module ha ( input   a, b,
            output  sum, cout);
 
  assign sum  = a ^ b;
  assign cout = a & b;
endmodule

// A top level design that contains N instances of half adder
module my_design 
	#(parameter N=4) 
		(	input [N-1:0] a, b,
			output [N-1:0] sum, cout);
			
	// Declare a temporary loop variable to be used during
	// generation and won't be available during simulation
	genvar i;
	
	// Generate for loop to instantiate N times
	generate 
		for (i = 0; i < N; i = i + 1) begin
          ha u0 (a[i], b[i], sum[i], cout[i]);
		end
	endgenerate
endmodule

module tb;
	parameter N = 2;
  reg  [N-1:0] a, b;
  wire [N-1:0] sum, cout;
  
  // Instantiate top level design with N=2 so that it will have 2
  // separate instances of half adders and both are given two separate
  // inputs
  my_design #(.N(N)) md( .a(a), .b(b), .sum(sum), .cout(cout));
  
  initial begin
    a <= 0;
    b <= 0;
    
    $monitor ("a=0x%0h b=0x%0h sum=0x%0h cout=0x%0h", a, b, sum, cout);
    
    #10 a <= 'h2;
    		b <= 'h3;
    #20 b <= 'h4;
    #10 a <= 'h5;
  end
endmodule