BCD tetrad adder using FAC

module fac(
  input x, y, ci,
  output z, co
  );
  assign {co, z} = x + y +ci;
endmodule

module bcd_add(
  input [3:0] x, y,
  input ci,
  output [3:0] z,
  output co
  );

  wire c1,c2,c3,c4,z1p,z2p,z3p,c2p,c3p;
  //de pe desen
  fac u0(
  .x(x[0]),
  .y(y[0]),
  .ci(ci),
  .z(z[0]),
  .co(c1)
  );

  fac u1(
  .x(x[1]),
  .y(y[1]),
  .ci(c1),
  .z(z1p),
  .co(c2)
  );

  fac u2(
  .x(x[2]),
  .y(y[2]),
  .ci(c2),
  .z(z2p),
  .co(c3)
  );

  fac u3(
  .x(x[3]),
  .y(y[3]),
  .ci(c3),
  .z(z3p),
  .co(c4)
  );

  assign co = (z1p & z2p) | (z2p & z3p) | c4;

  fac u4(
  .x(co),
  .y(z1p),
  .ci(1'b0),
  .z(z[1]),
  .co(c2p)
  );

  fac u5(
  .x(co),
  .y(z2p),
  .ci(c2p),
  .z(z[2]),
  .co(c3p)
  );

  assign z[3] = z3p ^ c3p;

endmodule



//-----------------------------------------------------------------------------


module bcd_add_tb
  (
  output reg [3:0] x, y,
  output reg ci,
  output [3:0] z,
  output co
  );

  bcd_add test(
  .x(x),
  .y(y),
  .ci(ci),
  .z(z),
  .co(co)
  );

  integer i, j, k;
  initial begin
    {x,y,ci} = 9'd0;
    for(i = 0; i < 10; i = i+1)
      for(j = 0; j < 10; j = j+1)
        for(k = 0; k < 2; k = k+1)
      #100 {x,y,ci} = {i[3:0], j[3:0], k[0]};
    end
  endmodule