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   <!-- <h1>Creating a very simple SimObject</h1> -->
   <dl>
   <dt>authors</dt>
   <dd>Jason Lowe-Power</dd>
 </dl>

 <h1 id="creating-a-very-simple-simobject">Creating a <em>very</em> simple SimObject</h1>

 <p>Almost all objects in gem5 inherit from the base SimObject type.
 SimObjects export the main interfaces to all objects in gem5. SimObjects
 are wrapped <code class="highlighter-rouge">C++</code> objects that are accessible from the <code class="highlighter-rouge">Python</code>
 configuration scripts.</p>

 <p>SimObjects can have many parameters, which are set via the <code class="highlighter-rouge">Python</code>
 configuration files. In addition to simple parameters like integers and
 floating point numbers, they can also have other SimObjects as
 parameters. This allows you to create complex system hierarchies, like
 real machines.</p>

 <p>In this chapter, we will walk through creating a simple “HelloWorld”
 SimObject. The goal is to introduce you to how SimObjects are created
 and the required boilerplate code for all SimObjects. We will also
 create a simple <code class="highlighter-rouge">Python</code> configuration script which instantiates our
 SimObject.</p>

 <p>In the next few chapters, we will take this simple SimObject and expand
 on it to include <a href="debugging-chapter">debugging support</a>, <a href="events-chapter">dynamic
 events</a>, and <a href="parameters-chapter">parameters</a>.</p>

 <blockquote>
   <p><strong>Using git branches</strong></p>

   <p>It is common to use a new git branch for each new feature you add to
 gem5.</p>

   <p>The first step when adding a new feature or modifying something in
 gem5, is to create a new branch to store your changes. Details on git
 branches can be found in the Git book_.</p>

   <p><code class="highlighter-rouge">{.sourceCode .sh}
 git checkout -b hello-simobject
 </code></p>
 </blockquote>

 <h2 id="step-1-create-a-python-class-for-your-new-simobject">Step 1: Create a Python class for your new SimObject</h2>

 <p>Each SimObject has a Python class which is associated with it. This
 Python class describes the parameters of your SimObject that can be
 controlled from the Python configuration files. For our simple
 SimObject, we are just going to start out with no parameters. Thus, we
 simply need to declare a new class for our SimObject and set it’s name
 and the C++ header that will define the C++ class for the SimObject.</p>

 <p>We can create a file, HelloObject.py, in <code class="highlighter-rouge">src/learning_gem5</code></p>

 <p>``` {.sourceCode .python}
 from m5.params import *
 from m5.SimObject import SimObject</p>

 <p>class HelloObject(SimObject):
     type = ‘HelloObject’
     cxx_header = “learning_gem5/hello_object.hh”</p>
 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
 You can find the complete file
 here \&lt;../\_static/scripts/part2/helloobject/HelloObject.py\&gt;.

 It is not required that the `type` be the same as the name of the class,
 but it is convention. The `type` is the C++ class that you are wrapping
 with this Python SimObject. Only in special circumstances should the
 `type` and the class name be different.

 The `cxx_header` is the file that contains the declaration of the class
 used as the `type` parameter. Again, the convention is to use the name
 of the SimObject with all lowercase and underscores, but this is only
 convention. You can specify any header file here.

 Step 2: Implement your SimObject in C++
 ---------------------------------------

 Next, we need to create `hello_object.hh` and `hello_object.cc` which
 will implement the hello object.

 We'll start with the header file for our `C++` object. By convention,
 gem5 wraps all header files in `#ifndef/#endif` with the name of the
 file and the directory its in so there are no circular includes.

 The only thing we need to do in the file is to declare our class. Since
 `HelloObject` is a SimObject, it must inherit from the C++ SimObject
 class. Most of the time, your SimObject's parent will be a subclass of
 SimObject, not SimObject itself.

 The SimObject class specifies many virtual functions. However, none of
 these functions are pure virtual, so in the simplest case, there is no
 need to implement any functions except for the constructor.

