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   <!-- <h1>Event-driven programming</h1> -->
   <dl>
   <dt>authors</dt>
   <dd>Jason Lowe-Power</dd>
 </dl>

 <h1 id="event-driven-programming">Event-driven programming</h1>

 <p>gem5 is an event-driven simulator. In this chapter, we will explore how
 to create and schedule events. We will be building from the simple
 <code class="highlighter-rouge">HelloObject</code> from hello-simobject-chapter.</p>

 <h2 id="creating-a-simple-event-callback">Creating a simple event callback</h2>

 <p>In gem5’s event-driven model, each event has a callback function in
 which the event is <em>processed</em>. Generally, this is a class that inherits
 from :cppEvent. However, gem5 provides a wrapper function for creating
 simple events.</p>

 <p>In the header file for our <code class="highlighter-rouge">HelloObject</code>, we simply need to declare a
 new function that we want to execute every time the event fires
 (<code class="highlighter-rouge">processEvent()</code>). This function must take no parameters and return
 nothing.</p>

 <p>Next, we add an <code class="highlighter-rouge">Event</code> instance. In this case, we will use an
 <code class="highlighter-rouge">EventFunctionWrapper</code> which allows us to execute any function.</p>

 <p>We also add a <code class="highlighter-rouge">startup()</code> function that will be explained below.</p>

 <p>``` {.sourceCode .c++}
 class HelloObject : public SimObject
 {
   private:
     void processEvent();</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>EventFunctionWrapper event;
 </code></pre></div></div>

 <p>public:
     HelloObject(HelloObjectParams *p);</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>void startup(); }; ```
 </code></pre></div></div>

 <p>Next, we must construct this event in the constructor of <code class="highlighter-rouge">HelloObject</code>.
 The <code class="highlighter-rouge">EventFuntionWrapper</code> takes two parameters, a function to execute
 and a name. The name is usually the name of the SimObject that owns the
 event. When printing the name, there will be an automatic
 “.wrapped_function_event” appended to the end of the name.</p>

 <p>The first parameter is simply a function that takes no parameters and
 has no return value (<code class="highlighter-rouge">std::function&lt;void(void)&gt;</code>). Usually, this is a
 simple lambda function that calls a member function. However, it can be
 any function you want. Below, we captute <code class="highlighter-rouge">this</code> in the lambda (<code class="highlighter-rouge">[this]</code>)
 so we can call member functions of the instance of the class.</p>

 <p>``` {.sourceCode .c++}
 HelloObject::HelloObject(HelloObjectParams *params) :
     SimObject(params), event([this]{processEvent();}, name())
 {
     DPRINTF(Hello, “Created the hello object\n”);
 }</p>
 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
 We also must define the implementation of the process function. In this
 case, we'll simply print something if we are debugging.

 ``` {.sourceCode .c++}
 void
 HelloObject::processEvent()
 {
     DPRINTF(Hello, "Hello world! Processing the event!\n");
 }
 </code></pre></div></div>

 <h2 id="scheduling-events">Scheduling events</h2>

 <p>Finally, for the event to be processed, we first have to <em>schedule</em> the
 event. For this we use the :cppschedule function. This function
 schedules some instance of an <code class="highlighter-rouge">Event</code> for some time in the future
 (event-driven simulation does not allow events to execute in the past).</p>

 <p>We will initially schedule the event in the <code class="highlighter-rouge">startup()</code> function we
 added to the <code class="highlighter-rouge">HelloObject</code> class. The <code class="highlighter-rouge">startup()</code> function is where
 SimObjects are allowed to schedule internal events. It does not get
 executed until the simulation begins for the first time (i.e. the
 <code class="highlighter-rouge">simulate()</code> function is called from a Python config file).</p>

 <p>``` {.sourceCode .c++}
 void
 HelloObject::startup()
 {
     schedule(event, 100);
 }</p>
 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
 Here, we simply schedule the event to execute at tick 100. Normally, you
 would use some offset from `curTick()`, but since we know the startup()
 function is called when the time is currently 0, we can use an explicit
 tick value.

 The output when you run gem5 with the "Hello" debug flag is now

     gem5 Simulator System.  http://gem5.org
     gem5 is copyrighted software; use the --copyright option for details.

     gem5 compiled Jan  4 2017 11:01:46
     gem5 started Jan  4 2017 13:41:38
     gem5 executing on chinook, pid 1834
     command line: build/X86/gem5.opt --debug-flags=Hello configs/learning_gem5/part2/run_hello.py

     Global frequency set at 1000000000000 ticks per second
           0: hello: Created the hello object
     Beginning simulation!
     info: Entering event queue @ 0.  Starting simulation...
         100: hello: Hello world! Processing the event!
     Exiting @ tick 18446744073709551615 because simulate() limit reached

 More event scheduling
 ---------------------

 We can also schedule new events within an event process action. For
 instance, we are going to add a latency parameter to the `HelloObject`
 and a parameter for how many times to fire the event. In the [next
 chapter](parameters-chapter) we will make these parameters accessible
 from the Python config files.

 To the HelloObject class declaration, add a member variable for the
 latency and number of times to fire.

