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<h1>
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<a href="." >不停计算的机器</a>
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<div class="panel panel-danger"><div class="panel-heading"><h5 class="panel-title" id="热身结束-进入真正的pa"><i class="fa fa-bullhorn"></i> 热身结束, 进入真正的PA</h5></div><div class="panel-body"><p>如果说PA1只是编程复习的热身运动, 那么PA2就是正片的开始了.
在PA2中, 你会感觉新代码和工程细节如潮水般袭来,
你需要不断地读讲义, RTFM, RTFSC, 并整理其中的细节,
一开始效率会比较低, 一旦掌握了这些细节, 后面就是重复性的工程活了.</p><p>此外, 如果说PA1中提到的"不要纠缠于和当前进度无关的代码"
是为了保护你的幼小心灵, 在PA2中你就需要勇敢地去探索了:
讲义不再像PA1那样详细指出每一处你值得阅读的代码.
事实上, PA中的所有代码都值得你去阅读, 在PA2结束之后, 你需要做到可以理解NEMU中的每一处细节.
随着你对这些代码细节的了解变得深入, 就算是调bug你也会觉得多了几分轻松.
相反, 如果你仍然抱着"测试能过就行", "这里不算分, 关我P事"的心态,
很快你就会发现连bug都不知道怎么调了.</p><p>所以, 不要再偷懒了.</p></div></div>
<h2 id="不停计算的机器">不停计算的机器</h2>
<p>在PA1中, 我们已经见识到最简单的计算机TRM的工作方式:</p>
<pre><code class="lang-c"><span class="hljs-keyword">while</span> (<span class="hljs-number">1</span>) {
从PC指示的存储器位置取出指令;
执行指令;
更新PC;
}
</code></pre>
<p>接下来我们就来谈谈这个过程, 也就是, CPU究竟是怎么执行一条指令的.
对于大部分指令来说, 执行它们都可以抽象成取指-译码-执行的<a href="http://en.wikipedia.org/wiki/Instruction_cycle" target="_blank">指令周期</a>.
为了使描述更加清晰, 我们借助指令周期中的一些概念来说明指令执行的过程.</p>
<h3 id="取指instruction-fetch-if">取指(instruction fetch, IF)</h3>
<p>要执行一条指令, 首先要拿到这条指令. 指令究竟在哪里呢?
还记得冯诺依曼体系结构的核心思想吗? 那就是"存储程序, 程序控制".
你以前听说这两句话的时候可能没有什么概念, 现在是实践的时候了.
这两句话告诉你, 指令在存储器中, 由PC指出当前指令的位置.
事实上, PC就是一个指针! 在计算机世界中, 指针的概念无处不在.
如果你觉得对指针的概念还不是很熟悉, 就要赶紧复习指针这门必修课啦.
取指令要做的事情自然就是将PC指向的指令从内存读入到CPU中.</p>
<h3 id="译码instruction-decode-id">译码(instruction decode, ID)</h3>
<p>在取指阶段, 计算机拿到了将要执行的指令.
让我们也来目睹一下指令的风采, 睁大眼睛一看, 竟然是个0和1组成的比特串!</p>
<pre><code>10111001 00110100 00010010 00000000 00000000
</code></pre><p>这究竟是什么鬼...
不过想想, 计算机也只是个巨大的数字电路, 它也只能理解0和1了.
但是, 这样的计算机又是如何理解这让人一头雾水的比特串的呢?</p>
<p>让我们先来回想一下指令是做什么的.
我们知道CPU是用来处理数据的, 指令则是用来指示CPU具体对什么数据进行什么样的处理.
也就是说, 我们只要让CPU从上面那串神秘的比特串中解读出处理的对象和处理的操作,
CPU就知道我们想让它做什么了.
所以相应地, CPU需要从指令中解读出"操作码"和"操作数"两部分信息.</p>
<p>于是, 为了让计算机明白指令的含义, 先驱想到了一个办法, 那就是你在数字电路课上学习过的查找表!
CPU拿到一条指令之后, 可以通过查表的方式得知这条指令的操作数和操作码.
这个过程叫译码.</p>
<p>当然, 译码逻辑实际上也并非只有一张查找表那么简单,
还需要根据不同的指令通过多路选择器选择不同的操作数.
回想一下, 计算机现在已经有存储器和寄存器了, 它们都可以存放操作数, 指令中也可以存放立即数.
也可能还有二次译码的处理...
不过无论再怎么复杂, 我们只需要知道, 这个过程终究也只是一些数字电路的事情,
毕竟所有需要的信息都在指令里面了, 没什么神秘的操作.</p>
<h3 id="执行execute-ex">执行(execute, EX)</h3>
<p>经过译码之后, CPU就知道当前指令具体要做什么了, 执行阶段就是真正完成指令的工作.
