	 N N N	N N N  N N N
 - - - - -|
 Deny	R |C C C  N N N	 N N N	N N N  N N N
 All	W |C C C  N N N	 N N N	N N N  N N N
	RW|C C C  N N N	 N N N	N N N  N N N
 - - - - -|
 Deny	R |2 C C  N N N	 Y N N	N N N  Y N N
 Write	W |C C C  N N N	 N N N	Y N N  Y N N
	RW|C C C  N N N	 N N N	N N N  Y N N
 - - - - -|
 Deny	R |C C C  N N N	 N Y N	N N N  N Y N
 Read	W |C C C  N N N	 N N N	N Y N  N Y N
	RW|C C C  N N N	 N N N	N N N  N Y N
 - - - - -|
 Deny	R |2 C C  N N N	 Y Y Y	N N N  Y Y Y
 None	W |C C C  N N N	 N N N	Y Y Y  Y Y Y
	RW|C C C  N N N	 N N N	N N N  Y Y Y
Legend: Y = open succeeds, N = open fails with error code 05h
	C = open fails, INT 24 generated
	1 = open succeeds if file read-only, else fails with error code
	2 = open succeeds if file read-only, else fails with INT 24
SeeAlso: #01636,#01404


(Table 01404)
Values for DOS 7.x file sharing behavior:
	  |	Second and subsequent Opens
 First	  |Compat    Deny      Deny	 Deny	   Deny
 Open	  |	     All       Write	 Read	   None
	  |R W RW A  R W RW A  R W RW A	 R W RW A  R W RW A
 - - - - -| - - - - - - - - - - - - - - - - - - - - - - - -
 Compat R |Y Y Y Y   N N N N   Y N N Y	 N N N Y   Y N N Y
	W |Y Y Y C   N N N N   N N N N	 N N N Y   Y N N Y
	RW|Y Y Y C   N N N N   N N N N	 N N N Y   Y N N Y
	NA|Y C C Y   N N N N   Y N N Y	 N N N Y   Y N N Y
 - - - - -|
 Deny	R |C C C C   N N N N   N N N N	 N N N N   N N N N
 All	W |C C C C   N N N N   N N N N	 N N N N   N N N N
	RW|C C C C   N N N N   N N N N	 N N N N   N N N N
	NA|C C C C   N N N N   N N N N	 N N N N   N N N N
 - - - - -|
 Deny	R |Y C C Y   N N N N   Y N N Y	 N N N Y   Y N N Y
 Write	W |C C C C   N N N N   N N N N	 Y N N Y   Y N N Y
	RW|C C C C   N N N N   N N N N	 N N N Y   Y N N Y
	NA|Y C C Y   N N N N   Y N N Y	 N N N Y   Y N N Y
 - - - - -|
 Deny	R |C C C C   N N N N   N Y N N	 N N N N   N Y N N
 Read	W |C C C C   N N N N   N N N N	 N Y N N   N Y N N
	RW|C C C C   N N N N   N N N N	 N N N N   N Y N N
	NA|Y Y Y Y   N N N N   Y Y Y Y	 N N N Y   Y Y Y Y
 - - - - -|
 Deny	R |Y Y Y Y   N N N N   Y Y Y Y	 N N N Y   Y Y Y Y
 None	W |C C C C   N N N N   N N N N	 Y Y Y Y   Y Y Y Y
	RW|C C C C   N N N N   N N N N	 N N N Y   Y Y Y Y
	NA|Y Y Y Y   N N N N   Y Y Y Y	 N N N Y   Y Y Y Y
Legend: R -> reading, W -> writing, RW -> both reading & writing,
	A/NA -> reading without access time update
	Y = open succeeds, N = open fails with error code 05h
	C = open fails, INT 24 generated
SeeAlso: #01403,#01636