 The constructor for all SimObjects assumes it will take a parameter
 object. This parameter object is automatically created by the build
 system and is based on the `Python` class for the SimObject, like the
 one we created above. The name for this parameter type is generated
 automatically from the name of your object. For our "HelloObject" the
 parameter type's name is "HelloObject\**Params*\*".

 The code required for our simple header file is listed below.

 ``` {.sourceCode .c++}
 #ifndef __LEARNING_GEM5_HELLO_OBJECT_HH__
 #define __LEARNING_GEM5_HELLO_OBJECT_HH__

 #include "params/HelloObject.hh"
 #include "sim/sim_object.hh"

 class HelloObject : public SimObject
 {
   public:
     HelloObject(HelloObjectParams *p);
 };

 #endif // __LEARNING_GEM5_HELLO_OBJECT_HH__
 </code></pre></div></div>

 <p>You can find the complete file
 here &lt;../_static/scripts/part2/helloobject/hello_object.hh&gt;.</p>

 <p>Next, we need to implement <em>two</em> functions in the <code class="highlighter-rouge">.cc</code> file, not just
 one. The first function, is the constructor for the <code class="highlighter-rouge">HelloObject</code>. Here
 we simply pass the parameter object to the SimObject parent and print
 “Hello world!”</p>

 <p>Normally, you would <strong>never</strong> use <code class="highlighter-rouge">std::cout</code> in gem5. Instead, you
 should use debug flags. In the <a href="debugging-chapter">next chapter</a>, we
 will modify this to use debug flags instead. However, for now, we’ll
 simply use <code class="highlighter-rouge">std::cout</code> because it is simple.</p>

 <p>``` {.sourceCode .c++}
 #include “learning_gem5/hello_object.hh”</p>

 <p>#include <iostream></iostream></p>

 <p>HelloObject::HelloObject(HelloObjectParams *params) :
     SimObject(params)
 {
     std::cout « “Hello World! From a SimObject!” « std::endl;
 }</p>
 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
 There is another function that we have to implement as well for the
 SimObject to be complete. We must implement one function for the
 parameter type that is implicitly created from the SimObject `Python`
 declaration, namely, the `create` function. This function simply returns
 a new instantiation of the SimObject. Usually this function is very
 simple (as below).

 ``` {.sourceCode .c++}
 HelloObject*
 HelloObjectParams::create()
 {
     return new HelloObject(this);
 }
 </code></pre></div></div>

 <p>You can find the complete file
 here &lt;../_static/scripts/part2/helloobject/hello_object.cc&gt;.</p>

 <p>If you forget to add the create function for your SimObject, you will
 get a linker error when you compile. It will look something like the
 following.</p>

 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>build/X86/python/m5/internal/param_HelloObject_wrap.o: In function `_wrap_HelloObjectParams_create':
 /local.chinook/gem5/gem5-tutorial/gem5/build/X86/python/m5/internal/param_HelloObject_wrap.cc:3096: undefined reference to `HelloObjectParams::create()'
 collect2: error: ld returned 1 exit status
 scons: *** [build/X86/gem5.opt] Error 1
 scons: building terminated because of errors.
 </code></pre></div></div>

 <p>This <code class="highlighter-rouge">undefined reference to `HelloObjectParams::create()'</code> means
 you need to implement the create function for your SimObject.</p>

 <h2 id="step-3-register-the-simobject-and-c-file">Step 3: Register the SimObject and C++ file</h2>

 <p>In order for the <code class="highlighter-rouge">C++</code> file to be compiled and the <code class="highlighter-rouge">Python</code> file to be
 parsed we need to tell the build system about these files. gem5 uses
 SCons as the build system, so you simply have to create a SConscript
 file in the directory with the code for the SimObject. If there is
 already a SConscript file for that directory, simply add the following
 declarations to that file.</p>