现在TRM只有加法器这一个执行部件, 必要的时候,
只需要往加法器输入两个源操作数, 就能得到执行的结果了.
之后还要把结果写回到目的操作数中, 可能是寄存器, 也可能是内存.</p>
<h3 id="更新pc">更新PC</h3>
<p>执行完一条指令之后, CPU就要执行下一条指令.
在这之前, CPU需要更新PC的值,
让PC加上刚才执行完的指令的长度, 即可指向下一条指令的位置.</p>
<hr>
<p>于是, 计算机不断地重复上述四个步骤, 不断地执行指令, 直到永远.</p>
<h2 id="yemu-一个简单的cpu模拟器">YEMU: 一个简单的CPU模拟器</h2>
<p>我们还是以ICS课本的1.1.3小节中介绍的简单计算机为例, 来展示如何用C语言实现一条指令的执行.
这个计算机有4个8位的寄存器, 一个4位PC, 以及一段16字节的内存.
它支持R型和M型两种指令格式, 4条指令. 其指令手册如下:</p>
<pre><code> 4 2 0
| | | +----+--+--+
mov rt,rs | R[rt] <- R[rs] | R-type | |0000|rt|rs|
| | | +----+--+--+
| | | +----+--+--+
add rt,rs | R[rt] <- R[rs] + R[rt] | R-type | |0001|rt|rs|
| | | +----+--+--+
| | | +----+--+--+
load addr | R[0] <- M[addr] | M-type | |1110| addr|
| | | +----+--+--+
| | | +----+--+--+
store addr | M[addr] <- R[0] | M-type | |1111| addr|
| | | +----+--+--+
</code></pre><p>我们可以根据指令手册用C语言编写出这个简单计算机的模拟器YEMU(袁妈模拟器):</p>
<pre><code class="lang-c"><span class="hljs-meta">#<span class="hljs-meta-keyword">include</span> <span class="hljs-meta-string"><stdint.h></span></span>
<span class="hljs-meta">#<span class="hljs-meta-keyword">include</span> <span class="hljs-meta-string"><stdio.h></span></span>
<span class="hljs-meta">#<span class="hljs-meta-keyword">define</span> NREG 4</span>
<span class="hljs-meta">#<span class="hljs-meta-keyword">define</span> NMEM 16</span>
<span class="hljs-comment">// 定义指令格式</span>
<span class="hljs-keyword">typedef</span> <span class="hljs-keyword">union</span> {
<span class="hljs-keyword">struct</span> { <span class="hljs-keyword">uint8_t</span> rs : <span class="hljs-number">2</span>, rt : <span class="hljs-number">2</span>, op : <span class="hljs-number">4</span>; } rtype;
<span class="hljs-keyword">struct</span> { <span class="hljs-keyword">uint8_t</span> addr : <span class="hljs-number">4</span> , op : <span class="hljs-number">4</span>; } mtype;
<span class="hljs-keyword">uint8_t</span> inst;
} <span class="hljs-keyword">inst_t</span>;
<span class="hljs-meta">#<span class="hljs-meta-keyword">define</span> DECODE_R(inst) uint8_t rt = (inst).rtype.rt, rs = (inst).rtype.rs</span>
<span class="hljs-meta">#<span class="hljs-meta-keyword">define</span> DECODE_M(inst) uint8_t addr = (inst).mtype.addr</span>
<span class="hljs-keyword">uint8_t</span> pc = <span class="hljs-number">0</span>; <span class="hljs-comment">// PC, C语言中没有4位的数据类型, 我们采用8位类型来表示</span>
<span class="hljs-keyword">uint8_t</span> R[NREG] = {}; <span class="hljs-comment">// 寄存器</span>
<span class="hljs-keyword">uint8_t</span> M[NMEM] = { <span class="hljs-comment">// 内存, 其中包含一个计算z = x + y的程序</span>
<span class="hljs-number">0</span>b11100110, <span class="hljs-comment">// load 6# | R[0] <- M[y]</span>
<span class="hljs-number">0</span>b00000100, <span class="hljs-comment">// mov r1, r0 | R[1] <- R[0]</span>
<span class="hljs-number">0</span>b11100101, <span class="hljs-comment">// load 5# | R[0] <- M[x]</span>
<span class="hljs-number">0</span>b00010001, <span class="hljs-comment">// add r0, r1 | R[0] <- R[0] + R[1]</span>
<span class="hljs-number">0</span>b11110111, <span class="hljs-comment">// store 7# | M[z] <- R[0]</span>
<span class="hljs-number">0</span>b00010000, <span class="hljs-comment">// x = 16</span>
<span class="hljs-number">0</span>b00100001, <span class="hljs-comment">// y = 33</span>
<span class="hljs-number">0</span>b00000000, <span class="hljs-comment">// z = 0</span>
};
<span class="hljs-keyword">int</span> halt = <span class="hljs-number">0</span>; <span class="hljs-comment">// 结束标志</span>
