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这是一本关于verilog编程语言的教程,对学习verilog语言有帮助

<li class="Element600">如果约束存在一个解的话，那么约束求解器必须找到这个解。求解器只有在问题被过约束并且没有随机值的组合能够满足约束的时候才可以失败。</li>
<li class="Element600">约束双向地影响。在这个例子中，<i>addr</i>的值的选择依赖于<i>atype</i>以及它怎样被约束，并且<i>atype</i>值的选择依赖于<i>addr</i>以及它怎样被约束。所有的表达式操作符都被双向地对待，包括蕴含操作符（<span style="color: #800080">-&gt;</span>）。</li>
<li class="Element600">约束仅仅支持两态值。四态值（<span style="color: #FF0000">X</span>或<span style="color: #FF0000">Z</span>）或四态操作符（例如，<span style="color: #800080">===</span>、<span style="color: #800080">!==</span>）是非法的并且会导致错误。</li>
</ul><p class="Element10">
有时我们希望关闭随机变量的约束。例如，为了故意地产生一个非法的地址（非字对齐的地址）： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00396');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00396"><pre class="Element12"><span style="color: #0000FF">task</span> exercise_illegal<span style="color: #800080">(</span>MyBus bus<span style="color: #800080">,</span> <span style="color: #0000FF">int</span> cycles<span style="color: #800080">);</span>
    <span style="color: #0000FF">int</span> res<span style="color: #800080">;</span>

    <span style="color: #008000">// 关闭字对齐约束</span>
    bus<span style="color: #800080">.</span>word_align<span style="color: #800080">.</span><span style="color: #0000FF">constraint_mode</span><span style="color: #800080">(</span><span style="color: #FF0000">0</span><span style="color: #800080">);</span>

    <span style="color: #0000FF">repeat</span> <span style="color: #800080">(</span>cycles<span style="color: #800080">)</span> <span style="color: #0000FF">begin</span>
        <span style="color: #008000">// 情况1: 限制到小地址</span>
        res <span style="color: #800080">=</span> bus<span style="color: #800080">.</span><span style="color: #0000FF">randomize</span><span style="color: #800080">()</span> <span style="color: #0000FF">with</span> <span style="color: #800080">{</span>addr<span style="color: #800080">[</span><span style="color: #FF0000">0</span><span style="color: #800080">] ||</span> addr<span style="color: #800080">[</span><span style="color: #FF0000">1</span><span style="color: #800080">];};</span>
        ...
    <span style="color: #0000FF">end</span>

    <span style="color: #008000">// 重新使能字对齐约束</span>
    bus<span style="color: #800080">.</span>word_align<span style="color: #800080">.</span><span style="color: #0000FF">constraint_mode</span><span style="color: #800080">(</span><span style="color: #FF0000">1</span><span style="color: #800080">);</span>
<span style="color: #0000FF">endtask</span></pre></div></div>
<p class="Element10">
<span style="color: #0000FF">constraint_mode</span><span style="color: #800080">()</span>可以用来使能或关闭一个对象中任何命名的约束。在这个例子中，字对齐约束被关闭，接着对象使用其它的约束来随机化以便将低位地址强制成非零值（不再字对齐）。&nbsp;</p>
<p class="Element10">
&nbsp;</p>
<p class="Element10">
使能或关闭约束的能力使得用户能够设计约束层次。在这些层次中，最底层的约束可以代表物理限制，这些物理限制通过公用的特性分组成命名的约束块，它们可以独立地使能或关闭。&nbsp;</p>
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&nbsp;</p>
<p class="Element10">
类似地，<span style="color: #0000FF">rand_mode</span><span style="color: #800080">()</span>方法可以用来使能或关闭任何随机变量。当一个随机变量被关闭的时候，它与其它非随机的变量具有完全一致的行为。&nbsp;</p>
<p class="Element10">
&nbsp;</p>
<p class="Element10">
偶尔我们也希望在随机化之前或之后立即执行一些操作。这种功能可以通过两个内建方法完成：<span style="color: #0000FF">pre_randomize</span><span style="color: #800080">()</span>和<span style="color: #0000FF">post_randomize</span><span style="color: #800080">()</span>，这两个方法在随机化之前或之后被自动调用。这些方法可以使用希望的功能来过载： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00397');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00397"><pre class="Element12"><span style="color: #0000FF">class</span> XYPair<span style="color: #800080">;</span>
    <span style="color: #0000FF">rand integer</span> x<span style="color: #800080">,</span> y<span style="color: #800080">;</span>
<span style="color: #0000FF">endclass</span>

<span style="color: #0000FF">class</span> MyXYPair <span style="color: #0000FF">extends</span> XYPair
    <span style="color: #0000FF">function void</span> <span style="color: #0000FF">pre_randomize</span><span style="color: #800080">();</span>
        <span style="color: #0000FF">super</span><span style="color: #800080">.</span><span style="color: #0000FF">pre_randomize</span><span style="color: #800080">();</span>
        <span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">&quot;Before randomize x=%0d, y=%0d&quot;</span><span style="color: #800080">,</span> x<span style="color: #800080">,</span> y<span style="color: #800080">);</span>
    <span style="color: #0000FF">endfunction</span>

<span style="color: #0000FF">    function void</span> <span style="color: #0000FF">post_randomize</span><span style="color: #800080">();</span>
        <span style="color: #0000FF">super</span><span style="color: #800080">.</span><span style="color: #0000FF">post_randomize</span><span style="color: #800080">();</span>
        <span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">&quot;After randomize x=%0d, y=%0d&quot;</span><span style="color: #800080">,</span> x<span style="color: #800080">,</span> y<span style="color: #800080">);</span>
    <span style="color: #0000FF">endfunction</span>
<span style="color: #0000FF">endclass</span></pre></div></div>
<p class="Element10">
缺省情况下，<span style="color: #0000FF">pre_randomize</span><span style="color: #800080">()</span>和<span style="color: #0000FF">post_randomize</span><span style="color: #800080">()</span>调用它们过载的父类方法。当<span style="color: #0000FF">pre_randomize</span><span style="color: #800080">()</span>或<span style="color: #0000FF">post_randomize</span><span style="color: #800080">()</span>被过载的时候，必须小心地调用父类方法，除非父类是一个基类（没有父类），否则应该调用基类方法。&nbsp;</p>
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&nbsp;</p>
<p class="Element10">
随机激励产生能力以及面向对象的基于约束的验证方法使得用户能够快速地开发覆盖了复杂功能的测试，并能够更好地保证设计的正确性。</p></div>
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<a href="00865.html" target="topic">第十二章 随机约束</a></p></div>
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<a href="00865.html" target="topic">第十二章 随机约束</a></div>
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