 <p>This file is simply a normal <code class="highlighter-rouge">Python</code> file, so you can write any
 <code class="highlighter-rouge">Python</code> code you want in this file. Some of the scripting can become
 quite complicated. gem5 leverages this to automatically create code for
 SimObjects and to compile the domain-specific languages like SLICC and
 the ISA language.</p>

 <p>In the SConscript file, there are a number of functions automatically
 defined after you import them. See the section on that…</p>

 <p>To get your new SimObject to compile, you simply need to create a new
 file with the name “SConscript” in the <code class="highlighter-rouge">src/learning_gem5</code> directory. In
 this file, you have to declare the SimObject and the <code class="highlighter-rouge">.cc</code> file. Below
 is the required code.</p>

 <p>``` {.sourceCode .python}
 Import(‘*’)</p>

 <p>SimObject(‘HelloObject.py’)
 Source(‘hello_object.cc’)</p>
 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
 You can find the complete file
 here \&lt;../\_static/scripts/part2/helloobject/SConscript\&gt;.

 Step 4: (Re)-build gem5
 -----------------------

 To compile and link your new files you simply need to recompile gem5.
 The below example assumes you are using the x86 ISA, but nothing in our
 object requires an ISA so, this will work with any of gem5's ISAs.

 ``` {.sourceCode .sh}
 scons build/X86/gem5.opt
 </code></pre></div></div>

 <h2 id="step-5-create-the-config-scripts-to-use-your-new-simobject">Step 5: Create the config scripts to use your new SimObject</h2>

 <p>Now that you have implemented a SimObject, and it has been compiled into
 gem5, you need to create or modify a <code class="highlighter-rouge">Python</code> config file to instantiate
 your object. Since your object is very simple a system object is not
 required! CPUs are not needed, or caches, or anything, except a <code class="highlighter-rouge">Root</code>
 object. All gem5 instances require a <code class="highlighter-rouge">Root</code> object.</p>

 <p>Walking through creating a <em>very</em> simple configuration script, first,
 import m5 and all of the objects you have compiled.</p>

 <p>``` {.sourceCode .python}
 import m5
 from m5.objects import *</p>
 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
 Next, you have to instantiate the `Root` object, as required by all gem5
 instances.

 ``` {.sourceCode .python}
 root = Root(full_system = False)
 </code></pre></div></div>

 <p>Now, you can instantiate the <code class="highlighter-rouge">HelloObject</code> you created. All you need to
 do is call the <code class="highlighter-rouge">Python</code> “constructor”. Later, we will look at how to
 specify parameters via the <code class="highlighter-rouge">Python</code> constructor. In addition to creating
 an instantiation of your object, you need to make sure that it is a
 child of the root object. Only SimObjects that are children of the
 <code class="highlighter-rouge">Root</code> object are instantiated in <code class="highlighter-rouge">C++</code>.</p>

