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   <div class="edit"><a href="https://github.com/gem5/new-website/tree/master/_pages/documentation/part3/cache-intro.md">Edit this page</a></div>
   <dl>
   <dt>authors</dt>
   <dd>Jason Lowe-Power</dd>
 </dl>

 <h1 id="msi-example-cache-protocol">MSI example cache protocol</h1>

 <p>Before we implement a cache coherence protocol, it is important to have
 a solid understanding of cache coherence. This section leans heavily on
 the great book <em>A Primer on Memory Consistency and Cache Coherence</em> by
 Daniel J. Sorin, Mark D. Hill, and David A. Wood which was published as
 part of the Synthesis Lectures on Computer Architecture in 2011
 (<a href="DOI:`10.2200/S00346ED1V01Y201104CAC016">DOI:`10.2200/S00346ED1V01Y201104CAC016</a>
 &lt;&lt;https://doi.org/10.2200/S00346ED1V01Y201104CAC016&gt;&gt;_).
 If you are unfamiliar with cache coherence, I strongly advise reading that book before continuing.</p>

 <p>In this chapter, we will be implementing an MSI protocol.
 (An MSI protocol has three stable states, modified with read-write permission, shared with read-only permission, and invalid with no permissions.)
 We will implement this as a three-hop directory protocol (i.e., caches can send data directly to other caches without going through the directory).
 Details for the protocol can be found in Section 8.2 of the *Primer on Memory Consistency and Cache Coherence* (pages 141-149).
 It will be helpful to print out Section 8.2 to reference as you are implementing the protocol.</p>

 <p>You can download an exceprt of Sorin et al. that contains Section 8.2 :download:here
 &lt;../../_static/external/Sorin_et-al_Excerpt_8.2.pdf&gt;.</p>

 <p>First steps to writing a protocol
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</p>

 <p>Let’s start by creating a new directory for our protocol: src/learning_gem5/MSI_protocol.
 In this directory, like in all gem5 source directories, we need to create a file for SCons to know what to compile.
 However, this time, instead of creating a SConscript` file, we are
 going to create a <code class="highlighter-rouge">SConsopts</code> file. (The <code class="highlighter-rouge">SConsopts</code> files are processed
 before the <code class="highlighter-rouge">SConscript</code> files and we need to run the SLICC compiler
 before SCons executes.)</p>

 <p>We need to create a <code class="highlighter-rouge">SConsopts</code> file with the following:</p>

