INT 6D - DECnet DOS (before 2.1) - DATA LINK LAYER PROGRAM
        AH = function
        ???
Return: ???
Note:   the installation check consists of testing for a signature area
          immediately preceding the interrupt handler
SeeAlso: INT 69/AH=0Ah,INT 6C"DECnet",INT 6E"DECnet"
Index:  installation check;DECnet DOS Data Link Layer

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

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