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<title>12.4 约束块</title>
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<a href="00865.html" target="topic">第十二章 随机约束</a>, <a href="00927.html" target="topic">主题</a></div>
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<a href="#" onmousedown="showPopup(this, 'popup00221');"><img src="seealsolink.png" border="0" alt="" title=""></a> SystemVerilog 3.1a语言参考手册</div>
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12.4 约束块</div>
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随机变量的值通过由约束块声明的约束表达式来确定。像任务、函数和变量一样，随机块是类的成员。在一个类中，约束块的名字必须是唯一的。 </p>
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声明约束块的语法如下： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00402"><pre class="Element12">constraint_declaration ::= <span style="color: #008000">// 引用自附录A.1.9</span>
[<strong><span style="color: #FF0000">static</span></strong>] <strong><span style="color: #FF0000">constraint</span></strong> constraint_identifier constraint_block
constraint_block ::= <strong><span style="color: #FF0000">{</span></strong>{constraint_block_item}<strong><span style="color: #FF0000">}</span></strong>
constraint_block_item ::=
<strong><span style="color: #FF0000">solve</span></strong> identifier_list <strong><span style="color: #FF0000">before</span></strong> identifier_list<span style="color: #FF0000">;</span>
| constraint_expression
constraint_expression ::=
expression_or_dist<strong><span style="color: #FF0000">;</span></strong>
| expression <strong><span style="color: #FF0000">–></span></strong> constraint_set
| <strong><span style="color: #FF0000">if</span></strong> <strong><span style="color: #FF0000">(</span></strong>expression<strong><span style="color: #FF0000">)</span></strong> constraint_set [<strong><span style="color: #FF0000">else</span></strong> constraint_set]
| <strong><span style="color: #FF0000">foreach</span></strong> <strong><span style="color: #FF0000">(</span></strong>array_identifier <strong><span style="color: #FF0000">[</span></strong>loop_variables<strong><span style="color: #FF0000">])</span></strong> constraint_set
constraint_set ::=
constraint_expression
| <strong><span style="color: #FF0000">{</span></strong>{constraint_expression}<strong><span style="color: #FF0000">}</span></strong>
dist_list ::= dist_item {<strong><span style="color: #FF0000">,</span></strong> dist_item}
dist_item ::= value_range [dist_weight]
dist_weight ::=
<strong><span style="color: #FF0000">:=</span></strong> expression
| <strong><span style="color: #FF0000">:/</span></strong> expression
constraint_prototype ::= [<strong><span style="color: #FF0000">static</span></strong>] <strong><span style="color: #FF0000">constraint</span></strong> constraint_identifier<strong><span style="color: #FF0000">;</span></strong>
extern_constraint_declaration ::=
[<strong><span style="color: #FF0000">static</span></strong>] <strong><span style="color: #FF0000">constraint</span></strong> class_scope constraint_identifier constraint_block
identifier_list ::= identifier{<strong><span style="color: #FF0000">,</span></strong> identifier}
expression_or_dist ::= expression [<strong><span style="color: #FF0000">dist{</span></strong>dist_list<strong><span style="color: #FF0000">}</span></strong>] <span style="color: #008000">// 引用自附录A.2.10</span>
loop_variables ::= [index_variable_identifier]{<strong><span style="color: #FF0000">,</span></strong> [index_variable_identifier]}
<span style="color: #008000">// 引用自附录A.6.8</span></pre></div></div>
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<strong>语法12-2 — 约束语法（摘录自附录A）</strong> </p>
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<i>constraint_identifier</i>是约束块的名字。使用<span style="color: #0000FF">constraint_mode</span><span style="color: #800080">()</span>（参见<a href="00086.html" target="topic">12.8节</a>）方法，这个名字可以用来使能或关闭一个约束。 </p>
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<i>constraint_block</i>是一个表达式语句的列表，它用来限制一个变量的取值范围或者定义变量间的关系。<i>constraint_expression</i>是任意SystemVerilog表达式，或者是约束专用操作符<span style="color: #800080">-></span>和<span style="color: #0000FF">dist</span>（参见<a href="00074.html" target="topic">12.4.4节</a>和<a href="00075.html" target="topic">12.4.5节</a>）中的一个。 </p>
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约束的说明状态对约束表达式强加了下列限制：</p>
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<li class="Element600">函数可以具有某种限制（参见<a href="00070.html" target="topic">12.4.11节</a>）。</li>
<li class="Element600">不允许使用具有边带效应的操作符，例如<span style="color: #800080">++</span>和<span style="color: #800080">--</span>。</li>
<li class="Element600">在排序约束中不能指定<span style="color: #0000FF">randc</span>变量（参见<a href="00079.html" target="topic">12.4.9节</a>中的<span style="color: #0000FF">solve</span>...<span style="color: #0000FF">before</span>）。</li>
<li class="Element600"><span style="color: #0000FF">dist</span>表达式不能出现在其它表达式中。</li>
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<a href="00865.html" target="topic">第十二章 随机约束</a></p></div>
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