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17.7.8 序列上的条件</div>
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序列经常发生在某种假设的条件下以获得正确的行为。一个逻辑条件必须保持为“真”，例如在处理一个事物的时候。而且，在处理一个事务的时候，是禁止出现某些值的 。这样的情形可以直接使用下面的结构来表达： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00638"><pre class="Element12">sequence_expr ::= <span style="color: #008000">// 引用自附录A.2.10</span>
...
| expression_or_dist <strong><span style="color: #FF0000">throughout</span></strong> sequence_expr</pre></div></div>
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<strong>语法17-10 — throughout结构的语法（摘录自附录A）</strong></p><p class="Element10">
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<i><span class="Element146">exp <span style="color: #0000FF">throughout</span> seq</span></i>结构是下列语句的缩写： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00639"><pre class="Element12"><span style="color: #800080">(</span>exp<span style="color: #800080">) [</span><span style="color: #FF0000">*0</span><span style="color: #800080">:</span><span style="color: #FF0000">$</span><span style="color: #800080">]</span> <span style="color: #0000FF">intersect</span> seq</pre></div></div>
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对于复合序列<i>exp <span style="color: #0000FF">throughout</span> seq</i>，只要<i><span class="Element146">seq</span></i>在一个有限的连续时钟标记间隔上匹配并且<i><span class="Element146">exp</span></i>在这个间隔的每一个时钟标记上为“真”，那么这个复合序列就在这段间隔上匹配。 </p>
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图17-11演示了如下的例子。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00640"><pre class="Element12"><span style="color: #0000FF">sequence</span> burst_rule1<span style="color: #800080">;</span>
<span style="color: #800080">@(</span><span style="color: #0000FF">posedge</span> mclk<span style="color: #800080">)</span>
<span style="color: #0000FF">$fell</span><span style="color: #800080">(</span>burst_mode<span style="color: #800080">) ##</span><span style="color: #FF0000">0</span>
<span style="color: #800080">(!</span>burst_mode<span style="color: #800080">)</span> <span style="color: #0000FF">throughout</span> <span style="color: #800080">(##</span><span style="color: #FF0000">2</span> <span style="color: #800080">((</span>trdy<span style="color: #800080">==</span><span style="color: #FF0000">0</span><span style="color: #800080">)&&(</span>irdy<span style="color: #800080">==</span><span style="color: #FF0000">0</span><span style="color: #800080">)) [</span><span style="color: #FF0000">*7</span><span style="color: #800080">]);</span>
<span style="color: #0000FF">endsequence</span></pre></div></div>
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<img src="Figure_17_11.png" border="0" alt="" title=""> </p>
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<strong>图17-11 — throughout限制失败的匹配</strong></p><p class="Element10">
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图17-11演示了起始于时钟标记2处针对序列<i><span class="Element146">burst_rule1</span></i>的计算尝试。由于信号<i><span class="Element146">burst_mode</span></i>在时钟标记1处为高并且在时钟标记2处为低，所以<i><span class="Element146"><span style="color: #0000FF">$fell</span><span style="color: #800080">(</span>burst_mode<span style="color: #800080">)</span></span></i>在时钟标记2处为“真”。为了实现<i><span class="Element146">burst_rule1</span></i>的匹配，要求<i><span class="Element146">burst_mode</span></i>的值在起始于时钟标记2处的子序列<i><span class="Element146"><span style="color: #800080">(##</span><span style="color: #FF0000">2</span> <span style="color: #800080">((</span>trdy<span style="color: #800080">==</span><span style="color: #FF0000">0</span><span style="color: #800080">)&&(</span>irdy<span style="color: #800080">==</span><span style="color: #FF0000">0</span><span style="color: #800080">)) [</span><span style="color: #FF0000">*7</span><span style="color: #800080">])</span></span></i>的一次匹配期间内为低。这个子序列在时钟标记2到时钟标记10之间匹配。然而，在时钟标记9处<i><span class="Element146">burst_mode</span></i>变为高，因此，根据<span style="color: #0000FF">througout</span>的规则，序列的匹配失败。 </p>
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相反，如果信号<i><span class="Element146">burst_mode</span></i>能够至少在时钟标记10处保持为低，那么从时钟标记2到时钟标记10之间<i><span class="Element146">burst_rule1</span></i>会有一次匹配，如图17-12所示。 </p><p class="Element10" style="text-align: center">
<img src="Figure_17_12.png" border="0" alt="" title=""> </p>
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<strong>图17-12 — throughout限制成功的匹配</strong> </p></div>
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