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何为Systemverilo的线程或进程？

线程或进程是作为独立实体执行的任何一段代码。在verilog中，每个初始块和always块都派生为独立的线程，从0点开始并行运行。fork连接块还创建并行运行的不同线程。

不同的fork-join之间的区别?

下图为不同fork join的区别

fork join	Finishes when all child threads are over
fork join_any	Finishes when any child thread gets over
fork join_none	Finishes soon after child threads are spawned

线程或进程应该用在测试平台的什么地方？

验证环境中的组件可能需要能够并发运行多个任务。例如，一个进程可能等待一些事情发生，而另一个进程则继续执行一些其他任务。它们都是通过fork…join语句作为单独的线程衍生出来的。例如，scoreboard可以并行生成不同的任务，以获取和验证来自测试台不同部分的数据。

Verilog fork join的局限?

只有当fork-join中派生的所有线程都完成时，fork…join之后的代码才执行。因此，scb必须等到fork-join中生成的所有线程都完成后才能继续。

fork join示例

Systemverilog 的fork…join会等待所有任务结束在执行后面的代码。

module tb_top;
   initial begin

   	  #1 $display ("[%0t ns] Start fork ...", $time);

   	  // Main Process: Fork these processes in parallel and wait untill all
   	  // of them finish
      fork
      	 // Thread1 : Print this statement after 5ns from start of fork
         #5 $display ("[%0t ns] Thread1: Orange is named after orange", $time);

         // Thread2 : Print these two statements after the given delay from start of fork
         begin
            #2 $display ("[%0t ns] Thread2: Apple keeps the doctor away", $time);
            #4 $display ("[%0t ns] Thread2: But not anymore", $time);
         end

         // Thread3 : Print this statement after 10ns from start of fork
         #10 $display ("[%0t ns] Thread3: Banana is a good fruit", $time);
      join

      // Main Process: Continue with rest of statements once fork-join is over
      $display ("[%0t ns] After Fork-Join", $time);
   end
endmodule
Simulation Log
ncsim> run
[1 ns] Start fork ...
[3 ns] Thread2: Apple keeps the doctor away
[6 ns] Thread1: Orange is named after orange
[7 ns] Thread2: But not anymore
[11 ns] Thread3: Banana is a good fruit
[11 ns] After Fork-Join
ncsim: *W,RNQUIE: Simulation is complete.

fork join_any示例

SystemVerilog fork join_any 会等到任何一个分支进程完成。

module tb_top;
   initial begin

   	  #1 $display ("[%0t ns] Start fork ...", $time);

   	  // Main Process: Fork these processes in parallel and wait until
      // any one of them finish
      fork
      	 // Thread1 : Print this statement after 5ns from start of fork
         #5 $display ("[%0t ns] Thread1: Orange is named after orange", $time);

         // Thread2 : Print these two statements after the given delay from start of fork
         begin
            #2 $display ("[%0t ns] Thread2: Apple keeps the doctor away", $time);
            #4 $display ("[%0t ns] Thread2: But not anymore", $time);
         end

         // Thread3 : Print this statement after 10ns from start of fork
         #10 $display ("[%0t ns] Thread3: Banana is a good fruit", $time);
      join_any

      // Main Process: Continue with rest of statements once fork-join is exited
      $display ("[%0t ns] After Fork-Join", $time);
   end
endmodule
Simulation Log
ncsim> run
[1 ns] Start fork ...
[3 ns] Thread2: Apple keeps the doctor away
[6 ns] Thread1: Orange is named after orange
[6 ns] After Fork-Join
[7 ns] Thread2: But not anymore
[11 ns] Thread3: Banana is a good fruit
ncsim: *W,RNQUIE: Simulation is complete.

fork join_none示例

SystemVerilog fork join_none 不会等待并立即退出块，允许分叉进程在后台运行。主线程恢复执行在 fork join_none 块之后的语句。

module tb_top;
   initial begin

   	  #1 $display ("[%0t ns] Start fork ...", $time);