 <p>``` {.sourceCode .python}
 root.hello = HelloObject()</p>
 <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
 Finally, you need to call `instantiate` on the `m5` module and actually
 run the simulation!

 ``` {.sourceCode .python}
 m5.instantiate()

 print("Beginning simulation!")
 exit_event = m5.simulate()
 print('Exiting @ tick {} because {}'
       .format(m5.curTick(), exit_event.getCause()))
 </code></pre></div></div>

 <p>You can find the complete file
 here &lt;../_static/scripts/part2/helloobject/run_hello.py&gt;.</p>

 <p>The output should look something 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 May  4 2016 11:37:41
 gem5 started May  4 2016 11:44:28
 gem5 executing on mustardseed.cs.wisc.edu, pid 22480
 command line: build/X86/gem5.opt configs/learning_gem5/run_hello.py

 Global frequency set at 1000000000000 ticks per second
 Hello World! From a SimObject!
 Beginning simulation!
 info: Entering event queue @ 0.  Starting simulation...
 Exiting @ tick 18446744073709551615 because simulate() limit reached
 </code></pre></div></div>

 <p>Congrats! You have written your first SimObject. In the next chapters,
 we will extend this SimObject and explore what you can do with
 SimObjects.</p>


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	<!-- <h1>Creating a very simple SimObject</h1> -->
	<dl>
	<dt>authors</dt>
	<dd>Jason Lowe-Power</dd>
	</dl>

	<h1 id="creating-a-very-simple-simobject">Creating a <em>very</em> simple SimObject</h1>

	<p>Almost all objects in gem5 inherit from the base SimObject type.
	SimObjects export the main interfaces to all objects in gem5. SimObjects
	are wrapped <code class="highlighter-rouge">C++</code> objects that are accessible from the <code class="highlighter-rouge">Python</code>
	configuration scripts.</p>

	<p>SimObjects can have many parameters, which are set via the <code class="highlighter-rouge">Python</code>
	configuration files. In addition to simple parameters like integers and
	floating point numbers, they can also have other SimObjects as
	parameters. This allows you to create complex system hierarchies, like
	real machines.</p>

	<p>In this chapter, we will walk through creating a simple “HelloWorld”
	SimObject. The goal is to introduce you to how SimObjects are created
	and the required boilerplate code for all SimObjects. We will also
	create a simple <code class="highlighter-rouge">Python</code> configuration script which instantiates our
	SimObject.</p>

	<p>In the next few chapters, we will take this simple SimObject and expand
	on it to include <a href="debugging-chapter">debugging support</a>, <a href="events-chapter">dynamic
	events</a>, and <a href="parameters-chapter">parameters</a>.</p>

	<blockquote>
	<p><strong>Using git branches</strong></p>

	<p>It is common to use a new git branch for each new feature you add to
	gem5.</p>

	<p>The first step when adding a new feature or modifying something in
	gem5, is to create a new branch to store your changes. Details on git
	branches can be found in the Git book_.</p>

	<p><code class="highlighter-rouge">{.sourceCode .sh}
	git checkout -b hello-simobject
	</code></p>
	</blockquote>

	<h2 id="step-1-create-a-python-class-for-your-new-simobject">Step 1: Create a Python class for your new SimObject</h2>

	<p>Each SimObject has a Python class which is associated with it. This
	Python class describes the parameters of your SimObject that can be
	controlled from the Python configuration files. For our simple
	SimObject, we are just going to start out with no parameters. Thus, we
	simply need to declare a new class for our SimObject and set it’s name
	and the C++ header that will define the C++ class for the SimObject.</p>

	<p>We can create a file, HelloObject.py, in <code class="highlighter-rouge">src/learning_gem5</code></p>

	<p>``` {.sourceCode .python}
	from m5.params import *
	from m5.SimObject import SimObject</p>

	<p>class HelloObject(SimObject):
	type = ‘HelloObject’
	cxx_header = “learning_gem5/hello_object.hh”</p>
	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
	You can find the complete file
	here \<../\_static/scripts/part2/helloobject/HelloObject.py\>.

	It is not required that the `type` be the same as the name of the class,
	but it is convention. The `type` is the C++ class that you are wrapping
	with this Python SimObject. Only in special circumstances should the
	`type` and the class name be different.

	The `cxx_header` is the file that contains the declaration of the class
	used as the `type` parameter. Again, the convention is to use the name
	of the SimObject with all lowercase and underscores, but this is only
	convention. You can specify any header file here.

	Step 2: Implement your SimObject in C++
	---------------------------------------

	Next, we need to create `hello_object.hh` and `hello_object.cc` which
	will implement the hello object.

	We'll start with the header file for our `C++` object. By convention,
	gem5 wraps all header files in `#ifndef/#endif` with the name of the
	file and the directory its in so there are no circular includes.

	The only thing we need to do in the file is to declare our class. Since
	`HelloObject` is a SimObject, it must inherit from the C++ SimObject
	class. Most of the time, your SimObject's parent will be a subclass of
	SimObject, not SimObject itself.

	The SimObject class specifies many virtual functions. However, none of
	these functions are pure virtual, so in the simplest case, there is no
	need to implement any functions except for the constructor.

	The constructor for all SimObjects assumes it will take a parameter
	object. This parameter object is automatically created by the build
	system and is based on the `Python` class for the SimObject, like the
	one we created above. The name for this parameter type is generated
	automatically from the name of your object. For our "HelloObject" the
	parameter type's name is "HelloObject\*Params\*".

	The code required for our simple header file is listed below.

