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<li class="toctree-l2"><a class="reference internal" href="../chapter4/3sv39-implementation-1.html">实现 SV39 多级页表机制（上）</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter7/2cmdargs-and-redirection.html">命令行参数与标准 I/O 重定向</a></li>
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<li class="toctree-l1"><a class="reference internal" href="../appendix-d/index.html">附录 D：RISC-V相关信息</a></li>
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<h2>执行应用程序<a class="headerlink" href="#id2" title="永久链接至标题"></a></h2>
<p>我们先从最简单的 Rust <code class="docutils literal notranslate"><span class="pre">Hello,</span> <span class="pre">world</span></code> 程序开始，用 Cargo 工具创建 Rust 项目。</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>cargo new os
</pre></div>
</div>
<p>此时，项目的文件结构如下：</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>tree os
<span class="go">os</span>
<span class="go">├── Cargo.toml</span>
<span class="go">└── src</span>
<span class="go"> └── main.rs</span>
<span class="go">1 directory, 2 files</span>
</pre></div>
</div>
<p>其中 <code class="docutils literal notranslate"><span class="pre">Cargo.toml</span></code> 中保存了项目的库依赖、作者信息等。</p>
<p>cargo 为我们准备好了 <code class="docutils literal notranslate"><span class="pre">Hello</span> <span class="pre">world!</span></code> 源代码：</p>
<div class="literal-block-wrapper docutils container" id="id6">
<div class="code-block-caption"><span class="caption-text">最简单的 Rust 应用</span><a class="headerlink" href="#id6" title="永久链接至代码"></a></div>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos">1</span><span class="k">fn</span> <span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos">2</span><span class="w"> </span><span class="fm">println!</span><span class="p">(</span><span class="s">"Hello, world!"</span><span class="p">);</span><span class="w"></span>
<span class="linenos">3</span><span class="p">}</span><span class="w"></span>
</pre></div>
</div>
</div>
<p>输入 <code class="docutils literal notranslate"><span class="pre">cargo</span> <span class="pre">run</span></code> 构建并运行项目：</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="go"> Compiling os v0.1.0 (/home/shinbokuow/workspace/v3/rCore-Tutorial-v3/os)</span>
<span class="go"> Finished dev [unoptimized + debuginfo] target(s) in 1.15s</span>
<span class="go"> Running `target/debug/os`</span>
<span class="go">Hello, world!</span>
</pre></div>
</div>
<p>我们在屏幕上看到了一行 <code class="docutils literal notranslate"><span class="pre">Hello,</span> <span class="pre">world!</span></code> ，但为了打印出 <code class="docutils literal notranslate"><span class="pre">Hello,</span> <span class="pre">world!</span></code>，我们需要的不止几行源代码。</p>
</div>
<div class="section" id="id3">
<h2>理解应用程序执行环境<a class="headerlink" href="#id3" title="永久链接至标题"></a></h2>
<p>在现代通用操作系统（如 Linux）上运行应用程序，需要多层次的执行环境栈支持：</p>
<div class="figure align-center" id="id7">
<img alt="../_images/app-software-stack.png" src="../_images/app-software-stack.png"/>
<p class="caption"><span class="caption-text">应用程序执行环境栈：图中的白色块自上而下表示各级执行环境，黑色块则表示相邻两层执行环境之间的接口。
下层作为上层的执行环境，支持上层代码运行。</span><a class="headerlink" href="#id7" title="永久链接至图片"></a></p>
</div>
<p>我们的应用程序通过调用标准库或第三方库提供的接口，仅需少量源代码就能完成复杂的功能；
