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<title>12.16 随机序列产生 — randsequence</title>
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12.16 随机序列产生 — randsequence</div>
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分析程序生成器，例如yacc，使用巴科斯-诺尔范式（BNF）或类似的符号来描述被分析语言的文法。因此文法被用来产生一个程序，它能够检查一个标记流是否代表了该语言中一个语法正确的表达方式。SystemVerilog的序列产生器反转了这个过程。它使用文法来随机地产生一个文法所描述的语言的正确的表达方式（也就是一个标记流）。随机序列发生器对于随机地产生结构化的激励序列（例如指令或网络流量模式）非常有用。&nbsp;</p>
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&nbsp;</p>
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序列发生器使用个<span style="color: #0000FF">randsequence</span>块内的一组规则和生成式。<span style="color: #0000FF">randsequence</span>块的语法如下： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00474');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00474"><pre class="Element12">statement_item ::=           <span style="color: #008000">// 引用自附录A.6.4</span>
    ...
  | randsequence_statement

randsequence_statement ::= <strong><span style="color: #FF0000">randsequence(</span></strong>[production_identifier]<strong><span style="color: #FF0000">)</span></strong>  <span style="color: #008000">// 引用自附录A.6.12</span>
        production {production}
    <strong><span style="color: #FF0000">endsequence</span></strong>

production ::= [function_data_type] production_name[<strong><span style="color: #FF0000">(</span></strong>tf_port_list<strong><span style="color: #FF0000">)</span></strong>]<strong><span style="color: #FF0000">:</span></strong>rs_rule{<strong><span style="color: #FF0000">|</span></strong>rs_rule}<strong><span style="color: #FF0000">;</span></strong>

rs_rule ::= rs_production_list [<strong><span style="color: #FF0000">:=</span></strong> expression[rs_code_block]]

rs_production_list ::=
    rs_prod {rs_prod}
  | <strong><span style="color: #FF0000">rand join</span></strong> [<strong><span style="color: #FF0000">(</span></strong>expression<strong><span style="color: #FF0000">)</span></strong>] production_item production_item {production_item}

rs_code_block ::= <strong><span style="color: #FF0000">{</span></strong>{data_declaration} {statement_or_null}<strong><span style="color: #FF0000">}</span></strong>

rs_prod ::=
    production_item
  | rs_code_block
  | rs_if_else
  | rs_repeat
  | rs_case

production_item ::= production_identifier[<strong><span style="color: #FF0000">(</span></strong>list_of_arguments<strong><span style="color: #FF0000">)</span></strong>]

rs_if_else ::= <strong><span style="color: #FF0000">if(</span></strong>expression<strong><span style="color: #FF0000">)</span></strong> production_item [<strong><span style="color: #FF0000">else</span></strong> production_item]

rs_repeat ::= <strong><span style="color: #FF0000">repeat(</span></strong>expression<strong><span style="color: #FF0000">)</span></strong> production_item

rs_case ::= <strong><span style="color: #FF0000">case(</span></strong>expression<strong><span style="color: #FF0000">)</span></strong> rs_case_item {rs_case_item} <strong><span style="color: #FF0000">endcase</span></strong>

rs_case_item ::=
    expression{<strong><span style="color: #FF0000">,</span></strong> expression}<strong><span style="color: #FF0000">:</span></strong>production_item
  | <strong><span style="color: #FF0000">default</span></strong> [<strong><span style="color: #FF0000">:</span></strong>] production_item</pre></div></div>
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<strong>语法12-12 — randsequence语法（摘录自附录A）</strong></p><p class="Element10">
&nbsp;</p>
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一个<span style="color: #0000FF">randsequence</span>文法有一个或多个生成式组成。每一个生成式都包含了一个名字和一个生成式条目的列表。生成式条目可以进一步分类成端接和非端接。非端接根据端接和其它的非端接定义。一个端接是一个不可分割的条目，除了与其关联的代码块外它不需要进一步的定义。最终，每一个非端接被分解成它的端接。一个生成式列表包含了一连串的生成式条目，它指示这些条目必须按顺序形成流。单个生成式可以包含以<span style="color: #800080">|</span>符号分割的多个生成式列表。以<span style="color: #800080">|</span>符号分割的生成式列表意味着一组选择，发生器能够随机地对它们做出选择。&nbsp;</p>
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&nbsp;</p>
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下面的简单的例子演示了一些基本概念： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00475');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00475"><pre class="Element12"><span style="color: #0000FF">randsequence</span><span style="color: #800080">(</span>main<span style="color: #800080">)</span>
    main   <span style="color: #800080">:</span> first second done<span style="color: #800080">;</span>
    first  <span style="color: #800080">:</span> add <span style="color: #800080">|</span> dec<span style="color: #800080">;</span>
    second <span style="color: #800080">:</span> pop <span style="color: #800080">|</span> push<span style="color: #800080">;</span>
    done   <span style="color: #800080">:</span> <span style="color: #800080">{</span><span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">&quot;done&quot;</span><span style="color: #800080">);};</span>
    add    <span style="color: #800080">:</span> <span style="color: #800080">{</span><span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">&quot;add&quot;</span><span style="color: #800080">);};</span>
    dec    <span style="color: #800080">:</span> <span style="color: #800080">{</span><span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">&quot;dec&quot;</span><span style="color: #800080">);};</span>
    pop    <span style="color: #800080">:</span> <span style="color: #800080">{</span><span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">&quot;pop&quot;</span><span style="color: #800080">);};</span>
    push   <span style="color: #800080">:</span> <span style="color: #800080">{</span><span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">&quot;push&quot;</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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生成式<span class="Element146">main</span>依据三个非端接（<span class="Element146">first</span>、<span class="Element146">second</span>和<span class="Element146">done</span>）定义。当<span class="Element146">main</span>被选择的时候，它产生序列<span class="Element146">first</span>，<span class="Element146">second</span>和<span class="Element146">done</span>。当<span class="Element146">first</span>被产生的时候，它被分解成它的生成式，它在<span class="Element146">add</span>和<span class="Element146">dec</span>之间指定一个随机选择。类似地，<span class="Element146">second</span>生成式在<span class="Element146">pop</span>和<span class="Element146">push</span>之间指定一个选择。所有其它的生成式都是端接；它们被它们的代码块完全指定，在这个例子中就是显示生成式名字。因此，文法会导致下列可能的结果： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00476"><pre class="Element12">add pop done
add push done
dec pop done
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