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<a href="#" onmousedown="showPopup(this, 'popup00376');"><img src="seealsolink.png" border="0" alt="" title=""></a> SystemVerilog 3.1a语言参考手册</div>
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18.3 编译单元的支持</div>
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通过使用编译单元，SystemVerilog支持分离的编译。SystemVerilog提供了下列的术语和定义：
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<li class="Element604">编译单元：a collection of one or more SystemVerilog source files compiled together</li>
<li class="Element604">编译单元作用域：a scope that is local to the compilation unit. It contains all declarations that lie outside of any other scope</li>
<li class="Element604"><span style="color: #0000FF">$unit</span>：用来显式地访问编译单元作用域中的标识符的名字</li>
</ul>The exact mechanism for defining which files constitute a compilation unit is tool specific. Tools shall provide a mechanism to specify the files that make up a compilation unit. Two extreme cases are:
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<li value="1" class="Element604">All files make a single compilation unit (in which case the declarations in the compilation-unit scope are accessible anywhere within the design)</li>
<li value="2" class="Element604">Each file is a separate compilation unit (in which case the declarations in each compilation-unit scope are accessible only within its corresponding file)</li>
</ol>The contents of files included using one or more ‘include directives become part of the compilation unit of the file they are included within. </p>
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If there is a declaration that is incomplete at the end of a file, then the compilation unit including that file will extend through each successive file until there are no incomplete declarations at the end of the group of files. </p>
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The default is that each file is a separate compilation unit. </p>
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A tool must also provide a mechanism (such as a command line switch) that specifies that all of the files compiled together are a single compilation unit. </p>
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There are other possible mappings of files to compilation units and the mechanism for defining them are tool specific and may not be portable. </p>
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The compilation-unit scope can contain any item that can be defined within a package. These items are in the compilation-unit scope name space. </p>
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The following items are visible in all compilation units: modules, macromodules, primitives, programs, interfaces, and packages. Items defined in the compilation-unit scope cannot be accessed by name from outside the compilation unit. Access to the items in a compilation-unit scope can be accessed using the PLI, which must provide an iterator to traverse all the compilation units. </p>
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In Verilog, compiler directives once seen by a tool apply to all forthcoming source text. This behavior shall be supported within a separately compiled unit; however, compiler directives from one separately compiled unit shall not affect other compilation units. This may result in a difference of behavior between compiling the units separately or as a single compilation unit containing the entire source. </p>
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When an identifier is referenced within a scope, SystemVerilog follows the Verilog name search rules:
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<li class="Element604">First, the nested scope is searched (1364-2001 12.6) (including nested module declarations), including any identifiers made available through package import declarations</li>
<li class="Element604">Next, the compilation-unit scope is searched (including any identifiers made available through package import declarations</li>
<li class="Element604">Finally, the instance hierarchy is searched (1364-2001 12.5)</li>
</ul>$unit is the name of the scope that encompasses a compilation unit. Its purpose is to allow the unambiguous reference to declarations at the outermost level of a compilation unit (i.e., those in the compilation-unit scope).This is done via the same scope resolution operator used to access package items. </p>
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For example: </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00800');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00800"><pre class="Element12">bit b;
task foo;
int b;
b = 5 + $unit::b; // $unit::b is the one outside
endtask</pre></div></div>
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The compilation-unit scope allows users to easily share declarations (e.g., types) across the unit of compilation, but without having to declare a package from which the declarations are subsequently imported. Thus, the compilation-unit scope is similar to an implicitly defined anonymous package. Because it has no name, the compilation-unit scope cannot be used with an import statement, and the identifiers declared within the scope are not accessible via hierarchical references. Within a particular compilation unit, however, the special name $unit can be used to explicitly access the declarations of its compilation-unit scope.</p></div>
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