INT 21 U - DOS 3.1+ internal - SERVER FUNCTION CALL
        AX = 5D00h
        DS:DX -> DOS parameter list (see below)
        DPL contains all register values for a call to INT 21h
Return: as appropriate for function being called
Notes:  does not check AH.  Out of range values will crash the system
        executes using specified computer ID and process ID
        sharing delay loops skipped
        a special sharing mode is enabled to handle FCBs opened across network
        wildcards are enabled for DELETE (AH=41h) and RENAME (AH=56h) under
          MS-DOS; under DR-DOS 3.41, wildcards corrupt the filesystem; and
          under DR-DOS 5.0-6.0, the call returns error code 03h due to improper
          support for the server function call (see below)
        an extra file attribute parameter is enabled for OPEN (AH=3Dh),
          DELETE (AH=41h), and RENAME (AH=56h)
        functions which take filenames require canonical names (as returned
          by AH=60h); this is apparently to prevent multi-hop file forwarding
BUGS:   the OS/2 2.0 DOS Boot Session incorrectly maps DOS drive letters,
          seemingly ignoring HPFS drives
        DR-DOS 5.0-6.0 merely recursively call INT 21 after loading the
          registers from the DPL, leading to problems for peer-to-peer
          networks
SeeAlso: AH=3Dh,AH=41h,AH=56h,AH=60h

Format of DOS parameter list:
Offset  Size    Description
 00h    WORD    AX
 02h    WORD    BX
 04h    WORD    CX
 06h    WORD    DX
 08h    WORD    SI
 0Ah    WORD    DI
 0Ch    WORD    DS
 0Eh    WORD    ES
 10h    WORD    reserved (0)
 12h    WORD    computer ID (0 = current system)
 14h    WORD    process ID (PSP segment on specified computer)
Note:   under Windows Enhanced mode, the computer ID is normally the virtual
          machine ID (see INT 2F/AX=1683h), though this can reportedly be
          changed by setting UniqueDOSPSP= in SYSTEM.INI

.