MCS-51单片机应用子程序：1.2.1 双字节无符号数减法

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