INT 16 U - Netroom POSTNET - GET OLD INTERRUPT VECTORS
        AX = 5758h
        BX = 5859h
Return: CF clear
        DX:BX -> saved copy of interrupt vector table
Note:   the installation check consists of calling this function and comparing
          BX against 5859h on return; if it has changed, POSTNET is installed
SeeAlso: AX=5758h/BX=5858h
Index:  installation check;Netroom POSTNET

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