Environment Variables in Verilog

Environment variables are a universal mechanism for conveying configuration information to programs. Let’s look at how to set, get, and list environment variables in Verilog.

module environment_variables;
  
  // Simulating environment variables using a SystemVerilog associative array
  string env[string];

  initial begin
    // To set a key/value pair, we use the associative array
    env["FOO"] = "1";
    
    // To get a value for a key, we access the associative array
    // This will return an empty string if the key isn't present
    $display("FOO: %s", env["FOO"]);
    $display("BAR: %s", env["BAR"]);

    $display();

    // To list all key/value pairs in the environment
    // We iterate through the associative array
    foreach (env[key]) begin
      $display("%s", key);
    end
  end

endmodule

In Verilog, we don’t have built-in support for environment variables like in other programming languages. However, we can simulate this behavior using a SystemVerilog associative array.

To set a key/value pair, we simply assign a value to the associative array using the key. To get a value for a key, we access the associative array using the key. If the key doesn’t exist, it will return an empty string by default.

To list all key/value pairs, we use a foreach loop to iterate through the associative array and display all keys.

Running this simulation would show that we pick up the value for FOO that we set in the program, but that BAR is empty.

FOO: 1
BAR: 

FOO

The list of keys in the environment will depend on what you’ve added to the associative array.

In a real Verilog simulation environment, you might use simulator-specific system functions to interact with actual environment variables. For example, in ModelSim, you could use $getenv and $setenv:

initial begin
  $setenv("FOO", "1");
  $display("FOO: %s", $getenv("FOO"));
  $display("BAR: %s", $getenv("BAR"));
end

This would allow you to interact with the actual environment variables of the system running the simulation.

Remember that the exact behavior and availability of these functions can vary between different Verilog simulators and synthesis tools.