INT 21 - DOS 2+ - "OPEN" - OPEN EXISTING FILE
        AH = 3Dh
        AL = access and sharing modes (see below)
        DS:DX -> ASCIZ 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 AH=59h)
Notes:  file pointer is set to start of file
        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 against the reserved field in the directory
          entry before allowing access
        sharing modes are only effective on local drives if SHARE is loaded
SeeAlso: AH=0Fh,AH=3Ch,AX=4301h,AX=5D00h,INT 2F/AX=1116h,INT 2F/AX=1226h

Bitfields for access and sharing modes:
 bits 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
 bit 3  reserved (0)
 bits 6-4 sharing mode (DOS 3+)
        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 FCB (only available during server call)
 bit 7  inheritance
        if set, file is private to current process and will not be inherited
          by child processes

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

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