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Add create-syscall page
mrezabqr Nov 6, 2019
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Fix indentions in code sections
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<h1>Adding New System Calls to Xv6</h1>

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<article class="post-content">
<h1 id="Description">What is a system call?</h1>
<p>As you all know, an operating system supports two modes; the kernel mode and the user mode.
When a program in user mode requires access to RAM or a hardware resource, it must ask the kernel to
provide access to that particular resource. This is done via a system call. When a program makes a
system call, the mode is switched from user mode to kernel mode.
There are many system calls in an operating system which executes different types of tasks when they
are called.
</p>

<h1 id="Add">Adding a custom system call in Xv6</h1>
<p>In order to define your own system call in Xv6, you need to make changes to 5 files. Namely, these
files are as follows.</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>1. syscall.h<br>2. syscall.c<br>3. sysproc.c<br>4. usys.S<br>5. user.h</code></pre></div></div>
<h1 id="write">Let’s write a system call to return the year Unix version 6 was released.</h1>
<p>We would start the procedure by editing <code class='highlighter-rouge'>syscall.h</code> in which a number is given to every system call.
This file already contains 21 system calls. In order to add the custom system call, the following
line needs to be added to this file.</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="color-code-key">#define</span> SYS_getyear <span class="color-code-datatype">22</span></code></pre></div></div>
<p>
Next, we need to add a pointer to the system call in the <code class='highlighter-rouge'>syscall.c</code> file. This file contains an array
of function pointers which uses the above-defined numbers (indexes) as pointers to system calls
which are defined in a different location. In order to add our custom system call, add the following
line to this file.
</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>[SYS_getyear] sys_getyear,</code></pre></div></div>
<h1 id="heppens">
What exactly happens in here?
</h1>
<p>The underlying meaning of the above two changes is as follows.</p>
<ul>
<li class="no-p-margin">
When the system call with number 22 is called by a user program, the function pointer
<code class='highlighter-rouge'>sys_getyear</code> which has the index <code class='highlighter-rouge'>SYS_getyear</code> or 22 will call the system call function
<p></p>
</li>
</ul>
<p>
Therefore, we need to implement the system call function. However, we do not implement the system
call function in the <code class="highlighter-rouge">syscall.c</code> file. Instead, we only add the function prototype in here and we
define the function implementation in a different file. The function prototype which needs to be
added to the syscall.c file is as follows.
</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class='color-code-key'>extern</span> <span class='color-code-datatype'>int</span> sys_getyear(<span class='color-code-datatype'>void</span>);</code></pre></div></div>
<p>
Next, we will implement the system call function. In order to do this, open the <code class='highlighter-rouge'>sysproc.c</code> file where
system call functions are defined.
</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class='color-code-comment'>// return the year of which the Unix version 6 was released</span><br><span class='color-code-datatype'>int</span><br>sys_getyear(<span class='color-code-datatype'>void</span>)<br>{ <br><span class='color-code-key'>return</span> <span class='color-code-datatype'>1975</span>;<br>}<br></code></pre></div></div>
<p>
The basic implementation of the system call is now complete. However, there are 2 more minor steps
remaining.
</p>

<h1 id="Add-interface">Add the interface for the system call</h1>
<p>In order for a user program to call the system call, an interface needs to be added. Therefore, we
need to edit the <code class="highlighter-rouge">usys.S</code> file where we should add the following line.
</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>SYSCALL(getyear)</code></pre></div></div>
<p>
Next, the <code class='highlighter-rouge'>user.h</code> file needs to be edited.
</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class='color-code-datatype'>int</span> getyear(<span class="color-code-datatype">void</span>);</code></pre></div></div>
<p>
This would be the function which the user program calls. This function will be mapped to the system
call with the number 22 which is defined as <code class='highlighter-rouge'>SYS_getyear</code> preprocessor directive.
</p>
<p>
If you have completed all of the above, you have successfully added a new system call to Xv6.
However, in order to test the functionality of this, you would need to add a user program which
calls this system call.
</p>
<p>
The user program could be as follows.
</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class='color-code-key'>#include</span><span class='color-code-datatype'> “types.h”</span><br/><span class='color-code-key'>#include</span> <span class='color-code-datatype'>“stat.h”</span><br/><span class='color-code-key'>#include</span> <span class='color-code-datatype'>“user.h”</span><br/><br/><span class='color-code-datatype'>int</span><br/>main(<span class='color-code-datatype'>void</span>)<br/>{<br/><span class='color-code-key'>printf</span>(1, <span class='color-code-comment'>“Unix V6 was released in the year %d\n”</span>, getyear());<br/><span class='color-code-key'>exit</span>();<br/>}<br/></code></pre></div></div>
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