INT 2A,4 - Execute NETBIOS Request

	AH = 04
	AL = 00  allow retry
	   = 01  no retry allowed
	ES:BX = pointer to Network Control Block (NCB)
	
	
	on return:
	AX = 00 if successful
	AH = 01 if error
	AL = error code if AH = 01


	- invokes NETBIOS handler and allows retry of operation if specified