INT 7C - SK-UPPS Data Link Interface API(Application Program[ming] Interface) The defined set of calls which a program may make to interact with or request services of the operating system or environment under which it is running.  Because the inputs and outputs of the calls are well-defined, a program using the API can continue using the identical calls even if the internal organization of the program providing the API changes. - REGISTER MULTICAST
	AX = 0009h
	BX = protocol ID (see #03849)
	ES:DI -> multicast address (6 bytes)
Return: AX = completion code (see #03846)
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