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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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<title>8.4 选择语句</title>
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8.4 选择语句</div>
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<div class="Element13"><div class="Element12" id="code00264"><pre class="Element12">conditional_statement ::=                                       <span style="color: #008000">// 引用自附录A.6.6</span>
    <strong><span style="color: #FF0000">if</span></strong> <strong><span style="color: #FF0000">(</span></strong>cond_predicate<strong><span style="color: #FF0000">)</span></strong> statement_or_null [<strong><span style="color: #FF0000">else</span></strong> statement_or_null]
  | unique_priority_if_statement

unique_priority_if_statement ::=
    [unique_priority] <strong><span style="color: #FF0000">if</span></strong> <strong><span style="color: #FF0000">(</span></strong>cond_predicate<strong><span style="color: #FF0000">)</span></strong> statement_or_null
        {<strong><span style="color: #FF0000">else if (</span></strong>cond_predicate<strong><span style="color: #FF0000">)</span></strong> statement_or_null}
        [<strong><span style="color: #FF0000">else</span></strong> statement_or_null]

unique_priority ::= <strong><span style="color: #FF0000">unique</span></strong> | <strong><span style="color: #FF0000">priority</span></strong>

cond_predicate ::=
    expression_or_cond_pattern {<strong><span style="color: #FF0000">&amp;&amp;</span></strong> expression_or_cond_pattern}

expression_or_cond_pattern ::=
    expression | cond_pattern

cond_pattern ::= expression <strong><span style="color: #FF0000">matches</span></strong> pattern

case_statement ::=                                           <span style="color: #008000">// 引用自附录 A.6.7</span>
    [unique_priority]case_keyword<strong><span style="color: #FF0000">(</span></strong>expression<strong><span style="color: #FF0000">)</span></strong> case_item {case_item} <strong><span style="color: #FF0000">endcase</span></strong>
  | [unique_priority]case_keyword<strong><span style="color: #FF0000">(</span></strong>expression<strong><span style="color: #FF0000">)</span></strong> <strong><span style="color: #FF0000">matches</span></strong> case_pattern_item {case_pattern_item}
        <strong><span style="color: #FF0000">endcase</span></strong>

case_keyword ::= <strong><span style="color: #FF0000">case</span></strong> | <strong><span style="color: #FF0000">casez</span></strong> | <strong><span style="color: #FF0000">casex</span></strong>

case_item ::=
    expression {<strong><span style="color: #FF0000">,</span></strong>expression} <strong><span style="color: #FF0000">:</span></strong> statement_or_null
  | <strong><span style="color: #FF0000">default</span></strong> [<strong><span style="color: #FF0000">:</span></strong>] statement_or_null

case_pattern_item ::=
    pattern [<strong><span style="color: #FF0000">&amp;&amp;</span></strong>expression ] <strong><span style="color: #FF0000">:</span></strong> statement_or_null
  | <strong><span style="color: #FF0000">default</span></strong> [<strong><span style="color: #FF0000">:</span></strong>] statement_or_null</pre></div></div>
<p class="Element10" style="text-align: center">
<strong>语法 8-3—选择语句语法（摘录自附录A）</strong></p><p class="Element10">
&nbsp;</p>
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在Verilog中，<span style="color: #0000FF">if</span> <span style="color: #800080">(<i></span>expression<span style="color: #800080"></i>)</span> 被计算成一个布尔值，因此，如果表达式的值是<span style="color: #FF0000">0</span>或<span style="color: #FF0000">X</span>，那么条件检查被认为是“假”的。&nbsp;</p>
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&nbsp;</p>
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SystemVerilog加入了关键字<span style="color: #0000FF">unique</span>和<span style="color: #0000FF">priority</span>，它们可以在<span style="color: #0000FF">if</span>之前使用。除非有一个显式的<span style="color: #0000FF">else</span>语句，在没有条件匹配的情况下，只要使用了这两个关键字，就会引起运行时错误。例如： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00265"><pre class="Element12"><span style="color: #0000FF">unique if</span> <span style="color: #800080">((</span>a<span style="color: #800080">==</span><span style="color: #FF0000">0</span><span style="color: #800080">) || (</span>a<span style="color: #800080">==</span><span style="color: #FF0000">1</span><span style="color: #800080">))</span> <span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">&quot;0 or 1&quot;</span><span style="color: #800080">);</span>
<span style="color: #0000FF">else if</span> <span style="color: #800080">(</span>a <span style="color: #800080">== </span><span style="color: #FF0000">2</span><span style="color: #800080">)</span> <span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">&quot;2&quot;</span><span style="color: #800080">);</span>
<span style="color: #0000FF">else if</span> <span style="color: #800080">(</span>a <span style="color: #800080">== </span><span style="color: #FF0000">4</span><span style="color: #800080">)</span> <span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">&quot;4&quot;</span><span style="color: #800080">);</span> <span style="color: #008000">// 值3,5,6,7会引起一个错误</span>

<span style="color: #0000FF">priority if</span> <span style="color: #800080">(</span>a<span style="color: #800080">[</span><span style="color: #FF0000">2</span><span style="color: #800080">:</span><span style="color: #FF0000">1</span><span style="color: #800080">]==</span><span style="color: #FF0000">0</span><span style="color: #800080">)</span> <span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">&quot;0 or 1&quot;</span><span style="color: #800080">);</span>
<span style="color: #0000FF">else</span> <span style="color: #0000FF">if</span> <span style="color: #800080">(</span>a<span style="color: #800080">[</span><span style="color: #FF0000">2</span><span style="color: #800080">]==</span><span style="color: #FF0000">0</span><span style="color: #800080">)</span> <span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">&quot;2 or 3&quot;</span><span style="color: #800080">);</span>
<span style="color: #0000FF">else</span> <span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">&quot;4 or 7&quot;</span><span style="color: #800080">);</span> <span style="color: #008000">// 覆盖了所有可能的其他值，因此没有错误。</span></pre></div></div>
<p class="Element10">
<span style="color: #0000FF">unique if</span>指示在一系列 <span style="color: #0000FF">if...else...if</span>条件中不应该有任何交迭，也就是说，这些条件是互斥的，这就使得表达式能够并行计算。如果软件工具发现有多于一个条件为“真”，那么它应该发布一条错误信息。另外，如果软件工具发现没有条件为“真”，或者可能没有条件为“真”，并且最后的<span style="color: #0000FF">if</span>没有对应的<span style="color: #0000FF">else</span>语句，软件工具也应该发布一条错误信息。&nbsp;</p>
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&nbsp;</p>
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<span style="color: #0000FF">priority if</span>指示一系列<span style="color: #0000FF">if...else...if</span>条件应该按列出的顺序计算。在前面的例子中，如果变量<i>a</i>的值为<span style="color: #FF0000">0</span>，那么它可以满足第一个条件和第二个条件，这就要求一个优先级逻辑。如果软件工具发现没有条件为“真”，或者可能没有条件为“真”，并且最后的<span style="color: #0000FF">if</span>没有对应的<span style="color: #0000FF">else</span>语句，软件工具也应该发布一条错误信息。&nbsp;</p>
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