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

 <p>all_protocols.extend([
 ‘MSI’,
 ])</p>

 <p>protocol_dirs.append(str(Dir(‘.’).abspath))
 ```</p>

 <p>We do two things in this file. First, we register the name of our
 protocol (<code class="highlighter-rouge">'MSI'</code>). Since we have named our protocol MSI, SCons will
 assume that there is a file named <code class="highlighter-rouge">MSI.slicc</code> which specifies all of the
 state machine files and auxiliary files. We will create that file after
 writing all of our state machine files. Second, the <code class="highlighter-rouge">SConsopts</code> files
 tells the SCons to look in the current directory for files to pass to
 the SLICC compiler.</p>

 <p>You can download the <code class="highlighter-rouge">SConsopts</code> file
 here &lt;../../_static/scripts/part3/MSI_protocol/SConsopts&gt;.</p>

 <h2 id="writing-a-state-machine-file">Writing a state machine file</h2>

 <p>The next step, and most of the effort in writing a protocol, is to
 create the state machine files. State machine files generally follow the
 outline:</p>

 <dl>
   <dt>Parameters</dt>
   <dd>These are the parameters for the SimObject that will be generated
 from the SLICC code.</dd>
   <dt>Declaring required structures and functions</dt>
   <dd>This section declares the states, events, and many other required
 structures for the state machine.</dd>
   <dt>In port code blocks</dt>
   <dd>Contain code that looks at incoming messages from the (<code class="highlighter-rouge">in_port</code>)
 message buffers and determines what events to trigger.</dd>
   <dt>Actions</dt>
   <dd>These are simple one-effect code blocks (e.g., send a message) that
 are executed when going through a transition.</dd>
   <dt>Transitions</dt>
   <dd>Specify actions to execute given a starting state and an event and
 the final state. This is the meat of the state machine definition.</dd>
 </dl>

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	<div class="container" id="doc-container">
	<div class="edit"><a href="https://github.com/gem5/new-website/tree/master/_pages/documentation/part3/cache-intro.md">Edit this page</a></div>
	<dl>
	<dt>authors</dt>
	<dd>Jason Lowe-Power</dd>
	</dl>

	<h1 id="msi-example-cache-protocol">MSI example cache protocol</h1>

	<p>Before we implement a cache coherence protocol, it is important to have
	a solid understanding of cache coherence. This section leans heavily on
	the great book <em>A Primer on Memory Consistency and Cache Coherence</em> by
	Daniel J. Sorin, Mark D. Hill, and David A. Wood which was published as
	part of the Synthesis Lectures on Computer Architecture in 2011
	(<a href="DOI:`10.2200/S00346ED1V01Y201104CAC016">DOI:`10.2200/S00346ED1V01Y201104CAC016</a>
	<<https://doi.org/10.2200/S00346ED1V01Y201104CAC016>>_).
	If you are unfamiliar with cache coherence, I strongly advise reading that book before continuing.</p>

	<p>In this chapter, we will be implementing an MSI protocol.
	(An MSI protocol has three stable states, modified with read-write permission, shared with read-only permission, and invalid with no permissions.)
	We will implement this as a three-hop directory protocol (i.e., caches can send data directly to other caches without going through the directory).
	Details for the protocol can be found in Section 8.2 of the Primer on Memory Consistency and Cache Coherence (pages 141-149).
	It will be helpful to print out Section 8.2 to reference as you are implementing the protocol.</p>

	<p>You can download an exceprt of Sorin et al. that contains Section 8.2 :download:here
	<../../_static/external/Sorin_et-al_Excerpt_8.2.pdf>.</p>

	<p>First steps to writing a protocol
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</p>

	<p>Let’s start by creating a new directory for our protocol: src/learning_gem5/MSI_protocol.
	In this directory, like in all gem5 source directories, we need to create a file for SCons to know what to compile.
	However, this time, instead of creating a SConscript` file, we are
	going to create a <code class="highlighter-rouge">SConsopts</code> file. (The <code class="highlighter-rouge">SConsopts</code> files are processed
	before the <code class="highlighter-rouge">SConscript</code> files and we need to run the SLICC compiler
	before SCons executes.)</p>

	<p>We need to create a <code class="highlighter-rouge">SConsopts</code> file with the following:</p>

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

	<p>all_protocols.extend([
	‘MSI’,
	])</p>

	<p>protocol_dirs.append(str(Dir(‘.’).abspath))
	```</p>

	<p>We do two things in this file. First, we register the name of our
	protocol (<code class="highlighter-rouge">'MSI'</code>). Since we have named our protocol MSI, SCons will
	assume that there is a file named <code class="highlighter-rouge">MSI.slicc</code> which specifies all of the
	state machine files and auxiliary files. We will create that file after
	writing all of our state machine files. Second, the <code class="highlighter-rouge">SConsopts</code> files
	tells the SCons to look in the current directory for files to pass to
	the SLICC compiler.</p>

	<p>You can download the <code class="highlighter-rouge">SConsopts</code> file
	here <../../_static/scripts/part3/MSI_protocol/SConsopts>.</p>

	<h2 id="writing-a-state-machine-file">Writing a state machine file</h2>

	<p>The next step, and most of the effort in writing a protocol, is to
	create the state machine files. State machine files generally follow the
	outline:</p>

	<dl>
	<dt>Parameters</dt>
	<dd>These are the parameters for the SimObject that will be generated
	from the SLICC code.</dd>
	<dt>Declaring required structures and functions</dt>
	<dd>This section declares the states, events, and many other required
	structures for the state machine.</dd>
	<dt>In port code blocks</dt>
	<dd>Contain code that looks at incoming messages from the (<code class="highlighter-rouge">in_port</code>)
	message buffers and determines what events to trigger.</dd>
	<dt>Actions</dt>
	<dd>These are simple one-effect code blocks (e.g., send a message) that
	are executed when going through a transition.</dd>
	<dt>Transitions</dt>
	<dd>Specify actions to execute given a starting state and an event and
	the final state. This is the meat of the state machine definition.</dd>
	</dl>

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