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<a href="#" onmousedown="showPopup(this, 'popup00227');"><img src="seealsolink.png" border="0" alt="" title=""></a> SystemVerilog 3.1a语言参考手册</div>
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12.4.6 if...else约束</div>
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SystemVerilog还支持<span style="color: #0000FF">if</span>...<span style="color: #0000FF">else</span>类型的约束。 </p>
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定义<span style="color: #0000FF">if</span>...<span style="color: #0000FF">else</span>约束的语法如下： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00414"><pre class="Element12">constraint_expression ::= <span style="color: #008000">// 引用自附录A.1.9</span>
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| <strong><span style="color: #FF0000">if (</span></strong>expression<strong><span style="color: #FF0000">)</span></strong> constraint_set [<strong><span style="color: #FF0000">else</span></strong> constraint_set]</pre></div></div>
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<strong>语法12-5 — <span style="color: #0000FF">if</span>...<span style="color: #0000FF">else</span>约束语法（摘录自附录A）</strong></p><p class="Element10">
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<i>expression</i>可以是任意integral SystemVerilog表达式。 </p>
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<i>constraint_set</i>代表了任意有效的约束或一个未命名的约束块。如果表达式为真，那么第一个约束或约束集合中的约束必须被满足，否则在可选的<span style="color: #0000FF">else</span>约束或约束块中的约束必须被满足。约束集合可以用来聚合多个约束。 </p>
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<span style="color: #0000FF">if</span>...<span style="color: #0000FF">else</span>类型的约束声明具有等价的蕴含约束，例如： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00415"><pre class="Element12"><span style="color: #0000FF">if</span> <span style="color: #800080">(</span>mode <span style="color: #800080">==</span> small<span style="color: #800080">)</span>
len <span style="color: #800080"><</span> <span style="color: #FF0000">10</span><span style="color: #800080">;</span>
<span style="color: #0000FF">else if</span> <span style="color: #800080">(</span>mode <span style="color: #800080">==</span> large<span style="color: #800080">)</span>
len <span style="color: #800080">></span> <span style="color: #FF0000">100</span><span style="color: #800080">;</span></pre></div></div>
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等价于 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00416"><pre class="Element12">mode <span style="color: #800080">==</span> small <span style="color: #800080">-></span> len <span style="color: #800080"><</span> <span style="color: #FF0000">10</span><span style="color: #800080">;</span>
mode <span style="color: #800080">==</span> large <span style="color: #800080">-></span> len <span style="color: #800080">></span> <span style="color: #FF0000">100</span><span style="color: #800080">;</span></pre></div></div>
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在这个例子中，<i>mode</i>的值意味着<i>len</i>小于10，或者大于100，或者未约束。 </p>
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与蕴含一样，<span style="color: #0000FF">if</span>...<span style="color: #0000FF">else</span>类型的约束也是双向的。在上面的声明中，<i>mode</i>约束的值约束了<i>len</i>的值，并且<i>len</i>约束的值约束了<i>mode</i>的值。 </p>
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因为一个<span style="color: #0000FF">if</span>...<span style="color: #0000FF">else</span>类型的约束声明的else部分是可选的，所以当在一个嵌套的if序列中忽略<span style="color: #0000FF">else</span>的时候会引起混淆。这个问题通过总是将<span style="color: #0000FF">else</span>与最近的前一个缺少<span style="color: #0000FF">else</span>的<span style="color: #0000FF">if</span>相关联来解决。在下面的例子中，<span style="color: #0000FF">else</span>匹配于内层的<span style="color: #0000FF">if</span>，就像缩进格式所指示的那样： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00417"><pre class="Element12"><span style="color: #0000FF">if</span> <span style="color: #800080">(</span>mode<span style="color: #800080"> != </span>large<span style="color: #800080">)</span>
<span style="color: #0000FF">if</span> <span style="color: #800080">(</span>mode<span style="color: #800080"> == </span>small<span style="color: #800080">)</span>
len <span style="color: #800080"><</span> <span style="color: #FF0000">10</span><span style="color: #800080">;</span>
<span style="color: #0000FF">else</span> <span style="color: #008000">// else应用于前一个if</span>
len <span style="color: #800080">></span> <span style="color: #FF0000">100</span><span style="color: #800080">;</span></pre></div></div>
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