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