SDSUB:
MOV	R4,#20H					
MOV	R5,#80H					
MOV	R6,#0FDH					
MOV	R7,#71H					
MOV	20H,R4					
MOV	21H,R5
ACALL	COMP				
MOV	R4,20H
MOV	R5,21H
MOV	20H,R6
MOV	21H,R7
ACALL	COMP					
CLR		C
MOV	A,R4
SUBB	A,20H				
MOV	A,R5
SUBB	A,21H
MOV	21H,A
JB		D2,LOOP				
ACALL	COMP				
MOV	R4,20H					
MOV	R5,21H					
RET								
LOOP:
MOV	R4,#0FFH					
MOV	R5, #0FFH	
RET
COMP:
CLR		C						
MOV	A,20H
RLC		A
MOV	20H,A
MOV	A,21H
RLC		A						
MOV	21H,A
JNC		C,LOOP1
ACALL	NEGNVM
LOOP1:
MOV	A,21H
RRC	A							
MOV	20H,A
RET
NEGNVM:							
MOV	R0,20H
MOV	R1,21H
CLR		C					
CLR		A						
MOV	20H,A
MOV	21H,A
MOV	A,20H
SUBB	A,R0						
MOV	20H,A
MOV	A,21H
NOP
SUBB	A,R1						
MOV	21H,A
RET