	``` {.sourceCode .c++}
	#ifndef __LEARNING_GEM5_HELLO_OBJECT_HH__
	#define __LEARNING_GEM5_HELLO_OBJECT_HH__

	#include "params/HelloObject.hh"
	#include "sim/sim_object.hh"

	class HelloObject : public SimObject
	{
	public:
	HelloObject(HelloObjectParams *p);
	};

	#endif // __LEARNING_GEM5_HELLO_OBJECT_HH__
	</code></pre></div></div>

	<p>You can find the complete file
	here <../_static/scripts/part2/helloobject/hello_object.hh>.</p>

	<p>Next, we need to implement <em>two</em> functions in the <code class="highlighter-rouge">.cc</code> file, not just
	one. The first function, is the constructor for the <code class="highlighter-rouge">HelloObject</code>. Here
	we simply pass the parameter object to the SimObject parent and print
	“Hello world!”</p>

	<p>Normally, you would <strong>never</strong> use <code class="highlighter-rouge">std::cout</code> in gem5. Instead, you
	should use debug flags. In the <a href="debugging-chapter">next chapter</a>, we
	will modify this to use debug flags instead. However, for now, we’ll
	simply use <code class="highlighter-rouge">std::cout</code> because it is simple.</p>

	<p>``` {.sourceCode .c++}
	#include “learning_gem5/hello_object.hh”</p>

	<p>#include <iostream></iostream></p>

	<p>HelloObject::HelloObject(HelloObjectParams *params) :
	SimObject(params)
	{
	std::cout « “Hello World! From a SimObject!” « std::endl;
	}</p>
	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
	There is another function that we have to implement as well for the
	SimObject to be complete. We must implement one function for the
	parameter type that is implicitly created from the SimObject `Python`
	declaration, namely, the `create` function. This function simply returns
	a new instantiation of the SimObject. Usually this function is very
	simple (as below).

	``` {.sourceCode .c++}
	HelloObject*
	HelloObjectParams::create()
	{
	return new HelloObject(this);
	}
	</code></pre></div></div>

	<p>You can find the complete file
	here <../_static/scripts/part2/helloobject/hello_object.cc>.</p>

	<p>If you forget to add the create function for your SimObject, you will
	get a linker error when you compile. It will look something like the
	following.</p>

	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>build/X86/python/m5/internal/param_HelloObject_wrap.o: In function `_wrap_HelloObjectParams_create':
	/local.chinook/gem5/gem5-tutorial/gem5/build/X86/python/m5/internal/param_HelloObject_wrap.cc:3096: undefined reference to `HelloObjectParams::create()'
	collect2: error: ld returned 1 exit status
	scons: *** [build/X86/gem5.opt] Error 1
	scons: building terminated because of errors.
	</code></pre></div></div>

	<p>This <code class="highlighter-rouge">undefined reference to `HelloObjectParams::create()'</code> means
	you need to implement the create function for your SimObject.</p>

	<h2 id="step-3-register-the-simobject-and-c-file">Step 3: Register the SimObject and C++ file</h2>

	<p>In order for the <code class="highlighter-rouge">C++</code> file to be compiled and the <code class="highlighter-rouge">Python</code> file to be
	parsed we need to tell the build system about these files. gem5 uses
	SCons as the build system, so you simply have to create a SConscript
	file in the directory with the code for the SimObject. If there is
	already a SConscript file for that directory, simply add the following
	declarations to that file.</p>

	<p>This file is simply a normal <code class="highlighter-rouge">Python</code> file, so you can write any
	<code class="highlighter-rouge">Python</code> code you want in this file. Some of the scripting can become
	quite complicated. gem5 leverages this to automatically create code for
	SimObjects and to compile the domain-specific languages like SLICC and
	the ISA language.</p>

	<p>In the SConscript file, there are a number of functions automatically
	defined after you import them. See the section on that…</p>

	<p>To get your new SimObject to compile, you simply need to create a new
	file with the name “SConscript” in the <code class="highlighter-rouge">src/learning_gem5</code> directory. In
	this file, you have to declare the SimObject and the <code class="highlighter-rouge">.cc</code> file. Below
	is the required code.</p>

	<p>``` {.sourceCode .python}
	Import(‘*’)</p>

	<p>SimObject(‘HelloObject.py’)
	Source(‘hello_object.cc’)</p>
	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
	You can find the complete file
	here \<../\_static/scripts/part2/helloobject/SConscript\>.

	Step 4: (Re)-build gem5
	-----------------------

	To compile and link your new files you simply need to recompile gem5.
	The below example assumes you are using the x86 ISA, but nothing in our
	object requires an ISA so, this will work with any of gem5's ISAs.

