题目：3位二进制加法器
Now that you know how to build a full adder, make 3 instances of it to create a 3-bit binary ripple-carry adder. The adder adds two 3-bit numbers and a carry-in to produce a 3-bit sum and carry out. To encourage you to actually instantiate full adders, also output the carry-out from each full adder in the ripple-carry adder. cout[2] is the final carry-out from the last full adder, and is the carry-out you usually see.

解题：

module top_module( 
    input [2:0] a, b,
    input cin,
    output [2:0] cout,
    output [2:0] sum );
    add add1(.a(a[0]),.b(b[0]),.cin(cin),.cout(cout[0]),.sum(sum[0]));
    add add2(.a(a[1]),.b(b[1]),.cin(cout[0]),.cout(cout[1]),.sum(sum[1]));
    add add3(.a(a[2]),.b(b[2]),.cin(cout[1]),.cout(cout[2]),.sum(sum[2]));
endmodule
module add(input a,b,cin,
           output cout,sum
          );
    assign sum=a^b^cin;
    assign cout=(a&b)|((a^b)&cin);
endmodule

结果正确：