[quick start of Digital IC Verification] 18. Basic grammar of SystemVerilog learning 5 (concurrent threads... Including practical exercises)

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One 、 Description of content

• Concurrent operation characteristics of logic simulation tools
• Threads ：fork...join | join_any | join_none
• wait fork
• disable fork

Two 、 Logic simulation tool Concurrent Handling characteristics

2.1、 Concurrency means

• For all concurrent threads , Within the current simulation time of the simulation tool , The scheduled events will be completed before the simulation step to the next simulation time

2.2、 Concurrent thread execution

• When a thread executes , Only meet wait Statement will stop
  • There are sub threads generated by the executing thread Execute by queue
• When the executing thread encounters Wait statement when , In the queue ready Threads in state can execute
• When all threads enter wait In the state of , Simulation time update
• Examples of waiting statements ：

@(router.cb);

wait(var_a == 1);

##1 router.cb.din <= 4’hf;

join_any;

join

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2.3、 Concurrent thread execution mode

• When a thread executes , All other threads enter the queue and wait

  • READY - Represents the thread executing within the current simulation time
  • WAIT - Indicates that the statement is blocked （ Thread cannot execute ）, When a waiting condition is encountered, you can continue to execute

• When the executing thread enters WAIT In the state of , Thread into WAIT Sequence , next READY The thread in state continues to execute

• When all threads enter WAIT In the state of , The simulation time steps to the next simulation cycle

3、 ... and 、 Concurrent thread thread

3.1、 Statement set

• Veilog There is a typical set of concurrent statements

  • initial sentence ： Only once in the entire simulation time ,initial Statements are concurrent
  • always sentence ： It can model combinational circuits and sequential circuits ,always Statements are concurrent
  • assign sentence ： It can model the combinational circuit ,assign Statements are concurrent
  • begin...end ： Sentence from top to bottom , Sequential execution
  • fork...join： sentence Parallel execution , Independent of statement order

• fork...join

  • fork...join Statement blocks can create processes that execute in parallel ; Two fork...join stay initial in begin..end Between , Two fork...join Is a serial relationship , If two fork...join The two outer edges are fork...join, Well, the two one. fork...join It's parallel .

3.2、fork-join Create concurrent threads

• Use fork-join Statement to create a concurrent thread

int a, b, c;
fork
	statement0; // Threads 1
	begin // Threads 2 
		statement1;// Threads 2.1
		statement2;// Threads 2.2
	end
join | join_any | join_none
statement	

 
  

 
  

   
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• fork-join Thread in 1 And thread 2 Is executed in parallel
• fork-join Statements are encapsulated in begin-end Statements in form a single child thread （ Threads 2.1 And thread 2.2）, And in the order of statements , From top to bottom
• Concurrent thread There is no fixed sequence of execution
• All child threads share the variables of the parent thread

3.3、join Options

notes ： The parent thread here refers to for...join Outside the statement block, below The thread of

• fork...join（ And The relationship between ）
  • When all the sub threads are finished , The parent thread will continue to execute
  • stay fork...join In the block , have access to begin...end The statement block encapsulates a separate thread , All statements in this thread are executed sequentially

• fork...join_any（ or The relationship between ）
  • When fork...join_any After any child thread in the statement block completes , The parent thread continues to execute

• fork...join_noe
  • Parent thread and for The sub threads in the statement block are executed in parallel
  • When the parent thread executes a blocking statement , The child thread starts to execute （ The parent thread executes first ）

• fork...join and begin...end Example

• fork...join_any and begin...end Example

• fork...join_none and begin...end Example

Q： The above statements are preceded by time units , If there is no time unit in front of some sentences ,fork How to execute the statement ？

• A： There is no time unit , The sequence of execution of these statements can be determined at the compilation stage , After the compilation phase is determined, it is executed in sequence at the actual run time .【 The actual project must have a time unit ！】

3.4、 Concurrent thread control

• wait fork
  • Wait for all fork Concurrent process execution completed
  • Before executing the parent thread , Ensure that all fork Parallel sub thread execution is completed
  • Wait for all fork Send all the sub threads to complete the execution

• without wait fork,exec5(); Need to wait until fork...join_any After any one of them is executed , and fork...join_none Need to wait until exec5() Only after the execution is completed .

• disable fork

  • Stop the execution of all parallel sub threads

3.5、 Test questions

• A: 2 individual ;B：2 individual ;C：1 individual ;D：1 individual

• Cannot simulate normally ： Threads 1 The clock is simulating 0 Act at all times , Threads 2 Is in 5 In time units , hold a become 5, Threads will not emulate to the 5 Time units ！

• a = 4;b = 8;
• Although the print results are parallel , But the simulator can't be fully parallel , The following can still use the above value ！（ There are tiny δ Time difference ）

• a = 7; b = 4;

• Generally, we will not be in for Use in circulation fork...join
  
• Just understand this mistake ！

X、 Practice

X.1、fork...join/join_any/join_none Demo

module thread();
initial begin
  fork
    // Thread 1
    begin
      #6;
      $display("******@%0t, fork join sub thread 1 begin", $time);
    end
    // Thread 2
    begin
      #5;
      $display("******@%0t, fork join sub thread 2 begin", $time);
    end
  join
  $display("******@%0t, fork join father thread begin", $time);
  #20;
  fork
    // Thread 1
    begin
      #6;
      $display("******@%0t, fork join_any sub thread 1 begin", $time);
    end
    // Thread 2
    begin
      #5;
      $display("******@%0t, fork join_any sub thread 2 begin", $time);
    end
  join_any
  $display("******@%0t, fork join_any father thread begin", $time);
  #20;
  fork
    // Thread 1
    begin
      #6;
      $display("******@%0t, fork join_none sub thread 1 begin", $time);
    end
    // Thread 2
    begin
      #5;
      $display("******@%0t, fork join_none sub thread 2 begin", $time);
    end
  join_none
  $display("******@%0t, fork join_none father thread begin", $time);
end
endmodule 
******@5000, fork join sub thread 2 begin
******@6000, fork join sub thread 1 begin
******@6000, fork join father thread begin
******@31000, fork join_any sub thread 2 begin
******@31000, fork join_any father thread begin
******@32000, fork join_any sub thread 1 begin
******@51000, fork join_none father thread begin
******@56000, fork join_none sub thread 2 begin
******@57000, fork join_none sub thread 1 begin