Format of SK-UPPS Memory Buffer (Mbuf):
Offset	Size	Description	)
 00h	DWORD	-> NEXT Mbuf (linked list)
 04h	WORD	OFFSET of actual data within DATA area
 06h  2 BYTEs	reserved (currently unused)
 08h	WORD	LENGTH of actual data found in DATA area starting at OFFSET
 0Ah  2 BYTEs	reserved (currently unused)
 0Ch	DWORD	-> DATA area (size can be obtained via AX=000Eh call)
 10h	WORD	PROTOCOL ID of protocol currently "owning" the Mbuf
 12h	BYTE	IN USE flag; nonzero if Mbuf still in use by DLI (see AX=0006h)
 13h	BYTE	RECEIVE STATUS (bit 0-7 only; see AX=0007h - Receive Upcall)