Parameters and Generate

Good RTL is reusable. Instead of hand-writing an 8-bit version and a 16-bit version of the same block, you write it once with a parameter and pick the size when you use it. Parameters and generate are how real designs stay scalable.

Parameters

A parameter is a constant you can set per instance. localparam is a constant you cannot override from outside, used for internal values you want named.

// A register whose width is a parameter
module reg_n #(parameter WIDTH = 8) (
  input  wire             clk,
  input  wire [WIDTH-1:0] d,
  output reg  [WIDTH-1:0] q
);
  always @(posedge clk) q <= d;
endmodule

// Choose the width when you instantiate it
reg_n #(.WIDTH(16)) u_reg16 (.clk(clk), .d(d16), .q(q16));

Generate: replicate hardware

A generate block builds repeated or conditional hardware at elaboration time. With a genvar loop you can instantiate N copies of something, like the bit-slices of an adder, without writing them out by hand.

// Build a WIDTH-bit ripple-carry adder from full adders
genvar i;
generate
  for (i = 0; i < WIDTH; i = i + 1) begin : adders
    full_adder fa (.a(a[i]), .b(b[i]), .cin(carry[i]),
                   .sum(sum[i]), .cout(carry[i+1]));
  end
endgenerate

Generate also has an if form, so you can include or leave out hardware based on a parameter, for example adding an optional pipeline stage only when a parameter asks for it.