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📁 这是一本关于verilog编程语言的教程,对学习verilog语言有帮助
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<p class="Element10">
访问这些隐含变量产生从对应的生成式返回的值。当执行的时候，上面的例子显示一个简单的三条目的随机序列：一个操作符跟着两个8位的值。操作符+、-、和*分别以5/8、2/8、和1/8的分布被选择。&nbsp;</p>
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&nbsp;</p>
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只有已经被产生的生成式的返回值（也就是访问它们的代码块的左侧）可以被重新获取。试图读取一个还没有产生的生成式的返回值会导致一个为定义的值。例如： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00497');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00497"><pre class="Element12">X <span style="color: #800080">:</span> A <span style="color: #800080">{</span><span style="color: #0000FF">int</span> y <span style="color: #800080">=</span> B<span style="color: #800080">;}</span> B<span style="color: #800080">;</span>                  <span style="color: #008000">// 对B的无效的访问</span>
X <span style="color: #800080">:</span> A <span style="color: #800080">{</span><span style="color: #0000FF">int</span> y <span style="color: #800080">=</span> A<span style="color: #800080">[</span><span style="color: #FF0000">2</span><span style="color: #800080">];}</span> B A<span style="color: #800080">;</span>             <span style="color: #008000">// 对A[2]的无效的访问</span>
X <span style="color: #800080">:</span> A <span style="color: #800080">{</span><span style="color: #0000FF">int</span> y <span style="color: #800080">=</span> A<span style="color: #800080">;}</span> B <span style="color: #800080">{</span><span style="color: #0000FF">int</span> j <span style="color: #800080">=</span> A <span style="color: #800080">+</span> B<span style="color: #800080">;};</span> <span style="color: #008000">// 有效</span></pre></div></div>
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&nbsp;</p>
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<span style="color: #0000FF">randsequence</span>产生的序列可以作为生成式产生的边带效应被直接驱动到一个系统当中，或者这个序列可以被产生以便进行后续的处理。例如，下列的函数产生并返回一个位于其参数给定范围内的随机数队列。第一个和最后一个队列条目分别对应于下边界和上边界。而且，队列的尺寸根据生成式的权重被随机地选择。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00498"><pre class="Element12"><span style="color: #0000FF">function int</span><span style="color: #800080">[</span><span style="color: #FF0000">$</span><span style="color: #800080">]</span> GenQueue<span style="color: #800080">(</span><span style="color: #0000FF">int</span> low<span style="color: #800080">,</span> <span style="color: #0000FF">int</span> high<span style="color: #800080">);</span>
    <span style="color: #0000FF">int</span><span style="color: #800080">[</span><span style="color: #FF0000">$</span><span style="color: #800080">]</span> q<span style="color: #800080">;</span>

    <span style="color: #0000FF">randsequence</span><span style="color: #800080">()</span>
        TOP          <span style="color: #800080">:</span> BOUND<span style="color: #800080">(</span>low<span style="color: #800080">)</span> LIST BOUND<span style="color: #800080">(</span>high<span style="color: #800080">);</span>
        LIST         <span style="color: #800080">:</span> LIST ITEM <span style="color: #800080">:=</span> <span style="color: #FF0000">8</span><span style="color: #800080">{</span>q <span style="color: #800080">= {</span>q<span style="color: #800080">,</span> ITEM<span style="color: #800080">};}</span>
                          <span style="color: #800080">|</span> ITEM <span style="color: #800080">:=</span> <span style="color: #FF0000">2</span><span style="color: #800080">{</span>q <span style="color: #800080">= {</span>q<span style="color: #800080">,</span> ITEM<span style="color: #800080">};}</span>
                          <span style="color: #800080">;</span>
        <span style="color: #0000FF">int</span> ITEM     <span style="color: #800080">:</span> <span style="color: #800080">{</span><span style="color: #0000FF">return</span> <span style="color: #0000FF">$urandom_range</span><span style="color: #800080">(</span>low<span style="color: #800080">,</span> high<span style="color: #800080">);};</span>
        BOUND<span style="color: #800080">(</span><span style="color: #0000FF">int</span> b<span style="color: #800080">)</span> <span style="color: #800080">:</span> <span style="color: #800080">{</span>q <span style="color: #800080">= {</span>q<span style="color: #800080">,</span> b<span style="color: #800080">};};</span>
    <span style="color: #0000FF">endsequence</span>

