INT 21 U - Sun PC-NFS - API???
        AX = 44E0h
        DS:DX -> ???
        SS:BP -> stack frame (see below)
Return: ???
Note:   this function is also supported by Beame&Whiteside's BWPCNFS shim; the
          description presented here was derived from that shim

Format of stack frame:
Offset  Size    Description
 00h    WORD    -> previous stack frame
 02h    DWORD   return address

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