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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

无条件约束(x<span style="color: #800080">+</span>y<span style="color: #800080"> == </span><span style="color: #FF0000">10</span>)会被产生。</p>
<ul class="Element630">
<li class="Element600">情况2：a为<span style="color: #FF0000">null</span></li>
</ul><p class="Element10">
这种情况总会导致错误而不管其它的条件。</p>
<ul class="Element630">
<li class="Element600">情况3：a为非空，b为非空，a.x为10，b.x为20</li>
</ul><p class="Element10">
所有的守卫子表达式计算成FALSE。&nbsp;</p>
<p class="Element10">
产生有条件约束<i><span style="color: #800080">(</span>x<span style="color: #800080">&lt;</span>y<span style="color: #800080">)</span> <span style="color: #800080">-&gt; (</span>x<span style="color: #800080">+</span>y<span style="color: #800080">) == </span><span style="color: #FF0000">10</span></i>。&nbsp;</p>
<p class="Element10">
&nbsp;</p>
<p class="Element10">
例子2： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00435');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00435"><pre class="Element12"><span style="color: #0000FF">class</span> D<span style="color: #800080">;</span>
    <span style="color: #0000FF">int</span> x<span style="color: #800080">;</span>
<span style="color: #0000FF">endclass</span>

<span style="color: #0000FF">class</span> C<span style="color: #800080">;</span>
    <span style="color: #0000FF">rand int</span> x<span style="color: #800080">, </span>y<span style="color: #800080">;</span>
    D a<span style="color: #800080">,</span> b<span style="color: #800080">;</span>
    <span style="color: #0000FF">constraint</span> c1 <span style="color: #800080">{(</span>x<span style="color: #800080"> &lt; </span>y<span style="color: #800080"> &amp;&amp; </span>a<span style="color: #800080">.</span>x<span style="color: #800080"> &gt; </span>b<span style="color: #800080">.</span>x<span style="color: #800080"> &amp;&amp; </span>a<span style="color: #800080">.</span>x<span style="color: #800080"> == </span><span style="color: #FF0000">5</span><span style="color: #800080">) -&gt; </span>x<span style="color: #800080">+</span>y<span style="color: #800080"> == </span><span style="color: #FF0000">10</span><span style="color: #800080">;}</span>
<span style="color: #0000FF">endclass</span></pre></div></div>
<p class="Element10">
&nbsp;</p>
<p class="Element10">
在上面的例子中，判决表达式为(x<span style="color: #800080"> &lt; </span>y)、(a<span style="color: #800080">.</span>x<span style="color: #800080"> &gt; </span>b<span style="color: #800080">.</span>x)、以及(a<span style="color: #800080">.</span>x<span style="color: #800080"> == </span><span style="color: #FF0000">5</span>)，它们被联合（<span style="color: #800080">&amp;&amp;</span>）连接起来。一些可能的情况为：</p>
<ul class="Element630">
<li class="Element600">情况1：a为非空，b为<span style="color: #FF0000">null</span>，a.x为<span style="color: #FF0000">6</span>。</li>
</ul><p class="Element10">
因为(a.x==5)为假，b.x产生一个错误的事实不会导致一个错误。&nbsp;</p>
<p class="Element10">
约束被消除。</p>
<ul class="Element630">
<li class="Element600">情况2：a为<span style="color: #FF0000">null</span></li>
</ul><p class="Element10">
这种情况总会导致错误而不管其它的条件。</p>
<ul class="Element630">
<li class="Element600">情况3：a为非空，b为非空，a.x为5，b.x为2。</li>
</ul><p class="Element10">
所有的守卫子表达式计算成TRUE，产生有条件约束<i><span style="color: #800080">(</span>x<span style="color: #800080">&lt;</span>y<span style="color: #800080">)</span> <span style="color: #800080">-&gt; (</span>x<span style="color: #800080">+</span>y<span style="color: #800080">) == </span><span style="color: #FF0000">10</span></i>。&nbsp;</p>
<p class="Element10">
&nbsp;</p>
<p class="Element10">
例子3： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00436');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00436"><pre class="Element12"><span style="color: #0000FF">class</span> D<span style="color: #800080">;</span>
    <span style="color: #0000FF">int</span> x<span style="color: #800080">;</span>
<span style="color: #0000FF">endclass</span>

