一、基础部分（70 ）

设计一个简易计算器，它具有下列运算功能：

1. 两个无符号的8位二进制数的相加；

2. 两个无符号的8位二进制数的相减；

3. 数值和运算符用4×4键盘输入，输入的值为十进制，其中A为“+”，B为“-”，C为“退格”E为“=”，

4. 数值用数码管以十进制形式显示，以加法为例，初始时显示全“0”，先输入被加数，输入时数字顺序是从左到右。例如，输入1、2、3应该在显示器上上显示“123”，在输入运算符，按下运算符键后，数码管显示全“0”，再输入加数，方法和前面一样，最后按下“=”，数码管显示运算结果

二、扩展部分（30 ）

在基础部分上增加乘法功能，实现两个无符号的4位二进制数的相乘，D为“*”（30 ）-First, the base portion (70 ) to design a simple calculator, it has the following expression: 1. Sum of two unsigned 8-bit binary number　subtraction of two unsigned 8-bit binary number　values ​ ​ and operators with 44 keyboard input, the input value is decimal, where A is the+ B for " -"　C " backspace"　E " ="　Numerical digital tube In addition, for example, when the initial display of " 0" , the first input is addend input digital sequence is from left to right is displayed in decimal form. For example, enter 1, 2 and 3 on the display should be displayed on the " 123"　digital display " 0" , the input operator, press the operator key, and then enter the addend, methods, and as before, and finally press under the " =" , digital display the result of the operation, expansion portion (30 ) increase in the base part multiplication function, the multiplication of two unsigned 4-bit binary number, D is the " *"　(