 ``` {.sourceCode .c++}
 class HelloObject : public SimObject
 {
   private:
     void processEvent();

     EventFunctionWrapper event;

     Tick latency;

     int timesLeft;

   public:
     HelloObject(HelloObjectParams *p);

     void startup();
 };
 </code></pre></div></div>

 <p>Then, in the constructor add default values for the <code class="highlighter-rouge">latency</code> and
 <code class="highlighter-rouge">timesLeft</code>.</p>

 <p>``` {.sourceCode .c++}
 HelloObject::HelloObject(HelloObjectParams *params) :
     SimObject(params), event([this]{processEvent();}, name()),
     latency(100), timesLeft(10)
 {
     DPRINTF(Hello, “Created the hello object\n”);
 }</p>
 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
 Finally, update `startup()` and `processEvent()`.

 ``` {.sourceCode .c++}
 void
 HelloObject::startup()
 {
     schedule(event, latency);
 }

 void
 HelloObject::processEvent()
 {
     timesLeft--;
     DPRINTF(Hello, "Hello world! Processing the event! %d left\n", timesLeft);

     if (timesLeft &lt;= 0) {
         DPRINTF(Hello, "Done firing!\n");
     } else {
         schedule(event, curTick() + latency);
     }
 }
 </code></pre></div></div>

 <p>Now, when we run gem5, the event should fire 10 times, and the
 simulation will end after 1000 ticks. The output should now look like
 the following.</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>gem5 Simulator System.  http://gem5.org
 gem5 is copyrighted software; use the --copyright option for details.

 gem5 compiled Jan  4 2017 13:53:35
 gem5 started Jan  4 2017 13:54:11
 gem5 executing on chinook, pid 2326
 command line: build/X86/gem5.opt --debug-flags=Hello configs/learning_gem5/part2/run_hello.py

 Global frequency set at 1000000000000 ticks per second
       0: hello: Created the hello object
 Beginning simulation!
 info: Entering event queue @ 0.  Starting simulation...
     100: hello: Hello world! Processing the event! 9 left
     200: hello: Hello world! Processing the event! 8 left
     300: hello: Hello world! Processing the event! 7 left
     400: hello: Hello world! Processing the event! 6 left
     500: hello: Hello world! Processing the event! 5 left
     600: hello: Hello world! Processing the event! 4 left
     700: hello: Hello world! Processing the event! 3 left
     800: hello: Hello world! Processing the event! 2 left
     900: hello: Hello world! Processing the event! 1 left
    1000: hello: Hello world! Processing the event! 0 left
    1000: hello: Done firing!
 Exiting @ tick 18446744073709551615 because simulate() limit reached
 </code></pre></div></div>

 <p>You can find the updated header file
 here &lt;../_static/scripts/part2/events/hello_object.hh&gt; and the
 implementation file
 here &lt;../_static/scripts/part2/events/hello_object.cc&gt;.</p>


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	<!-- <h1>Event-driven programming</h1> -->
	<dl>
	<dt>authors</dt>
	<dd>Jason Lowe-Power</dd>
	</dl>

	<h1 id="event-driven-programming">Event-driven programming</h1>

	<p>gem5 is an event-driven simulator. In this chapter, we will explore how
	to create and schedule events. We will be building from the simple
	<code class="highlighter-rouge">HelloObject</code> from hello-simobject-chapter.</p>

	<h2 id="creating-a-simple-event-callback">Creating a simple event callback</h2>

	<p>In gem5’s event-driven model, each event has a callback function in
	which the event is <em>processed</em>. Generally, this is a class that inherits
	from :cppEvent. However, gem5 provides a wrapper function for creating
	simple events.</p>

