Format of internal communications control block:
Offset	Size	Description	)
 00h	WORD	subfunction number
 02h	WORD	magic number (FEFEh)
 04h	WORD	(ret) result, should be set to 0 before calling
 06h	WORD	reserved (will be set to client DS)
 08h	WORD	caller's code segment if function 000Bh, 0 else
 0Ah	WORD	pointer to parameter block
 0Ch	WORD	reserved (segment of parameter block; will be set to client DS)
 0Eh	WORD	StreetTalk handle
SeeAlso: #03323,#03444 at INT 61/AX=000Fh