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<a href="#" onmousedown="showPopup(this, 'popup00345');"><img src="seealsolink.png" border="0" alt="" title=""></a> SystemVerilog 3.1a语言参考手册</div>
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17.7.10 一个序列结束点的检测和使用</div>
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SystemVerilog中提供了两种方法将一个复杂序列分解成较为简单的子序列。 </p>
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一种方法是通过引用名字来实例化一个命名序列。这样一个引用的计算要求命名序列从包围序列的计算期间引用被到达的时钟标记上开始匹配。例如： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00644');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00644"><pre class="Element12"><span style="color: #0000FF">sequence</span> s<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 style="color: #0000FF">endsequence</span>
<span style="color: #0000FF">sequence</span> rule<span style="color: #800080">;</span>
<span style="color: #800080">@(</span><span style="color: #0000FF">posedge</span> sysclk<span style="color: #800080">)</span>
trans <span style="color: #800080">##</span><span style="color: #FF0000">1</span> start_trans <span style="color: #800080">##</span><span style="color: #FF0000">1</span> s <span style="color: #800080">##</span><span style="color: #FF0000">1</span> end_trans<span style="color: #800080">;</span>
<span style="color: #0000FF">endsequence</span></pre></div></div>
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</p>
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序列<i><span class="Element146">s</span></i>从序列<i><span class="Element146">rule</span></i>中的<i><span class="Element146">start_trans</span></i>被计算的一个时钟标记之后开始计算。 </p>
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使用一个序列的另外一个方法是在另外一个序列中检测它的结束点。只要一个序列匹配的结束时钟标记到达，这个序列的结束点就到达，而不管匹配的起始时钟标记如何。通过使用ended方法，任何序列中结束点的到达都可以被检测到。 </p>
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<span style="color: #0000FF">ended</span>方法的语法如下： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00645"><pre class="Element12">sequence_instance<span style="color: #800080">.</span><span style="color: #0000FF">ended</span></pre></div></div>
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<span style="color: #0000FF">ended</span>是一个作用于一个序列的方法。这个方法的操作结果或者为“真”，或者为“假”。当<span style="color: #0000FF">ended</span>方法在一个表达式中被计算的时候，它检测其操作数序列在特定的时间点上是否到达了它的结束点。 </p>
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ended的结果并不依赖于其操作书序列匹配的起始点。这样的一个例子如下所示： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00646"><pre class="Element12"><span style="color: #0000FF">sequence</span> e1<span style="color: #800080">;</span>
<span style="color: #800080">@(</span><span style="color: #0000FF">posedge</span> sysclk<span style="color: #800080">)</span> <span style="color: #800080">$rose(</span>ready<span style="color: #800080">) ##</span><span style="color: #FF0000">1</span> proc1 <span style="color: #800080">##</span><span style="color: #FF0000">1</span> proc2<span style="color: #800080">;</span>
endsequence
<span style="color: #0000FF">sequence</span> rule<span style="color: #800080">;</span>
<span style="color: #800080">@(</span><span style="color: #0000FF">posedge</span> sysclk<span style="color: #800080">)</span> reset <span style="color: #800080">##</span><span style="color: #FF0000">1</span> inst <span style="color: #800080">##</span><span style="color: #FF0000">1</span> e1<span style="color: #800080">.</span><span style="color: #0000FF">ended</span> <span style="color: #800080">##</span><span style="color: #FF0000">1</span> branch_back<span style="color: #800080">;</span>
<span style="color: #0000FF">endsequence</span></pre></div></div>
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在这个例子中，序列<i><span class="Element146">e1</span></i>必须在<i><span class="Element146">inst</span></i>之后的一个时钟标记匹配。如果使用e1的一个实例来替代<span style="color: #0000FF">ended</span>方法，<i><span class="Element146">e1</span></i>的一个匹配必须在<i><span class="Element146">inst</span></i>之后的一个时钟标记开始。注意：<span style="color: #0000FF">ended</span>方法仅仅检测<i><span class="Element146">e1</span></i>的结束点，并且对<i><span class="Element146">e1</span></i>的起始点没有意义。<span style="color: #0000FF">ended</span>方法可以用于具有形式参数的序列。例如下面的声明： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00647"><pre class="Element12"><span style="color: #0000FF">sequence</span> e2<span style="color: #800080">(</span>a<span style="color: #800080">,</span>b<span style="color: #800080">,</span>c<span style="color: #800080">);</span>
<span style="color: #800080">@(</span><span style="color: #0000FF">posedge</span> sysclk<span style="color: #800080">)</span> <span style="color: #0000FF">$rose</span><span style="color: #800080">(</span>a<span style="color: #800080">)</span> <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 style="color: #0000FF">endsequence</span>
<span style="color: #0000FF">sequence</span> rule2<span style="color: #800080">;</span>
<span style="color: #800080">@(</span><span style="color: #0000FF">posedge</span> sysclk<span style="color: #800080">)</span> reset <span style="color: #800080">##</span><span style="color: #FF0000">1</span> inst <span style="color: #800080">##</span><span style="color: #FF0000">1</span> e2<span style="color: #800080">(</span>ready<span style="color: #800080">,</span>proc1<span style="color: #800080">,</span>proc2<span style="color: #800080">).</span><span style="color: #0000FF">ended</span>
<span style="color: #800080">##</span><span style="color: #FF0000">1</span> branch_back<span style="color: #800080">;</span>
<span style="color: #0000FF">endsequence</span></pre></div></div>
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<i><span class="Element146">rule2</span></i>等价于下面的<i><span class="Element146">rule2a</span></i>： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00648"><pre class="Element12"><span style="color: #0000FF">sequence</span> e2_instantiated<span style="color: #800080">;</span>
e2<span style="color: #800080">(</span>ready<span style="color: #800080">,</span> proc1<span style="color: #800080">,</span> proc2<span style="color: #800080">);</span>
<span style="color: #0000FF">endsequence</span>
<span style="color: #0000FF">sequence</span> rule2a<span style="color: #800080">;</span>
<span style="color: #800080">@(</span><span style="color: #0000FF">posedge</span> sysclk<span style="color: #800080">)</span> reset <span style="color: #800080">##</span><span style="color: #FF0000">1</span> inst <span style="color: #800080">##</span><span style="color: #FF0000">1</span> e2_instantiated<span style="color: #800080">.</span><span style="color: #0000FF">ended</span> <span style="color: #800080">##</span><span style="color: #FF0000">1</span> branch_back<span style="color: #800080">;</span>
<span style="color: #0000FF">endsequence</span></pre></div></div>
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在将局部变量传递到一个应用了<span style="color: #0000FF">ended</span>方法的序列实例的时候存在一些其它的限制。参见<a href="00186.html" target="topic">17.8节</a>。</p></div>
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