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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

<a href="00208.html" target="topic">18.5 模块声明</a>&nbsp;</div></td><td class="Element206" valign="top" width="50%">
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<a href="00209.html" target="topic">18.6 嵌套的模块</a>&nbsp;</div></td><td class="Element206" valign="top" width="50%">
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A module can be declared within another module. The outer name space is visible to the inner module, so that any name declared there can be used, unless hidden by a local name, provided the module is declared and instantiated in the same scope.<br><br>One purpose of nesting modules is to show the logical partitioning of a module without using ports. Names that are global are in the outermost scope, and names that are only used locally can be limited to local modules. &nbsp;</div></td></tr><tr>
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<a href="00210.html" target="topic">18.7 外部模块</a>&nbsp;</div></td><td class="Element206" valign="top" width="50%">
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To support separate compilation, extern declarations of a module can be used to declare the ports on a module without defining the module itself. An extern module declaration consists of the keyword extern followed by the module name and the list of ports for the module. Both list of ports syntax (possibly with parameters), and original Verilog style port declarations can be used. Note that the potential existence of defparams precludes the checking of the port connection information prior to elaboration time even for list of ports style declarations.<br><br>The following example demonstrates the usage of extern module declarations. &nbsp;</div></td></tr><tr>
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<a href="00211.html" target="topic">18.8 端口声明</a>&nbsp;</div></td><td class="Element206" valign="top" width="50%">
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<a href="00212.html" target="topic">18.9 端口表达式列表</a>&nbsp;</div></td><td class="Element206" valign="top" width="50%">
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Verilog 1364-2001 created a list_of_port_declarations alternate style which minimized the duplication of data used to specify the ports of a module. SystemVerilog adds an explicitly named port declaration to that style, allowing elements of arrays and structures, concatenations of elements, or aggregate expressions of elements declared in a module, interface or program to be specified on the port list.<br><br>Like explicitly named ports in a module port declaration, port identifiers exist in their own namespace for each port list. When port item is just a simple port identifier, that identifier is used as both a reference to an interface item... <a href="00212.html" target="topic">more</a>&nbsp;</div></td></tr><tr>
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<a href="00189.html" target="topic">18.10 时间单位与精度</a>&nbsp;</div></td><td class="Element206" valign="top" width="50%">
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SystemVerilog has a time unit and precision declaration which has the equivalent functionality of the ‘timescale compiler directives in Verilog-2001. Use of these declarations removes the file order dependencies problems with compiler directives. The time unit and precision can be declared by the timeunit and timeprecision keywords, respectively, and set to a time literal which must be a power of 10 units. For example: &nbsp;</div></td></tr><tr>
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<a href="00190.html" target="topic">18.11 模块实例</a>&nbsp;</div></td><td class="Element206" valign="top" width="50%">
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<a href="00195.html" target="topic">18.12 端口连接规则</a>&nbsp;</div></td><td class="Element206" valign="top" width="50%">
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通过允许所有变量数据类型都能够通过端口传递，SystemVerilog扩展了Verilog的端口连接能力。通过允许端口连接的两侧具有相同的兼容类型，并且通过允许对变量的连续赋值，SystemVerilog做到了这一点。SystemVerilog还产生了一个新的端口限定符类型，<span style="color: #0000FF">ref</span>，它允许共享的变量通过传递一个层次化的引用来跨越一个端口。&nbsp;</div></td></tr><tr>
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<a href="00201.html" target="topic">18.13 命名空间</a>&nbsp;</div></td><td class="Element206" valign="top" width="50%">
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SystemVerilog has eight name spaces for identifiers, two are global (definitions name space and package name space), two are global to the compilation unit (compilation unit name space and text macro name space) and four are local. The eight name spaces are described as follows:
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<li value="1" class="Element603">The definitions name space unifies all the non-nested module, macromodule, primitive, program, and interface identifiers defined outside of all other declarations. Once a name is used to define a module, macromodule, primitive, program, or interface within one compilation unit the name shall not be used again (in any compilation unit) to declare another non-nested module,... <a href="00201.html" target="topic">more</a>&nbsp;</div></td></tr><tr>
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<a href="00202.html" target="topic">18.14 层次化的名字</a>&nbsp;</div></td><td class="Element206" valign="top" width="50%">
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Hierarchical names are also called nested identifiers. They consist of instance names separated by periods, where an instance name can be an array element. The instance name $root refers to the top of the instantiated design and is used to unambiguously gain access to the top of the design. &nbsp;</div></td></tr></table></div></div>
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