<span class="hljs-comment">// 执行一条指令</span>
<span class="hljs-function"><span class="hljs-keyword">void</span> <span class="hljs-title">exec_once</span><span class="hljs-params">()</span> </span>{
<span class="hljs-keyword">inst_t</span> <span class="hljs-keyword">this</span>;
<span class="hljs-keyword">this</span>.inst = M[pc]; <span class="hljs-comment">// 取指</span>
<span class="hljs-keyword">switch</span> (<span class="hljs-keyword">this</span>.rtype.op) {
<span class="hljs-comment">// 操作码译码 操作数译码 执行</span>
<span class="hljs-keyword">case</span> <span class="hljs-number">0</span>b0000: { DECODE_R(<span class="hljs-keyword">this</span>); R[rt] = R[rs]; <span class="hljs-keyword">break</span>; }
<span class="hljs-keyword">case</span> <span class="hljs-number">0</span>b0001: { DECODE_R(<span class="hljs-keyword">this</span>); R[rt] += R[rs]; <span class="hljs-keyword">break</span>; }
<span class="hljs-keyword">case</span> <span class="hljs-number">0</span>b1110: { DECODE_M(<span class="hljs-keyword">this</span>); R[<span class="hljs-number">0</span>] = M[addr]; <span class="hljs-keyword">break</span>; }
<span class="hljs-keyword">case</span> <span class="hljs-number">0</span>b1111: { DECODE_M(<span class="hljs-keyword">this</span>); M[addr] = R[<span class="hljs-number">0</span>]; <span class="hljs-keyword">break</span>; }
<span class="hljs-keyword">default</span>:
<span class="hljs-built_in">printf</span>(<span class="hljs-string">"Invalid instruction with opcode = %x, halting...\n"</span>, <span class="hljs-keyword">this</span>.rtype.op);
halt = <span class="hljs-number">1</span>;
<span class="hljs-keyword">break</span>;
}
pc ++; <span class="hljs-comment">// 更新PC</span>
}
<span class="hljs-function"><span class="hljs-keyword">int</span> <span class="hljs-title">main</span><span class="hljs-params">()</span> </span>{
<span class="hljs-keyword">while</span> (<span class="hljs-number">1</span>) {
exec_once();
<span class="hljs-keyword">if</span> (halt) <span class="hljs-keyword">break</span>;
}
<span class="hljs-built_in">printf</span>(<span class="hljs-string">"The result of 16 + 33 is %d\n"</span>, M[<span class="hljs-number">7</span>]);
<span class="hljs-keyword">return</span> <span class="hljs-number">0</span>;
}
</code></pre>
<div class="panel panel-warning"><div class="panel-heading"><h5 class="panel-title" id="理解yemu如何执行程序"><i class="fa fa-edit"></i> 理解YEMU如何执行程序</h5></div><div class="panel-body"><p>YEMU可以看成是一个简化版的NEMU, 它们的原理是相通的,
因此你需要理解YEMU是如何执行程序的. 具体地, 你需要</p><ul>
<li>画出在YEMU上执行的加法程序的状态机</li>
<li>通过RTFSC理解YEMU如何执行一条指令</li>
</ul><p>思考一下, 以上两者有什么联系?</p></div></div>
<p>也许你会疑惑, 这个只能做加法的TRM, 究竟还能做些什么呢?
对于采用补码表示的计算机, 能做加法自然就能做减法.
如果再添加一条条件跳转指令<code>jnz r, addr</code>:
当寄存器<code>r</code>不为<code>0</code>时, PC跳转到<code>addr</code>处, TRM就大不一样了.
例如通过<code>jnz</code>和<code>dec</code>的组合可以实现循环,
循环执行<code>inc</code>可以实现任意数的加法, 循环执行加法可以实现乘法,
函数调用可以看成一种特殊的跳转, 递归本质上就是函数调用...
这下可不得了了, 没想到这个弱不禁风的TRM竟然深藏着擎天撼地的威力!
不过, 虽然这个只有几条指令的TRM可以解决所有可计算的问题, 但却低效得让人无法忍受.
为此, 先驱决定往TRM中加入更多高效的指令.</p>
<footer class="page-footer-ex"> <span class="page-footer-ex-copyright"> By <a href="https://sashimi-yzh.github.io/" target="_blank">Zihao Yu</a>, 采用<a href="http://creativecommons.org/licenses/by-nc-sa/3.0/cn/" target="_blank">知识共享 署名-非商业性使用-相同方式共享 3.0 中国大陆 许可协议</a>发布 </span>            <span class="page-footer-ex-footer-update"> 此页面修订于: 2023-02-25 08:25:35 </span> </footer>
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