INT 6E - DECnet DOS - DECnet NETWORK PROCESS API
Notes:  this is the main DECnet DOS access, and is described in Digital manual
          AA-EB46B-TV ("DECnet-DOS Programmer's Reference Manual")
        there is a signature/data area immediately prior to the interrupt
          handler which may be used as an installation check

Format of signature area:
Offset  Size    Description
 -5     BYTE    major version number
 -4     BYTE    minor version number
 -3   3 BYTEs   signature (ASCII "DNP")