How to pass value to `define N

I am using the iverilog simulator, and I want to pass value to N during compilation. Which command do I need to use, and can anyone help me with the usage of `define?

`define N

module Nbcd(A, B ,S);
 input [N*4-1:0] A,B;
 output[N*4-1:0] S;

genvar i;
  generate
   for(i=0; i<=N-1; i=i+1)
      bcd (.A(A[4*i+3:i*4]),
           .B(B[4*i+3:i*4]).
           .S(S[4*i+3:i*4])
          );
  endgenerate
endmodule

Testbench

module Nbcd();
  reg [N*4-1:0] A,B;
  wire [N*4-1:0] S;
  integer i;

  Nbcd U1 (.A(A),.B(B),.S(S));

  initial begin
    for (i=o; i<=N-1; i=i+1)
      begin
        A=i;
        b=i+1;        
    end
endmodule  
  

I believe that most simulators allow the -define command line option.

irun test.sv -define N=0

It is slightly different for iverilog use -Dkey=value ie

-DN=0

Usage

To use `define (tick defines) you need to place a tick before it, so you use `N when you want to use the command line supplied value, eg:

module Nbcd(A, B ,S);
 input [`N*4-1:0] A,B;
 output[`N*4-1:0] S;

I have a working example on EDA Playground, with iverilog version 0.9.7. The value is supplied via command line -DN=10 and displays this value in the simulation using $display("N : %d", `N);.

Tip

You may also want to add the code below, so that it can run without the command line option or remind users it is required:

`ifndef N
  `define N 0
`endif