WWW.TESTBENCH.IN - SystemVerilog Constructs

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Specman E
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FORK CONTROL

Wait Fork Statement

The wait fork statement is used to ensure that all immediate child subprocesses (processes created by the current process, excluding their descendants) have completed their execution.

EXAMPLE
  program main();

   initial begin
    #(10);
       $display(" BEFORE fork  time = %0d ",$time );
       fork
           begin
               # (20);
               $display(" time = %0d # 20  ",$time );
           end
           begin
               #(10);
               $display(" time = %0d # 10  ",$time );
           end
           begin
               #(5);
               $display(" time = %0d # 5  ",$time );
           end
         join_any
    $display(" time = %0d Outside the main fork ",$time );
   end
   endprogram

RESULTS

BEFORE fork  time = 10
time = 15 # 5
time = 15 Outside the main fork

In the above example, Simulation ends before the #10 and #20 gets executed. In some situations, we need to wait until all the threads got finished to start the next task. Using wait fork, will block the  till all the child processes complete.


EXAMPLE:
   program main();

   initial begin
    #(10);
       $display(" BEFORE fork  time = %0d ",$time );
       fork
           begin
               # (20);
               $display(" time = %0d # 20  ",$time );
           end
           begin
               #(10);
               $display(" time = %0d # 10  ",$time );
           end
           begin
               #(5);
               $display(" time = %0d # 5  ",$time );
           end
         join_any
    $display(" time = %0d Outside the main fork ",$time );
    wait fork ;
    $display(" time = %0d  After wait fork ",$time );

   end
   endprogram
RESULTS

BEFORE fork  time = 10
time = 15 # 5
time = 15 Outside the main fork
time = 20 # 10
time = 30 # 20
time = 30  After wait fork

Disable Fork Statement

The disable fork statement terminates all active descendants (subprocesses) of the calling process.

In other words, if any of the child processes have descendants of their own, the disable fork statement shall terminate them as well. Sometimes, it is required to kill the child processes after certain condition.

EXAMPLE
   program main();

   initial begin
    #(10);
       $display(" BEFORE fork  time = %0d ",$time );
       fork
           begin
               # (20);
               $display(" time = %0d # 20  ",$time );
           end
           begin
               #(10);
               $display(" time = %0d # 10  ",$time );
           end
           begin
               #(5);
               $display(" time = %0d # 5  ",$time );
           end
         join_any
    $display(" time = %0d Outside the main fork ",$time );
   end

   initial
   #100 $finish;
   endprogram
RESULTS

BEFORE fork  time = 10
time = 15 # 5
time = 15 Outside the main fork
time = 20 # 10
time = 30 # 20

In the following example, disable for kills the threads #10 and #20.

EXAMPLE
   program main();

   initial begin
    #(10);
       $display(" BEFORE fork  time = %0d ",$time );
       fork
           begin
               # (20);
               $display(" time = %0d # 20  ",$time );
           end
           begin
               #(10);
               $display(" time = %0d # 10  ",$time );
           end
           begin
               #(5);
               $display(" time = %0d # 5  ",$time );
           end
         join_any
    $display(" time = %0d Outside the main fork ",$time );
    disable fork;
    $display(" Killed the child processes");
   end

   initial
   #100 $finish;
   endprogram
RESULTS

BEFORE fork  time = 10
time = 15 # 5
time = 15 Outside the main fork
Killed the child processes

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