	``` {.sourceCode .sh}
	scons build/X86/gem5.opt
	</code></pre></div></div>

	<h2 id="step-5-create-the-config-scripts-to-use-your-new-simobject">Step 5: Create the config scripts to use your new SimObject</h2>

	<p>Now that you have implemented a SimObject, and it has been compiled into
	gem5, you need to create or modify a <code class="highlighter-rouge">Python</code> config file to instantiate
	your object. Since your object is very simple a system object is not
	required! CPUs are not needed, or caches, or anything, except a <code class="highlighter-rouge">Root</code>
	object. All gem5 instances require a <code class="highlighter-rouge">Root</code> object.</p>

	<p>Walking through creating a <em>very</em> simple configuration script, first,
	import m5 and all of the objects you have compiled.</p>

	<p>``` {.sourceCode .python}
	import m5
	from m5.objects import *</p>
	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
	Next, you have to instantiate the `Root` object, as required by all gem5
	instances.

	``` {.sourceCode .python}
	root = Root(full_system = False)
	</code></pre></div></div>

	<p>Now, you can instantiate the <code class="highlighter-rouge">HelloObject</code> you created. All you need to
	do is call the <code class="highlighter-rouge">Python</code> “constructor”. Later, we will look at how to
	specify parameters via the <code class="highlighter-rouge">Python</code> constructor. In addition to creating
	an instantiation of your object, you need to make sure that it is a
	child of the root object. Only SimObjects that are children of the
	<code class="highlighter-rouge">Root</code> object are instantiated in <code class="highlighter-rouge">C++</code>.</p>

	<p>``` {.sourceCode .python}
	root.hello = HelloObject()</p>
	<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
	Finally, you need to call `instantiate` on the `m5` module and actually
	run the simulation!

	``` {.sourceCode .python}
	m5.instantiate()

	print("Beginning simulation!")
	exit_event = m5.simulate()
	print('Exiting @ tick {} because {}'
	.format(m5.curTick(), exit_event.getCause()))
	</code></pre></div></div>

	<p>You can find the complete file
	here <../_static/scripts/part2/helloobject/run_hello.py>.</p>

	<p>The output should look something 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 May 4 2016 11:37:41
	gem5 started May 4 2016 11:44:28
	gem5 executing on mustardseed.cs.wisc.edu, pid 22480
	command line: build/X86/gem5.opt configs/learning_gem5/run_hello.py

	Global frequency set at 1000000000000 ticks per second
	Hello World! From a SimObject!
	Beginning simulation!
	info: Entering event queue @ 0. Starting simulation...
	Exiting @ tick 18446744073709551615 because simulate() limit reached
	</code></pre></div></div>

	<p>Congrats! You have written your first SimObject. In the next chapters,
	we will extend this SimObject and explore what you can do with
	SimObjects.</p>


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