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<a href="#" onmousedown="showPopup(this, 'popup00169');"><img src="seealsolink.png" border="0" alt="" title=""></a> SystemVerilog 3.1a语言参考手册</div>
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9.6 fork...join</div>
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<span style="color: #0000FF">fork</span>...<span style="color: #0000FF">join</span>能够从它的每一个并行语句中产生并发进程。 </p>
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<span style="color: #0000FF">fork</span>...<span style="color: #0000FF">join</span>块的声明语法如下： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00301');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00301"><pre class="Element12">par_block ::= <span style="color: #008000">// 引用自附录A.6.3</span>
<strong><span style="color: #FF0000">fork</span></strong> [<strong><span style="color: #FF0000">: </span></strong>block_identifier] {block_item_declaration} {statement_or_null}
join_keyword [<strong><span style="color: #FF0000">:</span></strong> block_identifier]
join_keyword ::= <strong><span style="color: #FF0000">join</span></strong> | <strong><span style="color: #FF0000">join_any</span></strong> | <strong><span style="color: #FF0000">join_none</span></strong></pre></div></div>
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<strong>语法 9-1—Fork...join块语法（摘录自附录A）</strong></p><p class="Element10">
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<span style="color: #0000FF">fork</span>...<span style="color: #0000FF">join</span>块可以指定一条或多条语句，每一条语句都应该作为并发进程执行。Verilog <span style="color: #0000FF">fork</span>...<span style="color: #0000FF">join</span>块总是引起执行fork语句的进程阻塞直到所有分支进程中止。通过加入<span style="color: #0000FF">join_any</span>和<span style="color: #0000FF">join_none</span>关键字，SystemVerilog提供了三种选择来指定父进程何时恢复执行。 </p>
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<strong>表 9-1: <span style="color: #0000FF">fork</span>...<span style="color: #0000FF">join</span>控制选项</strong> </p><div class="Element63">
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选项 </div></td><td class="Element65" valign="top" width="85%">
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描述 </div></td></tr><tr>
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<span style="color: #0000FF">join</span> </div></td><td class="Element67" valign="top" width="85%">
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父进程会阻塞直到这个分支产生的所有进程结束。 </div></td></tr><tr>
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<span style="color: #0000FF">join_any</span> </div></td><td class="Element67" valign="top" width="85%">
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父进程会阻塞直到这个分支产生的任意一个进程结束。 </div></td></tr><tr>
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<span style="color: #0000FF">join_none</span> </div></td><td class="Element67" valign="top" width="85%">
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父进程会继续与这个分支产生的所有进程并发执行。在父线程执行一条阻塞语句之前，产生的进程不会启动执行。 </div></td></tr></table></div></div>
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在定义一个<span style="color: #0000FF">fork</span>...<span style="color: #0000FF">join</span>块的时候，将整个分叉封装在一个<span style="color: #0000FF">begin</span>...<span style="color: #0000FF">end</span>块中会引起整个块作为单个进程执行，其中每条语句顺序地执行。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00302"><pre class="Element12"><span style="color: #0000FF">fork</span>
<span style="color: #0000FF"> begin</span>
statement1<span style="color: #800080">;</span> <span style="color: #008000">// 一个带有2条语句的进程</span>
statement2<span style="color: #800080">;</span>
<span style="color: #0000FF">end</span>
<span style="color: #0000FF">join</span></pre></div></div>
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在下面的例子中，包含两个进程分支，第一个等待20ns，第二个等待命名事件<i>eventA</i>被触发。因为指定了<span style="color: #0000FF">join</span>关键字，父进程应该阻塞直到这两个进程结束；也就是说，直到过了20ns并且<i>eventA</i>被触发。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00303"><pre class="Element12"><span style="color: #0000FF">fork</span>
<span style="color: #0000FF"> begin</span>
<span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">"First Block\n"</span><span style="color: #800080">);</span>
<span style="color: #800080">#</span><span style="color: #FF0000">20</span><span style="color: #0000FF">ns</span><span style="color: #800080">;</span>
<span style="color: #0000FF">end</span>
<span style="color: #0000FF"> begin</span>
<span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">"Second Block\n"</span><span style="color: #800080">);</span>
<span style="color: #800080">@</span>eventA<span style="color: #800080">;</span>
<span style="color: #0000FF">end</span>
<span style="color: #0000FF">join</span></pre></div></div>
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在一个<span style="color: #0000FF">fork</span>...<span style="color: #0000FF">join</span>语句的关联文中使用<span style="color: #0000FF">return</span>语句是非法的，并且会导致一个编译错误。例如： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00304"><pre class="Element12"><span style="color: #0000FF">task</span> wait_20<span style="color: #800080">;</span>
<span style="color: #0000FF">fork</span>
<span style="color: #800080">#</span><span style="color: #FF0000">20</span><span style="color: #800080">;</span>
<span style="color: #0000FF">return</span><span style="color: #800080">;</span> <span style="color: #008000">// 非法的：不能返回；任务位于另外一个进程中</span>
<span style="color: #0000FF">join_none</span>
<span style="color: #0000FF">endtask</span></pre></div></div>
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只要<span style="color: #0000FF">fork</span>...<span style="color: #0000FF">join</span>块作用范围内声明的自动变量进入它们的作用范围，在产生任何进程之前，这些自动变量应该被初始化成它们的初始值。这些变量对于循环结构产生的进程存储唯一的、每次都交互的数据非常有用。例如： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00305');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00305"><pre class="Element12"><span style="color: #0000FF">initial</span>
<span style="color: #0000FF"> for</span><span style="color: #800080">(</span><span style="color: #0000FF">int</span> j <span style="color: #800080">=</span> 1<span style="color: #800080">;</span> j <span style="color: #800080"><=</span> <span style="color: #FF0000">3</span><span style="color: #800080">;</span> <span style="color: #FF0000">++</span>j<span style="color: #800080">)</span>
<span style="color: #0000FF">fork</span>
<span style="color: #0000FF"> automatic</span> <span style="color: #0000FF">int</span> k <span style="color: #800080">=</span> j<span style="color: #800080">;</span> <span style="color: #008000">// 对每一个j值的本地拷贝</span>
<span style="color: #800080">#</span>k <span style="color: #0000FF">$write</span><span style="color: #800080">(</span><span style="color: #575D94">"%0d"</span><span style="color: #800080">,</span> k<span style="color: #800080">);</span>
<span style="color: #0000FF">begin</span>
<span style="color: #0000FF">automatic int</span> m <span style="color: #800080">=</span> j<span style="color: #800080">;</span> <span style="color: #008000">// m的值是不确定的</span>
...
<span style="color: #0000FF">end</span>
<span style="color: #0000FF"> join_none</span></pre></div></div>
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上面的例子的输出为：123。</p></div>
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