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// Description of half adder (see Fig 4-5b)
module halfadder (S,C,x,y);
input x,y;
output S,C;
//Instantiate primitive gates
xor (S,x,y);
and (C,x,y);
endmodule
//Description of full adder (see Fig 4-8)
module fulladder (S,C,x,y,z);
input x,y,z;
output S,C;
wire S1,D1,D2; //Outputs of first XOR and two AND gates
//Instantiate the halfadder
halfadder HA1 (S1,D1,x,y),
HA2 (S,D2,S1,z);
or g1(C,D2,D1);
endmodule
//Description of 4-bit adder
module _4bit_adder (S,C4,A,B,C0);
input [3:0] A,B;
input C0;
output [3:0] S;
output C4;
wire C1,C2,C3; //Intermediate carries
//Instantiate the fulladder
fulladder FA0 (S[0],C1,A[0],B[0],C0),
FA1 (S[1],C2,A[1],B[1],C1),
FA2 (S[2],C3,A[2],B[2],C2),
FA3 (S[3],C4,A[3],B[3],C3);
endmodule