Function 41h    Wait On External Event                          (Convertible) 
entry   AH      41h 
	AL      condition type 
		bits 0-2: condition to wait for 
			  0 any external event 
			  1 compare and return if equal 
			  2 compare and return if not equal 
			  3 test and return if not zero 
			  4 test and return if zero 
		bit 3:    reserved 
		bit 4:    1=port address, 0=user byte 
		bits 5-7: reserved 
	BH      condition compare or mask value 
		condition codes: 
		0       any external event 
		1       compare and return if equal 
		2       compare and return if not equal 
		3       test and return if not zero 
		4       test and return if zero 
	BL      timeout value times 55 milliseconds 
		0 if no time limit 
	DX      I/O port address (if AL bit 4 = 1) 
	ES:DI   pointer to user byte (if AL bit 4 = 0)