CONTRBYT	EQU	83H	 
APORT	EQU	20H
BPORT	EQU	21H
CPORT	EQU	22H
CNTPORT	EQU	23H
		...
		MOV	A,#CONTRBYT
		MOV	R0,#CNTPORT
		MOVX @R0,A	
		MOV  R0,#BPORT	
		MOVX A,@R0	
		DEC  R0		
		MOVX	@R0,A	
		MOV	R0,#CPORT	
		MOVX	A,@R0	
		ANL	A,#0FH	
		SWAP	A		
		MOVX	@R0,A