<code class="docutils literal notranslate"><span class="pre">Hello,</span> <span class="pre">world!</span></code> 程序调用的 <code class="docutils literal notranslate"><span class="pre">println!</span></code> 宏就是由 Rust 标准库 std 和 GNU Libc 等提供的。
这些库属于应用程序的 <strong>执行环境</strong> (Execution Environment)，而它们的实现又依赖于操作系统提供的系统调用。</p>
</div>
<div class="section" id="id4">
<h2>平台与目标三元组<a class="headerlink" href="#id4" title="永久链接至标题"></a></h2>
<p>编译器在编译、链接得到可执行文件时需要知道，程序要在哪个 <strong>平台</strong> (Platform) 上运行，
<strong>目标三元组</strong> (Target Triplet) 描述了目标平台的 CPU 指令集、操作系统类型和标准运行时库。</p>
<p>我们研究一下现在 <code class="docutils literal notranslate"><span class="pre">Hello,</span> <span class="pre">world!</span></code> 程序的目标三元组是什么：</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>rustc --version --verbose
<span class="go"> rustc 1.61.0-nightly (68369a041 2022-02-22)</span>
<span class="go"> binary: rustc</span>
<span class="go"> commit-hash: 68369a041cea809a87e5bd80701da90e0e0a4799</span>
<span class="go"> commit-date: 2022-02-22</span>
<span class="go"> host: x86_64-unknown-linux-gnu</span>
<span class="go"> release: 1.61.0-nightly</span>
<span class="go"> LLVM version: 14.0.0</span>
</pre></div>
</div>
<p>其中 host 一项表明默认目标平台是 <code class="docutils literal notranslate"><span class="pre">x86_64-unknown-linux-gnu</span></code>，
CPU 架构是 x86_64，CPU 厂商是 unknown，操作系统是 linux，运行时库是 gnu libc。</p>
<p>接下来，我们希望把 <code class="docutils literal notranslate"><span class="pre">Hello,</span> <span class="pre">world!</span></code> 移植到 RISC-V 目标平台 <code class="docutils literal notranslate"><span class="pre">riscv64gc-unknown-none-elf</span></code> 上运行。</p>
<div class="admonition note">
<p class="admonition-title">注解</p>
<p><code class="docutils literal notranslate"><span class="pre">riscv64gc-unknown-none-elf</span></code> 的 CPU 架构是 riscv64gc，厂商是 unknown，操作系统是 none，
elf 表示没有标准的运行时库。没有任何系统调用的封装支持，但可以生成 ELF 格式的执行程序。
我们不选择有 linux-gnu 支持的 <code class="docutils literal notranslate"><span class="pre">riscv64gc-unknown-linux-gnu</span></code>，是因为我们的目标是开发操作系统内核，而非在 linux 系统上运行的应用程序。</p>
</div>
</div>
<div class="section" id="id5">
<h2>修改目标平台<a class="headerlink" href="#id5" title="永久链接至标题"></a></h2>
<p>将程序的目标平台换成 <code class="docutils literal notranslate"><span class="pre">riscv64gc-unknown-none-elf</span></code>，试试看会发生什么：</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>cargo run --target riscv64gc-unknown-none-elf
<span class="go"> Compiling os v0.1.0 (/home/shinbokuow/workspace/v3/rCore-Tutorial-v3/os)</span>
<span class="go">error[E0463]: can't find crate for `std`</span>
<span class="go"> |</span>
<span class="go"> = note: the `riscv64gc-unknown-none-elf` target may not be installed</span>
</pre></div>
</div>
<p>报错的原因是目标平台上确实没有 Rust 标准库 std，也不存在任何受 OS 支持的系统调用。
这样的平台被我们称为 <strong>裸机平台</strong> (bare-metal)。</p>
<p>幸运的是，除了 std 之外，Rust 还有一个不需要任何操作系统支持的核心库 core，
它包含了 Rust 语言相当一部分核心机制，可以满足本门课程的需求。
有很多第三方库也不依赖标准库 std，而仅仅依赖核心库 core。</p>
<p>为了以裸机平台为目标编译程序，我们要将对标准库 std 的引用换成核心库 core。</p>
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<ul>
<li><a class="reference internal" href="#">应用程序执行环境与平台支持</a><ul>
<li><a class="reference internal" href="#id2">执行应用程序</a></li>
<li><a class="reference internal" href="#id3">理解应用程序执行环境</a></li>
<li><a class="reference internal" href="#id4">平台与目标三元组</a></li>
<li><a class="reference internal" href="#id5">修改目标平台</a></li>
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