<li><a href="xcs/config.html">Optimization Schemes</a></li>

<h2><a class="anchor" aria-hidden="true" id="types-of-modules"></a><a href="#types-of-modules" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Types of Modules</h2>

<h3><a class="anchor" aria-hidden="true" id="source-modules"></a><a href="#source-modules" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Source Modules</h3>

<p><strong>Source Modules</strong> are XCSL files that are meant to be executed as program (source) code. In source modules, all XCSL code outside of function declarations is treated as executable program code.</p>

<p>For example, see the following XCSL code:</p>

<pre><code class="hljs"> let sum <span class="hljs-symbol">x</span> <span class="hljs-symbol">y</span> = <span class="hljs-symbol">x</span> + <span class="hljs-symbol">y</span>;;

sum <span class="hljs-number">1</span> <span class="hljs-number">2</span>

<p>A source module would declare function <em>sum</em>, along with its proposed behavior, into the module tree. Then it would execute ( <em>sum 1 2</em> ) and ultimately, return 3 to the shell.</p>

<h3><a class="anchor" aria-hidden="true" id="header-modules"></a><a href="#header-modules" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Header Modules</h3>

<p><strong>Header Modules (aka Submodules)</strong> are organizational files containing loadable XCSL functions. They differ from <em>Source Modules</em>, as none of the code outside of function declarations is ever executed.</p>

<p>Recall the previous example module:</p>

<pre><code class="hljs"> let sum <span class="hljs-symbol">x</span> <span class="hljs-symbol">y</span> = <span class="hljs-symbol">x</span> + <span class="hljs-symbol">y</span>;;

sum <span class="hljs-number">1</span> <span class="hljs-number">2</span>

<p>While a source module would execute ( <em>sum 1 2</em> ), a submodule would not. It would <strong>only</strong> declare the sum function for other modules' use.</p>

<h3><a class="anchor" aria-hidden="true" id="tether-modules"></a><a href="#tether-modules" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Tether Modules</h3>

<p><strong>Tether Modules</strong> are application interfaces for other modules. These allow an application/module to provide a subset of publicly-callable functions by any other (tethered) module in the system.</p>

<p>Tether modules must be <em>activated</em> for their services to be used. When done with a tether module, it is best practice to <em>deactivate</em> the module. See following example, where the file is named &quot;TMod.xt&quot;:</p>

<pre><code class="hljs"> <span class="hljs-built_in">activate</span> TMod

<span class="hljs-built_in">deactivate</span> TMod

<p>Within TMod (and all other tether modules), functionality can be <em>offered</em> to other modules.</p>

<pre><code class="hljs"> offer difference <span class="hljs-symbol">x</span> <span class="hljs-symbol">y</span> = <span class="hljs-symbol">x</span> - <span class="hljs-symbol">y</span>

<p>Then other modules could use the following <em>request</em> to use the <em>diff</em> function.</p>

<pre><code class="hljs css language-ocaml"> request <span class="hljs-type">TMod</span> difference <span class="hljs-number">2</span> <span class="hljs-number">1</span>

<p>This request would call TMod, then TMod would calculate the difference and return the result.</p>

<p>Though this may seem esoteric in the given examples, tether modules play a major role in multithreading (discussed later).</p>

<h3><a class="anchor" aria-hidden="true" id="system-modules"></a><a href="#system-modules" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>System Modules</h3>

<p><strong>System Modules</strong> do not exist as source code within the user environment; instead, they come defined within the XCS ISO image. System functionality can be referenced using the following format:</p>

<pre><code class="hljs">request<span class="hljs-built_in"> System </span>MODULE

<p>These modules are typically ones that are required for XCSL to operate normally, e.g. memory allocator, process scheduler. Below find one of the most useful system functions:</p>

<pre><code class="hljs">request<span class="hljs-built_in"> System </span>kill pid

<p>Where <em>pid</em> is some process ID number.</p>

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