Fork join

SystemVerilog
1

why is the below printing only thread 2 ?

module tb;
	initial begin
      $display ("[%0t] Main Thread: Fork join going to start", $time);
		fork
          display (20, "Thread1_0");
          display (30, "Thread1_1");
          display (10, "Thread2");              
		join
      $display ("[%0t] Main Thread: Fork join has finished", $time);
	end
 
  task display (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
[10] Thread2
[20] Thread2
[30] Thread2
[30] Main Thread: Fork join has finished

2

In reply to diptishe:

Because your task has a static lifetime, the arguments have static lifetimes as well. That means there is only one space allocated for the arguments for all concurrent invocations of the tasks. You need to declare the task with an automatic lifetime, making it reentrant.

task automatic display (int _time, string t_name);

Tasked defined in classes do not have this problem since they always have automatic lifetimes.

3

In reply to diptishe:

Your task is static and not automatic


module tb;
initial begin
$display ("[%0t] Main Thread: Fork join going to start", $time);
fork
display (20, "Thread1_0");
display (30, "Thread1_1");
display (10, "Thread2");
join
$display ("[%0t] Main Thread: Fork join has finished", $time);
end

 task automatic display (int _time, string t_name);
#(_time) $display ("[%0t] %s", $time, t_name);
endtask
endmodule
[0] Main Thread: Fork join going to start
# [10] Thread2
# [20] Thread1_0
# [30] Thread1_1
# [30] Main Thread: Fork join has finished
# exit

Ben Cohen
http://www.systemverilog.us/ ben@systemverilog.us
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  • SVA Handbook 4th Edition, 2016 ISBN 978-1518681448
  • A Pragmatic Approach to VMM Adoption 2006 ISBN 0-9705394-9-5
  • Using PSL/SUGAR for Formal and Dynamic Verification 2nd Edition, 2004, ISBN 0-9705394-6-0
  • Real Chip Design and Verification Using Verilog and VHDL, 2002 isbn 978-1539769712
  • Component Design by Example ", 2001 ISBN 0-9705394-0-1
  • VHDL Coding Styles and Methodologies, 2nd Edition, 1999 ISBN 0-7923-8474-1
  • VHDL Answers to Frequently Asked Questions, 2nd Edition ISBN 0-7923-8115
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