	<p>In the header file for our <code class="highlighter-rouge">HelloObject</code>, we simply need to declare a
	new function that we want to execute every time the event fires
	(<code class="highlighter-rouge">processEvent()</code>). This function must take no parameters and return
	nothing.</p>

	<p>Next, we add an <code class="highlighter-rouge">Event</code> instance. In this case, we will use an
	<code class="highlighter-rouge">EventFunctionWrapper</code> which allows us to execute any function.</p>

	<p>We also add a <code class="highlighter-rouge">startup()</code> function that will be explained below.</p>

	<p>``` {.sourceCode .c++}
	class HelloObject : public SimObject
	{
	private:
	void processEvent();</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>EventFunctionWrapper event;
	</code></pre></div></div>

	<p>public:
	HelloObject(HelloObjectParams *p);</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>void startup(); }; ```
	</code></pre></div></div>

	<p>Next, we must construct this event in the constructor of <code class="highlighter-rouge">HelloObject</code>.
	The <code class="highlighter-rouge">EventFuntionWrapper</code> takes two parameters, a function to execute
	and a name. The name is usually the name of the SimObject that owns the
	event. When printing the name, there will be an automatic
	“.wrapped_function_event” appended to the end of the name.</p>

	<p>The first parameter is simply a function that takes no parameters and
	has no return value (<code class="highlighter-rouge">std::function<void(void)></code>). Usually, this is a
	simple lambda function that calls a member function. However, it can be
	any function you want. Below, we captute <code class="highlighter-rouge">this</code> in the lambda (<code class="highlighter-rouge">[this]</code>)
	so we can call member functions of the instance of the class.</p>

	<p>``` {.sourceCode .c++}
	HelloObject::HelloObject(HelloObjectParams *params) :
	SimObject(params), event([this]{processEvent();}, name())
	{
	DPRINTF(Hello, “Created the hello object\n”);
	}</p>
	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
	We also must define the implementation of the process function. In this
	case, we'll simply print something if we are debugging.

	``` {.sourceCode .c++}
	void
	HelloObject::processEvent()
	{
	DPRINTF(Hello, "Hello world! Processing the event!\n");
	}
	</code></pre></div></div>

	<h2 id="scheduling-events">Scheduling events</h2>

	<p>Finally, for the event to be processed, we first have to <em>schedule</em> the
	event. For this we use the :cppschedule function. This function
	schedules some instance of an <code class="highlighter-rouge">Event</code> for some time in the future
	(event-driven simulation does not allow events to execute in the past).</p>

	<p>We will initially schedule the event in the <code class="highlighter-rouge">startup()</code> function we
	added to the <code class="highlighter-rouge">HelloObject</code> class. The <code class="highlighter-rouge">startup()</code> function is where
	SimObjects are allowed to schedule internal events. It does not get
	executed until the simulation begins for the first time (i.e. the
	<code class="highlighter-rouge">simulate()</code> function is called from a Python config file).</p>

	<p>``` {.sourceCode .c++}
	void
	HelloObject::startup()
	{
	schedule(event, 100);
	}</p>
	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
	Here, we simply schedule the event to execute at tick 100. Normally, you
	would use some offset from `curTick()`, but since we know the startup()
	function is called when the time is currently 0, we can use an explicit
	tick value.

