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
        wildcards are enabled for DELETE (AH=41h) and RENAME (AH=56h)
        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
SeeAlso: 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)