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<a href="#" onmousedown="showPopup(this, 'popup00226');"><img src="seealsolink.png" border="0" alt="" title=""></a> SystemVerilog 3.1a语言参考手册</div>
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12.4.5 蕴含</div>
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约束提供了两种结构来声明有条件（判定的）的关系：蕴含和<span style="color: #0000FF">if</span>...<span style="color: #0000FF">else</span>。 </p>
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蕴含操作符（<span style="color: #800080">-></span>）可以用来声明隐含了一个约束的表达式。 </p>
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定义一个蕴含约束的语法如下： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00411"><pre class="Element12">constraint_expression ::= <span style="color: #008000">// 引用自附录A.1.9</span>
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| expression <strong><span style="color: #FF0000">–></span></strong> constraint_set
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<strong>语法12-4 — 约束蕴含语法（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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蕴含操作符在布尔表达式上等价于：<i>a</i> <span style="color: #800080">-></span> <i>b</i>等价于<span style="color: #800080">(!<i></span>a</i> <span style="color: #800080">||</span> <i>b<span style="color: #800080"></i>)</span>。它表明：如果表达式为真，那么产生的随机数被其后的约束（或约束集合）所约束。否则，产生的随机数是未施加约束的。 </p>
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<i>constraint_set</i>代表任意有效的约束或一个未命名的约束集合。如果表达式为真，那么约束集合中的所有约束也必须被满足。 </p>
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例如： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00412"><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：如果<i>mode</i> <span style="color: #800080">==</span> <i>small</i>，那么<i>len</i>的值应该被约束成小于<span style="color: #FF0000">10</span>；如果<i>mode</i> <span style="color: #800080">==</span> <i>large</i>，那么<i>len</i>的值应该被约束成大于<span style="color: #FF0000">100</span>；否则，<i>len</i>的值是未约束的。 </p>
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在下面的例子中： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00413"><pre class="Element12"><span style="color: #0000FF">bit</span> <span style="color: #800080">[</span><span style="color: #FF0000">3</span><span style="color: #800080">:</span><span style="color: #FF0000">0</span><span style="color: #800080">]</span> a<span style="color: #800080">, </span>b<span style="color: #800080">;</span>
<span style="color: #0000FF">constraint</span> c <span style="color: #800080">{(</span>a<span style="color: #800080"> == </span><span style="color: #FF0000">0</span><span style="color: #800080">) -> (</span>b<span style="color: #800080"> == </span><span style="color: #FF0000">1</span><span style="color: #800080">);}</span></pre></div></div>
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无论是<i>a</i>还是<i>b</i>都是4位的，因此<i>a</i>和<i>b</i>的组合会有256种可能。约束<i>c</i>表明，<i>a<span style="color: #800080"></i>==</span><span style="color: #FF0000">0</span>意味着<i>b<span style="color: #800080"></i>==</span><span style="color: #FF0000">1</span>，因此排除了15种组合：{0,0}, {0,2}, … {0,15}。因此，<i>a<span style="color: #800080"></i>==</span><span style="color: #FF0000">0</span>的概率为1/(256-15)或1/241。</p></div>
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