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<title>Xv6 introduction « Stop Talking, Start Doing - 停止空想，开始行动</title>
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<h1 class="post-title">Xv6 introduction</h1>
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<p class="post-meta">Jul 28, 2017</p>
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<article class="post-content">
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<p>I hate xv6, a stupid, useless education-oriented system. In this article, I will generally talk about how to implement system call to this operating system.</p>
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<hr />
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<h2 id="xv6-systemcall">Xv6 Systemcall</h2>
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<p>To invoke a system call, we have to first define a user mode function to be the interface of the kernel instruction in file <em>user.h</em>.</p>
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<div class="language-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kt">void</span> <span class="n">function</span> <span class="p">(</span><span class="kt">void</span><span class="p">);</span>
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</code></pre></div></div>
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<p>This interface-like function will then pass the function name, in this case function, to <em>usys.S</em>. When using user mode function in programs, <em>usys.S</em> will generate a reference to SYS_function and push system call number of this function into %eax. After that, system can know from <em>syscall.c</em> and determining whether this system call is available. We must define same name system function and add it into <em>syscall.h</em> and <em>syscall.c</em>.</p>
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<div class="language-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp">#define SYS_function ## // ## is the system call number
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</span><span class="p">[</span><span class="n">SYS_function</span><span class="p">]</span> <span class="n">sys_function</span> <span class="c1">// real system function name</span>
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<span class="k">extern</span> <span class="kt">int</span> <span class="n">sys_function</span><span class="p">(</span><span class="kt">void</span><span class="p">);</span> <span class="c1">// real system function declaration</span>
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</code></pre></div></div>
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<p>After adding these sentences to syscall files, we can implement real function in specific place where you want to make the function works well.</p>
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<p>Sometimes, we need to pass variables among system calls. In this case, variables’ values are not necessary and even can’t be pass directly into system_function. When invoke a system call function, all variables of this system call will be pushed into current process’ stack. In file <em>syscall.c</em>, multiple functions are provided to get these variables from the process. I won’t waste time on explaining how to use these functions especially when elegant and detailed comments were written in source codes. However, I will explain concepts and how process organized and works in xv6 in future articles.</p>
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<img class="avatar" src="/static/avatar.jpg" alt="Pengzhan Hao - 碾子">
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<div class="col-box-title name">Pengzhan Hao - 碾子</div>
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<a href="https://github.com/codersherlock">GitHub</a>
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<a href="https://twitter.com/haopengzhan">Twitter</a>
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<a href="mailto:haopengzhan@gmail.com">Email</a>
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<div class="col-box-title">Newest Posts</div>
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<ul class="post-list">
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<li><a class="post-link" href="/archivers/intro-xv6">Xv6 introduction</a></li>
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<li><a class="post-link" href="/archivers/some-of-my-previews-exper-work">Some of my previews experiment works: 2016</a></li>
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<li><a class="post-link" href="/archivers/charles-is-not-a-good-tool">Using charles proxy to monitor mobile SSL traffics</a></li>
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<li><a class="post-link" href="/archivers/hello">Stop Talking is the worst title of one blog</a></li>
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