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module full_adder(a,b,cin,sum,cout);
  
input a,b,cin;
output sum, cout;
reg sum, cout;

always @(a or b or cin)

  begin
    sum = a^b^cin;
     cout = (a & b) | (a & cin) | (b & cin);
 end
endmodule

module full_adder_4bit(a,b,cin,sum,cout);
  
input[3:0] a,b;
input cin;
output[3:0] sum;
output cout;
wire c1,c2,c3;

// instantiation
full_adder FA1(a[0],b[0],cin,sum[0],c1);
full_adder FA2(a[1],b[1],c1,sum[1],c2);
full_adder FA3(a[2],b[2],c2,sum[2],c3);
full_adder FA4(a[3],b[3],c3,sum[3],cout);


endmodule
  
module test_adder;

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

full_adder_4bit dut(a,b,cin,sum,cout);

initial

  begin
    a=4'b0000;
    b=4'b0000;
    cin = 1'b0;
    #50;
    
    a=4'b0101;
    b=4'b1010;
    // sum = 1111 et cout = 0
    #50;
    
    a=4'b1111;
    b=4'b0001;
    // sum = 0000 et cout = 1
    #50;

    a=4'b0000;
    b=4'b1111;
    cin = 1'b1;
    // sum = 0000 et cout = 1
    #50;
 
    a=4'b0110;
    b=4'b0001;
    // sum = 1000 et cout = 0
    
  end
 
endmodule