<span style="color: #0000FF">class</span> C<span style="color: #800080">;</span>
    <span style="color: #0000FF">rand int</span> x<span style="color: #800080">, </span>y<span style="color: #800080">;</span>
    D a<span style="color: #800080">,</span> b<span style="color: #800080">;</span>
    <span style="color: #0000FF">constraint</span> c1 <span style="color: #800080">{(</span>x<span style="color: #800080"> &lt; </span>y<span style="color: #800080"> &amp;&amp; (</span>a<span style="color: #800080">.</span>x<span style="color: #800080"> &gt; </span>b<span style="color: #800080">.</span>x<span style="color: #800080"> || </span>a<span style="color: #800080">.</span>x<span style="color: #800080"> == </span><span style="color: #FF0000">5</span><span style="color: #800080">)) -&gt; </span>x<span style="color: #800080">+</span>y<span style="color: #800080"> == </span><span style="color: #FF0000">10</span><span style="color: #800080">;}</span>
<span style="color: #0000FF">endclass</span></pre></div></div>
<p class="Element10">
在上面的例子中，判决表达式为<span style="color: #800080">(</span>x<span style="color: #800080"> &lt; </span>y<span style="color: #800080">)</span>以及<span style="color: #800080">(</span>a<span style="color: #800080">.</span>x<span style="color: #800080"> &gt; </span>b<span style="color: #800080">.</span>x<span style="color: #800080"> || </span>a<span style="color: #800080">.</span>x<span style="color: #800080"> == </span><span style="color: #FF0000">5</span><span style="color: #800080">)</span>，有一个表达式使用分离（<span style="color: #800080">||</span>）连接起来。一些可能的情况如下：</p>
<ul class="Element630">
<li class="Element600">情况1：<i>a</i>为非空，<i>b</i>为<span style="color: #FF0000">null</span>，<i>a<span style="color: #800080">.</span>x</i>为<span style="color: #FF0000">5</span>。</li>
</ul><p class="Element10">
守卫表达式计算成(ERROR || a.x==5)，进一步计算成(ERROR || TRUE)。&nbsp;</p>
<p class="Element10">
守卫子表达式计算成TRUE。&nbsp;</p>
<p class="Element10">
产生有条件约束<i><span style="color: #800080">(</span>x<span style="color: #800080">&lt;</span>y<span style="color: #800080">)</span> <span style="color: #800080">-&gt; (</span>x<span style="color: #800080">+</span>y<span style="color: #800080">) == </span><span style="color: #FF0000">10</span></i>。</p>
<ul class="Element630">
<li class="Element600">情况2：<i>a</i>为非空，<i>b</i>为<span style="color: #FF0000">null</span>，<i>a<span style="color: #800080">.</span>x</i>为8。</li>
</ul><p class="Element10">
守卫表达式计算成(ERROR || FALSE)，并产生一个错误。</p>
<ul class="Element630">
<li class="Element600">情况3：<i>a</i>为<span style="color: #FF0000">null</span></li>
</ul><p class="Element10">
这种情况总会导致错误而不管其它的条件。</p>
<ul class="Element630">
<li class="Element600">情况4：<i>a</i>为非空，<i>b</i>为非空，<i>a<span style="color: #800080">.</span>x</i>为<span style="color: #FF0000">5</span>，<i>b<span style="color: #800080">.</span>x</i>为<span style="color: #FF0000">2</span>。</li>
</ul><p class="Element10">
所有的守卫子表达式计算成TRUE。&nbsp;</p>
<p class="Element10">
产生有条件约束<i><span style="color: #800080">(</span>x<span style="color: #800080">&lt;</span>y<span style="color: #800080">)</span> <span style="color: #800080">-&gt; (</span>x<span style="color: #800080">+</span>y<span style="color: #800080">) == </span><span style="color: #FF0000">10</span></i>。</p></div>
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