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17.10 系统函数</div>
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断言一般被用来计算一个设计实现的某些特定属性，例如一个特定的信号是否为“热点（one-hot）”。SystemVerilog中包含下列系统函数来促进这些基本的断言功能：
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<li class="Element601"><i><span class="Element146"><span style="color: #0000FF">$onehot</span><span style="color: #800080">(</span><expression><span style="color: #800080">)</span></span></i> ：如果表达式中只有一位为高，那么返回“真”。</li>
<li class="Element601"><i><span class="Element146"><span style="color: #0000FF">$onehot0</span><span style="color: #800080">(</span><expression><span style="color: #800080">)</span></span></i> ：如果表达式中至少有一位为高，那么返回“真”。</li>
<li class="Element601"><i><span class="Element146"><span style="color: #0000FF">$isunknown</span><span style="color: #800080">(</span><expression><span style="color: #800080">)</span></span></i> ：如果表达式任何一位为<span style="color: #FF0000">X</span>或<span style="color: #FF0000">Z</span>，那么返回“真”，等价于：<i><span class="Element146"><span style="color: #800080">^</span><expression> <span style="color: #800080">===</span> <span style="color: #800080">'</span><span style="color: #0000FF">b</span><span style="color: #FF0000">x</span></span></i>。</li>
</ul>所有上述的系统函数都具有一个<span style="color: #0000FF">bit</span>类型的返回值。<span style="color: #FF0000">1</span><span style="color: #800080">'</span><span style="color: #0000FF">b</span><span style="color: #FF0000">1</span>的返回值指示“真”，<span style="color: #FF0000">1</span><span style="color: #800080">'</span><span style="color: #0000FF">b</span><span style="color: #FF0000">0</span>的返回值指示“假”。 </p>
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为布尔表达式提供的另外一个有用的函数是<span style="color: #0000FF">$countones</span>，它可以计算一个位向量表达式中<span style="color: #FF0000">1</span>的数目。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00666"><pre class="Element12"><span style="color: #0000FF">$countones</span><span style="color: #800080">(</span>expression<span style="color: #800080">)</span></pre></div></div>
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值为<span style="color: #FF0000">X</span>和<span style="color: #FF0000">Z</span>的位不会被计数到<span style="color: #FF0000">1</span>的数目。</p></div>
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