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上面的序列在接下来的两个时钟标记内寻找<i><span class="Element146">frame</span></i>信号的上升沿。在<i><span class="Element146">frame</span></i>反转为高后，<i><span class="Element146">irdy</span></i>信号也必须在一个时钟标记之后反转为高。图17-14演示了在时钟标记6处的计算尝试。<span class="Element146"><span style="color: #800080">`data_end_exp</span></span>在时钟标记6处被确认。接下来，<i><span class="Element146">frame</span></i>信号在时钟标记7处反转为高。由于它满足由<span style="color: #800080">[</span><span style="color: #FF0000">1</span><span style="color: #800080">:</span><span style="color: #FF0000">2</span><span style="color: #800080">]</span>所施加的约束，所以它满足这个序列并且继续进行下一步的计算。在时钟标记8处，<i><span class="Element146">irdy</span></i>信号被计算。信号<i><span class="Element146">irdy</span></i>在时钟标记8处跃变为高，对于开始于时钟标记6处的计算尝试，它完全满足序列规范。 </p>
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<img src="Figure_17_14.png" border="0" alt="" title=""> </p>
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<strong>图17-14 — 有条件的序列</strong> </p><p class="Element10">
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通常，断言与一些先决条件相关联，因此所执行的检查也仅仅发生在某些特性的条件下。正像前一个例子所看到的那样，|->操作符提供了这种能力来说明在计算它们的结果特性之前序列必须满足的先决条件。下面的例子修改了前一个例子，通过删除了结果的先决条件，我们可以看到它对断言结果的影响。如图17-15所示。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00682"><pre class="Element12"><span style="color: #0000FF">property</span> data_end_rule2<span style="color: #800080">;</span>
<span style="color: #800080">@(</span><span style="color: #0000FF">posedge</span> mclk<span style="color: #800080">)</span>
<span style="color: #800080">##[</span><span style="color: #FF0000">1</span><span style="color: #800080">:</span><span style="color: #FF0000">2</span><span style="color: #800080">]</span> <span style="color: #0000FF">$rose</span><span style="color: #800080">(</span>frame<span style="color: #800080">) ##</span><span style="color: #FF0000">1</span> <span style="color: #0000FF">$rose</span><span style="color: #800080">(</span>irdy<span style="color: #800080">);</span>
<span style="color: #0000FF">endproperty</span></pre></div></div>
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<img src="Figure_17_15.png" border="0" alt="" title=""> </p>
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<strong>图17-15 — 没有先决条件产生的结果</strong></p><p class="Element10">
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上述特性在每一个时钟标记处进行计算。对于时钟标记1处的计算，在时钟标记1或时钟标记2处没有出现<span class="Element146">frame</span>信号的上升沿，所以特性在时钟标记1失败。类似的，在时钟标记2、3和4处特性也是失败的。对于起始于时钟标记5和6处的计算尝试，时钟标记7处发生的<span class="Element146">frame</span>信号的上升沿使得特性能够得到进一步的检查。根据规范，序列在时钟标记8处结束，对于起始于5和6处的计算尝试会产生一次匹配。因为<span class="Element146"><span style="color: #0000FF">$rose</span><span style="color: #800080">(</span>frame<span style="color: #800080">)</span></span>没有再次发生，所以所有后续的序列匹配计算尝试都是失败的。 </p>
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从图17-15中可以看出，将检查的先决条件<span class="Element146"><span style="color: #800080">‘data_end_exp</span></span>从断言中删除会导致与验证目标不相关的失败。从确认的立场来看，确定先决条件并用其过滤掉不恰当或无关的条件是非常重要的。 </p>
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蕴含操作中前项是一个序列的例子如下所示： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00683"><pre class="Element12"><span style="color: #800080">(</span>a <span style="color: #800080">##</span><span style="color: #FF0000">1</span> b <span style="color: #800080">##</span><span style="color: #FF0000">1</span> c<span style="color: #800080">) |-> (</span>d <span style="color: #800080">##</span><span style="color: #FF0000">1</span> e<span style="color: #800080">)</span></pre></div></div>
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如果序列<span class="Element146"><span style="color: #800080">(</span>a <span style="color: #800080">##</span><span style="color: #FF0000">1</span> b <span style="color: #800080">##</span><span style="color: #FF0000">1</span> c<span style="color: #800080">)</span></span>匹配，那么序列<span style="color: #800080">(</span>d <span style="color: #800080">##</span><span style="color: #FF0000">1</span> e<span style="color: #800080">)</span>也必须匹配。换句话说，如果序列<span style="color: #800080">(</span>a <span style="color: #800080">##</span><span style="color: #FF0000">1</span> b <span style="color: #800080">##</span><span style="color: #FF0000">1</span> c<span style="color: #800080">)</span>没有匹配，那么结果为“真”。 </p>
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蕴含的另外一个例子如下： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00684');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00684"><pre class="Element12"><span style="color: #0000FF">property</span> p16<span style="color: #800080">;</span>
<span style="color: #800080">(</span>write_en <span style="color: #800080">&</span> data_valid<span style="color: #800080">)</span> <span style="color: #800080">##</span><span style="color: #FF0000">0</span>
<span style="color: #800080">(</span>write_en <span style="color: #800080">&& (</span>retire_address<span style="color: #800080">[</span><span style="color: #FF0000">0</span><span style="color: #800080">:</span><span style="color: #FF0000">4</span><span style="color: #800080">]==</span>addr<span style="color: #800080">)) [*</span><span style="color: #FF0000">2</span><span style="color: #800080">] |-></span>
<span style="color: #800080">##[</span><span style="color: #FF0000">3</span><span style="color: #800080">:</span><span style="color: #FF0000">8</span><span style="color: #800080">]</span> write_en <span style="color: #800080">&& !</span>data_valid <span style="color: #800080">&&(</span>write_address<span style="color: #800080">[</span><span style="color: #FF0000">0</span><span style="color: #800080">:</span><span style="color: #FF0000">4</span><span style="color: #800080">]==</span>addr<span style="color: #800080">);</span>
<span style="color: #0000FF">endproperty</span></pre></div></div>
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作为一种选择，这个特性可以被编码成一个嵌套的蕴含： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00685');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00685"><pre class="Element12"><span style="color: #0000FF">property</span> p16_nested<span style="color: #800080">;</span>
<span style="color: #800080">(</span>write_en <span style="color: #800080">&</span> data_valid<span style="color: #800080">)</span> <span style="color: #800080">|-></span>
<span style="color: #800080">(</span>write_en <span style="color: #800080">&& (</span>retire_address<span style="color: #800080">[</span><span style="color: #FF0000">0</span><span style="color: #800080">:</span><span style="color: #FF0000">4</span><span style="color: #800080">]==</span>addr<span style="color: #800080">)) [*</span><span style="color: #FF0000">2</span><span style="color: #800080">] |-></span>
<span style="color: #800080">##[</span><span style="color: #FF0000">3</span><span style="color: #800080">:</span><span style="color: #FF0000">8</span><span style="color: #800080">]</span> write_en <span style="color: #800080">&& !</span>data_valid <span style="color: #800080">&&(</span>write_address<span style="color: #800080">[</span><span style="color: #FF0000">0</span><span style="color: #800080">:</span><span style="color: #FF0000">4</span><span style="color: #800080">]==</span>addr<span style="color: #800080">);</span>
<span style="color: #0000FF">endproperty</span></pre></div></div>
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多时钟序列的蕴含操作将在<a href="00151.html" target="topic">17.12节</a>中解释。</p></div>
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