one digit BCD ADDER

module bcdadd(a,b,cin,s,cout);  //Main module of one digit BCD adder

  input [3:0]a;

  input [3:0]b;

  input cin;

  output[3:0]s;

  output cout;

  wire [3:0]w;

  wire y,c0,c1,c2,c3,c4,c5;

  fulladd m1(a[0],b[0],cin,w[0],c0);

  fulladd m2(a[1],b[1],c0,w[1],c1);

  fulladd m3(a[2],b[2],c1,w[2],c2);

  fulladd m4(a[3],b[3],c2,w[3],c3);

  assign y=c3|(w[3]&(w[2]|w[1]));

  halfadd m5(w[1],y,s[1],c4);

  fulladd m6(w[2],y,c4,s[2],c5);

  halfadd m7(w[3],c5,s[3],cout);

  assign s[0]=w[0];

endmodule

                                                      

module fulladd(a,b,cin,s,cout); //submodule for fulladder

  input a,b;

  input cin;

  output s;

  output cout;

  wire w1,w2,w3;

  halfadd g1(a,b,w1,w2);

  halfadd g2(w1,cin,s,w3);

  assign cout=w3|w2;

endmodule

module halfadd(a,b,s,cout);  //submodule for halfadder

  input a,b;

  output s,cout;

assign s=a^b;

assign cout=a&b;

endmodule