fork join

fork join语句可以多个线程同时并发执行，最后要等待所有的线程完成。如下图所示：

只有当进程3执行完成后，才会跳转到statement-3执行。

module fork_join;

initial begin
    $display("------------------------------------------");
    fork
        //process 1
        begin
            $display($time,"\t Process 1 Started");
            #5;
            $display($time,"\t Process 1 Finished");
        end 

        //process 2
        begin
            $display($time, "\t Process 2 Started");
            #20;
            $display($time, "\t Process 2 Finished");
        end
    join
    $display($time, "\t Outside Fork-Join");
    $display("------------------------------------------");
    $finish;
end


endmodule
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其仿真输出结果为：

fork join_any

语句的并行执行，但是当其中任意一个进程结束后，都会执行fork join_any后面的语句。如下图示，当process -2 执行完成之后，就会执行statement - 3。但是没有执行完的进程还是会继续执行。

module fork_join;

initial begin
    $display("------------------------------------------");
    fork
        //process 1
        begin
            $display($time,"\t Process 1 Started");
            #5;
            $display($time,"\t Process 1 Finished");
        end 

        //process 2
        begin
            $display($time, "\t Process 2 Started");
            #20;
            $display($time, "\t Process 2 Finished");
        end
    join_any
    $display($time, "\t Outside Fork-Join");
    $display("------------------------------------------");
    $finish;
end


endmodule
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仿真输出结果是：

fork join_none

module fork_join_none;
  initial begin
    $display("-----------------------------------------------------------------");
    fork
      //Process-1
      begin
        $display($time,"\tProcess-1 Started");
        #5;
        $display($time,"\tProcess-1 Finished");
      end
      //Process-2
      begin
        $display($time,"\tProcess-2 Startedt");
        #20;
        $display($time,"\tProcess-2 Finished");
      end
    join_none
  
    $display($time,"\tOutside Fork-Join_none");
    $display("-----------------------------------------------------------------");
  end
endmodule
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其输出结果为：

0 Outside Fork-Join_none
-----------------------------------------------------------------
0 Process-1 Started
0 Process-2 Startedt
5 Process-1 Finished
20 Process-2 Finished
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wait fork

通过fork-join_any，我们执行完其中一个进程就能执行fork-join_any后面的语句，同时没完成的进程也会继续执行。问题是，当后面的语句出现如结束仿真的系统函数$finish时，很可能fork内部的进程没做完，就会结束仿真。

module wait_fork;
 
  initial begin
    $display("-----------------------------------------------------------------");
    fork
      //Process-1
      begin
        $display($time,"\tProcess-1 Started");
        #5;
        $display($time,"\tProcess-1 Finished");
      end
 
      //Process-2
      begin
        $display($time,"\tProcess-2 Started");
        #20;
        $display($time,"\tProcess-2 Finished");
      end
    join_any
    $display("-----------------------------------------------------------------");
    $finish; //ends the simulation
  end
endmodule
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仿真结果如下：

-----------------------------------------------------------------
0 Process-1 Started
0 Process-2 Started
5 Process-1 Finished
-----------------------------------------------------------------
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这应该避免，为了解决这样的问题，我们可以使用wait fork。示例如下:

module wait_fork;

initial begin

$display("----------------------------------------------------------");
fork
    //process 1
    begin
        $display($time, "\t Process-1 Started");
        #5;
        $display($time, "\t Process 1 Finished");
    end

    //process 2
    begin
        $display($time, "\t Process 2 Started");
        #20;
        $display($time, "\t Process 2 Finshed");
    end
join_any
//wait fork;  // waiting for the completion of active fork threads

$display("----------------------------------------------------------");
$finish;   // ends the simulation
end

endmodule
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其输出结果为：

disable fork

在fork的block中，杀死或者终止一些活跃的进程。

module  disable_fork;

initial begin
    $display("----------------------------------------------------------");
    
    //fork-1
    fork
        //process-1
        begin
            $display($time,"\t Process-1 of fork-1 started");
            #5;
            $display($time,"\t Process-1 of fork-1 Finished");
        end

        //process-2
        begin
            $display($time, "\t Process-2 of fork-1 started");
            #20;
            $display($time, "\t Process-2 of fork-1 Finished");
        end
    join_any

    //fork-2
    fork
        //process-1
        begin
            $display($time,"\t Process-1 of fork-2 started");
            #5;
            $display($time,"\t Process-1 of fork-2 Finished");
        end

        //process-2
        begin
            $display($time, "\t Process-2 of fork-2 started");
            #20;
            $display($time, "\t Process-2 of fork-2 Finished");
        end
    join_none
    disable fork;
    $display("----------------------------------------------------------");
    $display($time,"\t After disable-fork");
    $display("----------------------------------------------------------");
end

endmodule
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其输出结果为：