INT 7C - SK-UPPS Data Link Interface API - REGISTER MULTICAST
        AX = 0009h
        BX = protocol ID (see AX=0007h)
        ES:DI -> multicast address (6 bytes)
Return: AX = completion code (see AX=0001h)
Notes:  The broadcast address is always enabled.
        Set bit 9 of PCB's protocol mode field in REGISTER PROTOCOL call
          (AX=0007h) to receive all multicast frames.
SeeAlso: AX=000Ah,AX=0014h

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