SystemVerilog fork join_none

There is a third type of fork join in SystemVerilog which is fork and join_none.

A fork and join_none will allow the main thread to resume execution of further statements that lie after the fork regardless of whether the forked threads finish. If five threads are launched, the main thread will resume execution immediately while all the five threads remain running in the background.

Syntax


	fork
		// Thread 1
		// Thread 2
		// ...
		// Thread N
	join_none

fork join_none Example


module tb;
	initial begin
      $display ("[%0t] Main Thread: Fork join going to start", $time);
		fork
          print (20, "Thread1_0");
          print (30, "Thread1_1");
          print (10, "Thread2");              
		join_none
      $display ("[%0t] Main Thread: Fork join has finished", $time);
	end
  
  // Note that we need automatic task
  task automatic print (int _time, string t_name);
    #(_time) $display ("[%0t] %s", $time, t_name);
  endtask
endmodule
 Simulation Log
ncsim> run
[0] Main Thread: Fork join going to start
[0] Main Thread: Fork join has finished
[10] Thread2
[20] Thread1_0
[30] Thread1_1
ncsim: *W,RNQUIE: Simulation is complete.

Nested fork join_none


module tb;
	initial begin
      $display ("[%0t] Main Thread: Fork join going to start", $time);
		fork
          begin
			fork
              print (20, "Thread1_0");
              print (30, "Thread1_1");
            join_none
            $display("[%0t] Nested fork has finished", $time);
          end
          print (10, "Thread2");              
        join_none
      $display ("[%0t] Main Thread: Fork join has finished", $time);
	end
  
  // Note that we need automatic task
  task automatic print (int _time, string t_name);
    #(_time) $display ("[%0t] %s", $time, t_name);
  endtask
endmodule
 Simulation Log
ncsim> run
[0] Main Thread: Fork join going to start
[0] Main Thread: Fork join has finished
[0] Nested fork has finished
[10] Thread2
[20] Thread1_0
[30] Thread1_1
ncsim: *W,RNQUIE: Simulation is complete.

Why do we need automatic task ?

Without automatic keyword, the same display task with different string tags will produce the same display message. This is because multiple threads call the same task and share the same variable in tool simulation memory. In order for different threads to initiate different copies of the same task, automatic keyword has to be used.


module tb;
  initial begin
      $display ("[%0t] Main Thread: Fork join going to start", $time);
    fork
          print (20, "Thread1_0");
          print (30, "Thread1_1");
          print (10, "Thread2");              
    join_none
      $display ("[%0t] Main Thread: Fork join has finished", $time);
  end
 
  // Note that this is not an automatic task, its static
  task print (int _time, string t_name);
    #(_time) $display ("[%0t] %s", $time, t_name);
  endtask
endmodule
 Simulation Log
ncsim> run
[0] Main Thread: Fork join going to start
[0] Main Thread: Fork join has finished
[10] Thread2
[20] Thread2
[30] Thread2
ncsim: *W,RNQUIE: Simulation is complete.