module rca3( a, b, cin, cout, sum);
input [2:0]   a, b;
input            cin;

output  [2:0]  sum;
output            cout;

wire     [1:0]   m;


fa    g1 (.cout(m[0]), .sum(sum[0]), .a(a[0]), .b(b[0]), .cin(cin));
fa    g2 (.cout(m[1]), .sum(sum[1]), .a(a[1]), .b(b[1]), .cin(m[0]) );
fa    g3  (.cout(cout), .sum(sum[2]), .a(a[2]), .b(b[2]), .cin(m[1]) );

endmodule


`timescale 1ns / 1ns
module rca3_tb;

reg[2:0]     a,  b;
reg             cin;
wire [2:0]  sum;
wire           cout;
wire [3:0]   res;

assign res  = { cout, sum };

rca3   uu ( a, b, cin, cout, sum );




initial begin
          a = 2;  b= 4;  cin = 0;
  #10  a = 3;  b = 3; cin = 1;
  #10  a=  5;  b =6;  cin = 1;
  #10  a=  7;  b = 7; cin = 1;  

  #10 $stop;
end

initial $monitor($time, "ns, a=%d, b=%d, cin = %d, addition result = %d", a, b, cin, res);

endmodule