thread execution

module tb;
  parameter time CYCLE = 10ns;

  logic clk;

  initial begin
    clk <= 0;

    forever begin
      #(CYCLE/2) clk ^= 1'b1;
    end
  end

  always @(posedge clk) begin
    clk = 0;
    $display("first thread");
  end

  always @(posedge clk) begin
    clk = 0;
    $display("second thread");
  end

  initial begin
    #1000ns;
    $stop;
  end
endmodule