DE2 练习源码2-4

代码说明：

Part IV In part II we discussed the conversion of binary numbers into decimal digits. It is sometimes useful to build circuits that use this method of representing decimal numbers, in which each decimal digit is represented using four bits. This scheme is known as the binary coded decimal (BCD) representation. As an example, the decimal value 59 is encoded in BCD form as 0101 1001. You are to design a circuit that adds two BCD digits. The inputs to the circuit are BCD numbers A and B, plus a carry-in, cin. The output should be a two-digit BCD sum S1S0. Note that the largest sum that needs to be handled by this circuit is S