	The output when you run gem5 with the "Hello" debug flag is now

	gem5 Simulator System. http://gem5.org
	gem5 is copyrighted software; use the --copyright option for details.

	gem5 compiled Jan 4 2017 11:01:46
	gem5 started Jan 4 2017 13:41:38
	gem5 executing on chinook, pid 1834
	command line: build/X86/gem5.opt --debug-flags=Hello configs/learning_gem5/part2/run_hello.py

	Global frequency set at 1000000000000 ticks per second
	0: hello: Created the hello object
	Beginning simulation!
	info: Entering event queue @ 0. Starting simulation...
	100: hello: Hello world! Processing the event!
	Exiting @ tick 18446744073709551615 because simulate() limit reached

	More event scheduling
	---------------------

	We can also schedule new events within an event process action. For
	instance, we are going to add a latency parameter to the `HelloObject`
	and a parameter for how many times to fire the event. In the [next
	chapter](parameters-chapter) we will make these parameters accessible
	from the Python config files.

	To the HelloObject class declaration, add a member variable for the
	latency and number of times to fire.

	``` {.sourceCode .c++}
	class HelloObject : public SimObject
	{
	private:
	void processEvent();

	EventFunctionWrapper event;

	Tick latency;

	int timesLeft;

	public:
	HelloObject(HelloObjectParams *p);

	void startup();
	};
	</code></pre></div></div>

	<p>Then, in the constructor add default values for the <code class="highlighter-rouge">latency</code> and
	<code class="highlighter-rouge">timesLeft</code>.</p>

	<p>``` {.sourceCode .c++}
	HelloObject::HelloObject(HelloObjectParams *params) :
	SimObject(params), event([this]{processEvent();}, name()),
	latency(100), timesLeft(10)
	{
	DPRINTF(Hello, “Created the hello object\n”);
	}</p>
	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
	Finally, update `startup()` and `processEvent()`.

	``` {.sourceCode .c++}
	void
	HelloObject::startup()
	{
	schedule(event, latency);
	}

	void
	HelloObject::processEvent()
	{
	timesLeft--;
	DPRINTF(Hello, "Hello world! Processing the event! %d left\n", timesLeft);

	if (timesLeft <= 0) {
	DPRINTF(Hello, "Done firing!\n");
	} else {
	schedule(event, curTick() + latency);
	}
	}
	</code></pre></div></div>

	<p>Now, when we run gem5, the event should fire 10 times, and the
	simulation will end after 1000 ticks. The output should now look like
	the following.</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>gem5 Simulator System. http://gem5.org
	gem5 is copyrighted software; use the --copyright option for details.

	gem5 compiled Jan 4 2017 13:53:35
	gem5 started Jan 4 2017 13:54:11
	gem5 executing on chinook, pid 2326
	command line: build/X86/gem5.opt --debug-flags=Hello configs/learning_gem5/part2/run_hello.py

	Global frequency set at 1000000000000 ticks per second
	0: hello: Created the hello object
	Beginning simulation!
	info: Entering event queue @ 0. Starting simulation...
	100: hello: Hello world! Processing the event! 9 left
	200: hello: Hello world! Processing the event! 8 left
	300: hello: Hello world! Processing the event! 7 left
	400: hello: Hello world! Processing the event! 6 left
	500: hello: Hello world! Processing the event! 5 left
	600: hello: Hello world! Processing the event! 4 left
	700: hello: Hello world! Processing the event! 3 left
	800: hello: Hello world! Processing the event! 2 left
	900: hello: Hello world! Processing the event! 1 left
	1000: hello: Hello world! Processing the event! 0 left
	1000: hello: Done firing!
	Exiting @ tick 18446744073709551615 because simulate() limit reached
	</code></pre></div></div>

	<p>You can find the updated header file
	here <../_static/scripts/part2/events/hello_object.hh> and the
	implementation file
	here <../_static/scripts/part2/events/hello_object.cc>.</p>


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