1 基本定义

        在 SystemVerilog 中，事件是一种用于线程同步的机制。它就像是一个信号旗，不同的线程（如 initial 块、always 块等产生的进程）可以等待这个信号，当信号被触发（就像旗帜被升起）时，等待这个事件的线程就可以继续执行。

        事件主要用于协调不同的并发进程之间的操作顺序，确保某些操作在其他操作完成之后才开始。

        event是一个静态对象句柄，用于在两个或多个并发活动进程之间进行同步。一个进程将触发event，另一个thread的等待event。event

• 可以分配或其它event变量进行比较
  • 可以分配给null
  • 当分配给另一个event时，两个变量都指向同一个同步对象
• 可以传递给队列、函数、任务

• event over;                  // a new event is created called over
  event over_again = over;     // over_again becomes an alias to over
  event empty      = null;     // event variable with no synchronization object
  

2 事件的声明和触发

1. 声明：事件的声明非常简单，使用event关键字。例如：event my_event;就声明了一个名为my_event的事件。

   触发：可以使用->操作符来触发一个事件。例如，在一个进程中：

initial begin#10;-> my_event;
}

3 等待事件

       1  使用@操作符可以实现阻塞式等待一个事件。例如：

initial begin$display("等待事件触发...");@my_event;$display("事件已触发，继续执行。");
}

        2  可以使用wait(my_event.triggered)来实现非阻塞式等待。例如：

initial beginforkbegin$display("非阻塞等待事件触发...");wait(my_event.triggered);$display("事件已触发，非阻塞等待完成。");endbegin#5;-> my_event;}join
}

4 使用示例

module tb;// Created an event variable that processes can use to trigger and waitevent event_a;// Thread1: Triggers the event using "->" operatorinitial begin#20 -> event_a;$display ("[%0t] Thread1：triggered event_a", $time);end// Thread2: Waits for the event using "@" operatorinitial begin$display ("[%0t] Thread2：waiting for trigger", $time);@(event_a);$display ("[%0t] Thread2：received event_a trigger", $time);end// Thread3: Waits for the event using ".triggered"initial begin$display ("[%0t] Thread3：waiting for trigger", $time);wait(event_a.triggered);$display ("[%0t] Thread2：received event_a trigger", $time);end
endmodule
//----------------------------------------------------------------//
//                           display log                          //
//----------------------------------------------------------------//ncsim> run
[0] Thread2: Waiting for trigger
[0] Thread3: Waiting for trigger
[20] Thread1: triggered event_a
[20] Thread2: received event_a trigger
[20] Thread3: received event_a trigger
ncsim: *W,RNQUIE: Simulation is complete.

4.1 wait(event_a.triggered)和@(event_a);

 event的状态在整个事件步长中持续存在，直到模拟进行。因此，如果event和event的触发器同时发生，则将存在竞争条件，该属性有助于避免这种情况。

module tb;// Created an event variable that processes can use to trigger and waitevent event_a;// Thread1: Triggers the event using "->" operator at 20nsinitial begin#20 -> event_a;$display ("[%0t] Thread1：triggered event_a", $time);end// Thread2: Starts waiting for the event using "@" operator at 20nsinitial begin$display ("[%0t] Thread2：waiting for trigger", $time);#20 @(event_a);$display ("[%0t] Thread2：received event_a trigger", $time);end// Thread3: Starts waiting for the event using ".triggered" at 20nsinitial begin$display ("[%0t] Thread3：waiting for trigger", $time);#20 wait(event_a.triggered);$display ("[%0t] Thread2：received event_a trigger", $time);end
endmodule//----------------------------------------------------------------//
//                           display log                          //
//----------------------------------------------------------------//
请注意，由于@和->操作之间的争用条件，Thread2从未收到触发器。
ncsim> run
[0] Thread2: Waiting for trigger
[0] Thread3: Waiting for trigger
[20] Thread1: triggered event_a
[20] Thread3: received event_a trigger
ncsim: *W,RNQUIE: Simulation is complete.

4.2 wait order

等待按给定顺序触发事件，如果任何事件无序执行，则发出错误。

module tb;// Declare three events that can be triggered aeparatelyevent a, b, c;// This block triggers each event one by oneinitial begin#10 -> a;#10 -> b;#10 -> c;end// This block waits until each event is triggered in the given orderinitial beginwait_order (a,b,c)$display ("Events were executed in the correct order");else$display ("Events were NOT executed in the correct order !");end
endmodule//----------------------------------------------------------------//
//                           display log                          //
//----------------------------------------------------------------//
Compiler version J-2014.12-SP1-1: Runtime version J-2014.12-SP1-1
Events were executed in the correct order

4.4 Merging Events

当一个event变量分配给另一个event变量时，等待第一个event触发的所有threads都将等待第二个变量触发。

module tb;// Create event variabllesevent event_a, event_b;initial beginfork// Thread1: waits for event_a to be triggeredbeginwait(event_a.triggered);$display ("[%0t] Thread1: Wait for event_a is over", $time);end// Thread2: waits for event_b to be triggeredbeginwait(event_b.triggered);$display ("[%0t] Thread2: Wait for event_b is over", $time);end// Thread3: triggers event_a at 20ns#20 ->event_a;// Thread4: triggers event_b at 20ns#20 ->event_b;// Thread5: Assigns event_b to event_a at 10nsbegin// Comment code below and try again to see Thread2 finish later#10 event_b = event_a;endjoinend
endmodule
//----------------------------------------------------------------//
//                           display log                          //
//----------------------------------------------------------------//
ncsim> run
[20] Thread1: Wait for event_a is over
[20] Thread2: Wait for event_b is over
ncsim: *W,RNQUIE: Simulation is complete.

4.4 验证环境中的使用