   	  // Main Process: Fork these processes in parallel and exits immediately
      fork
      	 // Thread1 : Print this statement after 5ns from start of fork
         #5 $display ("[%0t ns] Thread1: Orange is named after orange", $time);

         // Thread2 : Print these two statements after the given delay from start of fork
         begin
            #2 $display ("[%0t ns] Thread2: Apple keeps the doctor away", $time);
            #4 $display ("[%0t ns] Thread2: But not anymore", $time);
         end

         // Thread3 : Print this statement after 10ns from start of fork
         #10 $display ("[%0t ns] Thread3: Banana is a good fruit", $time);
      join_none

      // Main Process: Continue with rest of statements once fork-join is exited
      $display ("[%0t ns] After Fork-Join", $time);
   end
endmodule
Simulation Log
ncsim> run
[1 ns] Start fork ...
[1 ns] After Fork-Join
[3 ns] Thread2: Apple keeps the doctor away
[6 ns] Thread1: Orange is named after orange
[7 ns] Thread2: But not anymore
[11 ns] Thread3: Banana is a good fruit
ncsim: *W,RNQUIE: Simulation is complete.

SystemVerilog disable fork join

在前文中，讨论了启动并行线程的不同方法。现在我们将看到如何禁用分叉线程。所有从fork…join启动的活动线程都可以通过调用disable fork来终止。

为何需要禁用一个fork？

对于给定的例子，在仿真开始时会发生以下事情:

1. 主线程执行初始块并找到一个fork join_any块
2. 它将并行启动三个线程，并等待其中任何一个线程完成
3. 线程首先完成，因为延迟最少
4. 恢复执行主线程
   Threadz和Thread3仍然在运行，即使主线程已经脱离了fork join_any块。

module tb_top;

	initial begin
		// Fork off 3 sub-threads in parallel and the currently executing main thread
		// will finish when any of the 3 sub-threads have finished.
		fork

			// Thread1 : Will finish first at time 40ns
			#40 $display ("[%0t ns] Show #40 $display statement", $time);

			// Thread2 : Will finish at time 70ns
			begin
				#20 $display ("[%0t ns] Show #20 $display statement", $time);
				#50 $display ("[%0t ns] Show #50 $display statement", $time);
			end

			// Thread3 : Will finish at time 60ns
			#60 $display ("[%0t ns] TIMEOUT", $time);
		join_any

		// Display as soon as the fork is done
		$display ("[%0tns] Fork join is done", $time);
   end
endmodule
Simulation Log
ncsim> run
[20 ns] Show #20 $display statement
[40 ns] Show #40 $display statement
[40ns] Fork join is done
[60 ns] TIMEOUT
[70 ns] Show #50 $display statement
ncsim: *W,RNQUIE: Simulation is complete.
ncsim> exit

当fork被禁用时会发生什么?

上面的例子是相同的，最后添加了disable fork。由于禁用了fork, Thread2和Thread3被杀死了

module tb_top;

	initial begin
   		// Fork off 3 sub-threads in parallel and the currently executing main thread
      	// will finish when any of the 3 sub-threads have finished.
		fork

         // Thread1 : Will finish first at time 40ns
         #40 $display ("[%0t ns] Show #40 $display statement", $time);

         // Thread2 : Will finish at time 70ns
         begin
            #20 $display ("[%0t ns] Show #20 $display statement", $time);
            #50 $display ("[%0t ns] Show #50 $display statement", $time);
         end

         // Thread3 : Will finish at time 60ns
          #60 $display ("[%0t ns] TIMEOUT", $time);
      join_any

      // Display as soon as the fork is done
      $display ("[%0tns] Fork join is done, let's disable fork", $time);

      disable fork;
   end
endmodule
Simulation Log
ncsim> run
[20 ns] Show #20 $display statement
[40 ns] Show #40 $display statement
[40ns] Fork join is done, let's disable fork
ncsim: *W,RNQUIE: Simulation is complete.
ncsim> exit