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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
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23.4 表达式尺寸系统函数</div>
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<div class="Element13"><div class="Element12" id="code00931"><pre class="Element12">size_function ::=        <span style="color: #008000">// 没有包含在附录A中</span>
    <strong><span style="color: #FF0000">$bits(</span></strong>expression<strong><span style="color: #FF0000">)</span></strong>
  | <strong><span style="color: #FF0000">$bits(</span></strong>type_identifier<strong><span style="color: #FF0000">)</span></strong></pre></div></div>
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<strong>语法23-3 — 尺寸函数语法（没有包含在附录A中）</strong></p><p class="Element10">
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<span style="color: #0000FF">$bits</span>系统函数返回将一个表达式作为位流保存时所需要的位数。参见3.16节，一个有效类型定义的位流强制类型转换。一个四态值作为一位计数。对于下面的声明： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00932"><pre class="Element12"><span style="color: #0000FF">logic</span> <span style="color: #800080">[</span><span style="color: #FF0000">31</span><span style="color: #800080">:</span><span style="color: #FF0000">0</span><span style="color: #800080">]</span> foo<span style="color: #800080">;</span></pre></div></div>
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那么<span style="color: #0000FF">$bits</span><span style="color: #800080">(</span>foo<span style="color: #800080">)</span>应该返回32，尽管软件实现会采用多于32位的存储空间来代表四态值。对于下面的声明： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00933"><pre class="Element12"><span style="color: #0000FF">typedef struct</span> <span style="color: #800080">{</span>
    <span style="color: #0000FF">logic</span> valid<span style="color: #800080">;</span>
    <span style="color: #0000FF">bit</span> <span style="color: #800080">[</span><span style="color: #FF0000">8</span><span style="color: #800080">:</span><span style="color: #FF0000">1</span><span style="color: #800080">]</span> data<span style="color: #800080">;</span>
<span style="color: #800080">}</span> MyType<span style="color: #800080">;</span></pre></div></div>
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表达式<span style="color: #0000FF">$bits</span><span style="color: #800080">(</span>MyType<span style="color: #800080">)</span> 应返回9，也就是类型为<i>MyType</i>的变量所需要的数据位的数目。当用于固定尺寸类型的时候，<span style="color: #0000FF">$bits</span>函数可以作为一个确立时常量使用。 因此，它可以用在其它类型或变量的声明中。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00934"><pre class="Element12"><span style="color: #0000FF">typedef bit</span><span style="color: #800080">[</span><span style="color: #0000FF">$bits</span><span style="color: #800080">(</span>MyType<span style="color: #800080">):</span><span style="color: #FF0000">1</span><span style="color: #800080">]</span> MyBits<span style="color: #800080">;</span> <span style="color: #008000">// 与typedef bit [9:1] MyBits;相同</span>
MyBits b<span style="color: #800080">;</span></pre></div></div>
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变量<i>b</i>可以用来保留类型为<i>MyType</i>的变量的位模式而不会丢失任何信息。当作用于一个当前为空的动态尺寸类型的时候，<span style="color: #0000FF">$bits</span>系统函数返回<span style="color: #FF0000">0</span>。将<span style="color: #0000FF">$bits</span>系统函数直接与动态尺寸类型标识符一起使用是错误的。</p></div>
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