    GenQueue <span style="color: #800080">=</span> q<span style="color: #800080">;</span>
<span style="color: #0000FF">endfunction</span></pre></div></div>
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&nbsp;</p>
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当函数<span class="Element146">GenQueue</span>中的<span class="Element146">randsequence</span>执行的时候，它产生<span class="Element146">TOP</span>生成式，<span class="Element146">TOP</span>生成式会产生三个生成式：具有参数<span class="Element146">low</span>的<span class="Element146">BOUND</span>、<span class="Element146">LIST</span>和具有参数<span class="Element146">high</span>的<span class="Element146">BOUND</span>。<span class="Element146">BOUND</span>生成式简单地将它的参数附加到队列当中。<span class="Element146">LIST</span>生成式包含了一个加权的<span class="Element146">LIST ITEM</span>生成式和一个<span class="Element146">ITEM</span>生成式。产生<span class="Element146">LIST ITEM</span>生成式的概率为80%，它会引起<span class="Element146">LIST</span>生成式被递归地产生，因此延缓了<span class="Element146">ITEM</span>生成式的产生。<span class="Element146">LIST ITEM</span>和<span class="Element146">ITEM</span>的选择会被重复直到选择<span class="Element146">ITEM</span>（它会中止<span class="Element146">LIST</span>生成式）。每次产生<span class="Element146">ITEM</span>生成式的时候，它会在指定的范围内产生一个随机数，这个随机数会在以后附加到队列。&nbsp;</p>
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&nbsp;</p>
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下面的例子使用一个<span style="color: #0000FF">randsequence</span>块为一个<span class="Element146">DSL</span>包网络产生随机流量。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00499"><pre class="Element12"><span style="color: #0000FF">class</span> DSL<span style="color: #800080">;</span> ... <span style="color: #0000FF">endclass</span>  <span style="color: #008000">// 产生有效DSL包的类</span>

<span style="color: #0000FF">randsequence</span> <span style="color: #800080">(</span>STREAM<span style="color: #800080">)</span>
    STREAM               <span style="color: #800080">:</span> GAP DATA <span style="color: #800080">:=</span> <span style="color: #FF0000">80</span>
                         <span style="color: #800080">|</span> DATA <span style="color: #800080">:=</span> <span style="color: #FF0000">20</span><span style="color: #800080">;</span>
    DATA                 <span style="color: #800080">:</span> PACKET<span style="color: #800080">(</span><span style="color: #FF0000">0</span><span style="color: #800080">) :=</span> <span style="color: #FF0000">94</span><span style="color: #800080">{</span>transmit<span style="color: #800080">(</span>PACKET<span style="color: #800080">);}</span>
                         <span style="color: #800080">|</span> PACKET<span style="color: #800080">(</span><span style="color: #FF0000">1</span><span style="color: #800080">) :=</span> <span style="color: #FF0000">6</span><span style="color: #800080">{</span>transmit<span style="color: #800080">(</span>PACKET<span style="color: #800080">);};</span>
    DSL PACKET <span style="color: #800080">(</span><span style="color: #0000FF">bit</span> bad<span style="color: #800080">)</span> <span style="color: #800080">:</span> <span style="color: #800080">{</span> DSL d <span style="color: #800080">=</span> <span style="color: #0000FF">new</span><span style="color: #800080">;</span>
                             <span style="color: #0000FF">if </span><span style="color: #800080">(</span>bad<span style="color: #800080">)</span> d<span style="color: #800080">.</span>crc <span style="color: #800080">^= </span><span style="color: #FF0000">23</span><span style="color: #800080">;</span>  <span style="color: #008000">// 损坏的crc</span>
                             <span style="color: #0000FF">return</span> d<span style="color: #800080">;</span>
                           <span style="color: #800080">};</span>
    GAP                  <span style="color: #800080">:</span> <span style="color: #800080">{##{</span><span style="color: #0000FF">$urandom_range</span><span style="color: #800080">(</span><span style="color: #FF0000">1</span><span style="color: #800080">, </span><span style="color: #FF0000">20</span><span style="color: #800080">)};};</span>
<span style="color: #0000FF">endsequence</span></pre></div></div>
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&nbsp;</p>
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在这个例子中，流量包含了一个数据包（好的和坏的）和及其间隙的流。第一个生成式<span class="Element146">STREAM</span>指出：在80%的时间里流量包含了一个<span class="Element146">GAP</span>然后跟着一些<span class="Element146">DATA</span>，在20%的时间里流量仅仅包含了<span class="Element146">DATA</span>（没有<span class="Element146">GAP</span>）。第二个生成式<span class="Element146">DATA</span>指出：所有的数据包中有94%是好的数据包，另外余下的6%是坏的数据包。<span class="Element146">PACKET</span>生成式实现了<span class="Element146">DSL</span>包的产生；如果生成式参数是<span style="color: #FF0000">1</span>，那么会通过损坏一个有效的<span class="Element146">DSL</span>包的<span class="Element146">crc</span>来产生一个坏包。最后，<span class="Element146">GAP</span>生成式通过在<span style="color: #FF0000">1</span>到<span style="color: #FF0000">20</span>之间等待一个随机的周期数目来实现发送间隔。</p></div>
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