组合逻辑电路-arithmetic circuits

Create a half adder. A half adder adds two bits (with no carry-in) and produces a sum and carry-out.

module top_module( 
    input a, b,
    output cout, sum );
    assign sum=a^b;
    assign cout=a&b;//{cout,sum}=a+b;
endmodule

Create a full adder. A full adder adds three bits (including carry-in) and produces a sum and carry-out. 

module top_module( 
    input a, b, cin,
    output cout, sum );
    assign {cout,sum} = a + b + cin;
endmodule

Now that you know how to build a full adder, make 3 instances of it to create a 3-bit binary