单片机按键实现计算器加减乘除；case 0x7d: key='F'; P1 = tab[15];break;// /????
case 0xeb: key='9'; a = 9;P1 = tab[9];break;
case 0xdb: key='6'; a = 6;P1 = tab[6];break;
case 0xbb: key='3'; a = 3;P1 = tab[3];break;
case 0x7b: key='E';res += (mul*a);P1 = tab[res];break;//=
case 0xe7: key='A';res = a;mul = 1; break;//+
case 0xd7: key='B';res = a;mul = -1;break;//-
case 0xb7: key='C';res = 0; a = 0;P1 = tab[0];break;//clr
case 0x77: key='D';mul =a;break;//*
