DSUB:
MOV	R4,#20H					
MOV	R5,#80H					
MOV	R6,#0ADH					
MOV	R7,#71H					
MOV	20H,R4
MOV	21H,R5
CLR		C
MOV	A,R4
SUBB	A,20H					
MOV	20H,A
MOV	A,R5
SUBB	A,21H
MOV	21H,A
JB		D2,LOOP					
MOV	R4,20H					
MOV	R5,21H					
RET								
LOOP:
MOV	R4,#0FFH					
MOV	R5, #0FFH	
RET