4x4 bit Wallace Tree Multiplier Implementation in Verilog

//Half Adder Code:

`timescale 1ns / 1ps
module half_add( input a,  input b,  output s,  output c  );

assign s = a ^ b;
assign c = a & b;

endmodule

//Full Adder Code:

`timescale 1ns / 1ps
module full_add( input a,  input b,  input ci,  output s,  output co );
 
assign s = (a ^ b) ^ ci;
assign co = (a & b) ^ (ci & (a ^ b));
    
endmodule

//Wallace 4 Code:

`timescale 1ns / 1ps

module wallace4(input [3:0] A, input [3:0] B, output [7:0] P);

integer i;
wire s11,s12,s13,s14,s15,s22,s23,s24,s25,s26,s32,s34,s35,s36,s37;
wire c11,c12,c13,c14,c15,c22,c23,c24,c25,c26,c32,c34,c35,c36,c37;
reg [3:0] pp0,pp1,pp2,pp3 ;

//Calculation of Partial Product
always @(A or B)
begin
    for(i=0;i<4;i=i+1) begin
pp0[i] <= A[i] & B[0];
pp1[i] <= A[i] & B[1];
pp2[i] <= A[i] & B[2];
pp3[i] <= A[i] & B[3];
    end     
end

assign P[0] = pp0[0];
assign P[1] = s11;
assign P[2] = s22;
assign P[3] = s32;
assign P[4] = s34;
assign P[5] = s35;
assign P[6] = s36;
assign P[7] = s37;

//first stage
half_add ha11 (pp0[1],pp1[0],s11,c11);
full_add fa12 (pp0[2],pp1[1],pp2[0],s12,c12);
full_add fa13 (pp0[3],pp1[2],pp2[1],s13,c13);
full_add fa14 (pp1[3],pp2[2],pp3[1],s14,c14);
half_add ha15 (pp2[3],pp3[2],s15,c15);

//second stage
half_add ha21 (c11,s12,s22,c22);
full_add fa22 (pp3[0],c12,s13,s23,c23);
full_add fa23 (c13,c23,s14,s24,c24);
full_add fa24 (c14,c24,s15,s25,c25);
full_add fa25 (c15,c25,pp3[3],s26,c26);

//third stage
half_add ha31 (c22,s23,s32,c32);
half_add ha32 (c32,s24,s34,c34);
half_add ha33 (c34,s25,s35,c35);
half_add ha34 (c35,s26,s36,c36);
half_add ha35 (c36,c26,s37,c37);

endmodule

//Simulation Results: