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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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12.16.3 case生成式语句</div>
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一个生成式可以使用<span style="color: #0000FF">case</span>生成式语句从一组可选项中选择。<span style="color: #0000FF">case</span>生成式语句的语法如下： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00481"><pre class="Element12">rs_case ::= <strong><span style="color: #FF0000">case</span></strong> <strong><span style="color: #FF0000">(</span></strong>expression<strong><span style="color: #FF0000">)</span></strong> rs_case_item {rs_case_item} <strong><span style="color: #FF0000">endcase</span></strong>

rs_case_item ::=
    expression {<strong><span style="color: #FF0000">,</span></strong> expression} : production_item
  | <strong><span style="color: #FF0000">default</span></strong> [<strong><span style="color: #FF0000">:</span></strong>] production_item</pre></div></div>
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&nbsp;</p>
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除了下述的说明外，<span style="color: #0000FF">case</span>生成式语句类似于过程化的<span style="color: #0000FF">case</span>语句。<span style="color: #0000FF">case</span>表达式被计算，它的值与每一个<span style="color: #0000FF">case</span>条目表达式（它们以指定的顺序计算和比较）进行比较。产生的生成式是与<span style="color: #0000FF">case</span>表达式匹配的第一个<span style="color: #0000FF">case</span>条目表达式对应的生成式。如果没有找到匹配的<span style="color: #0000FF">case</span>条目表达式，那么产生与可选的<span style="color: #0000FF">default</span>条目相对应的生成式。&nbsp;</p>
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&nbsp;</p>
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在一个<span style="color: #0000FF">case</span>生成式语句中具有多个缺省语句是非法的。使用逗号分割多个<span style="color: #0000FF">case</span>条目表达式能够共享相同的生成式。例如： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00482"><pre class="Element12"><span style="color: #0000FF">randsequence</span><span style="color: #800080">()</span>
    SELECT <span style="color: #800080">:</span> <span style="color: #0000FF">case</span> <span style="color: #800080">(</span>device <span style="color: #800080">&amp;</span> <span style="color: #FF0000">7</span><span style="color: #800080">)</span>
                 <span style="color: #FF0000">0</span>       <span style="color: #800080">:</span> NETWORK
                 <span style="color: #FF0000">1</span><span style="color: #800080">,</span> <span style="color: #FF0000">2</span>    <span style="color: #800080">:</span> DISK
                 <span style="color: #0000FF">default</span> <span style="color: #800080">:</span> MEMORY
             <span style="color: #0000FF">endcase</span><span style="color: #800080">;</span>
    ...
<span style="color: #0000FF">endsequence</span></pre></div></div>
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&nbsp;</p>
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上面的例子定义了具有一个<span style="color: #0000FF">case</span>语句的生成式<span class="Element146">SELECT</span>。case表达式<span class="Element146"><span style="color: #800080">(</span>device <span style="color: #800080">&amp;</span> <span style="color: #FF0000">7</span><span style="color: #800080">)</span></span>被计算并与两个<span style="color: #0000FF">case</span>条目表达式进行比较。如果表达式与<span style="color: #FF0000">0</span>匹配，那么产生生成式<span class="Element146">NETWORK</span>；如果与<span style="color: #FF0000">1</span>或<span style="color: #FF0000">2</span>匹配，那么产生<span class="Element146">DISK</span>生成式；否则产生<span class="Element146">MOMORY</span>生成式。</p></div>
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