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<HTML><HEAD> <META NAME="GENERATOR" CONTENT="Adobe PageMill 2.0 Mac"> <LINK REL="STYLESHEET" HREF="ch02.css"> <TITLE> 2.7 Identifiers, keywords, and system names </TITLE></HEAD><BODY BGCOLOR="#ffffff"><P><A NAME="pgfId=262"></A><HR ALIGN=LEFT></P><P><A HREF="ch02.htm">Chapter start</A> <A HREF="ch02.6.htm">Previous page</A></P><H1>2.7 Identifiers, keywords, and system names</H1><P><P CLASS="Body"><A NAME="pgfId=264"></A>An <I>identifier</I> is usedto give an object a unique name so it can be referenced. An identifier shallbe any sequence of letters, digits, dollar signs (<CODE> $</CODE> ), andthe underscore characters (<CODE> _</CODE> ).</P><P><P CLASS="Body"><A NAME="pgfId=265"></A>The first character of an identifiershall not be a digit or <CODE>$</CODE> ; it can be a letter or an underscore.Identifiers shall be case sensitive.</P><P><P CLASS="Body"><A NAME="pgfId=267"></A>Examples:</P><PRE><A NAME="pgfId=268"></A> shiftreg_abusa_index error_condition merge_ab_bus3n$657</PRE><P><P CLASS="Note"><A NAME="pgfId=247"></A>NOTE--Implementations may seta limit on the maximum length of identifiers, but shall at least be 1024characters. If an identifier exceeds implementation specified length limit,an error shall be reported.</P><P><P CLASS="SubSection"><A NAME="pgfId=271"></A>Escaped identifiers</P><P><P CLASS="Body"><A NAME="pgfId=188"></A><I>Escaped identifiers</I> shallstart with the backslash character (<CODE> \</CODE> ) and end with whitespace (space, tab, newline). They provide a means of including any of theprintable ASCII characters in an identifier (the decimal values 33 through126, or 21 through 7E in hexadecimal).</P><P><P CLASS="Body"><A NAME="pgfId=246"></A>Neither the leading back-slashcharacter nor the terminating white space is considered to be part of theidentifier.<EM> Therefore, an escaped identifier</EM> <CODE>\cpu3</CODE>is treated same as a non-escaped identifier <CODE>cpu3</CODE> .</P><P><P CLASS="Body"><A NAME="pgfId=329"></A>Examples:</P><PRE><A NAME="pgfId=273"></A> \busa+index\-clock\***error-condition***\net1/\net2\{a,b}\a*(b+c)</PRE><P><P CLASS="SubSection"><A NAME="pgfId=280"></A>Keywords</P><P><P CLASS="Body"><A NAME="pgfId=178"></A><I>Keywords</I> are predefinednon-escaped identifiers that are used to define the language constructs.A Verilog HDL keyword preceded by an escape character is not interpretedas a keyword.</P><P><P CLASS="Body"><A NAME="pgfId=228"></A>All keywords are defined in lowercaseonly. Annex B gives a list of all keywords defined.</P><P><P CLASS="SubSection"><A NAME="pgfId=230"></A>The $keyword construct</P><P><P CLASS="Body"><A NAME="pgfId=285"></A>The <B>$</B> character introducesa language construct that enables development of user-defined tasks andfunctions. A name following the <B>$</B> is interpreted as a <I>system task</I>or a <I>system function</I> .</P><P><P CLASS="Body"><A NAME="pgfId=244"></A>The syntax for a system taskor function is as follows:</P><PRE><A NAME="pgfId=5373"></A> system_task_or_function ::=<A NAME="pgfId=5375"></A> <B>$</B><I>system_task_</I>identifier<B> </B>[<B> (</B> list_of_arguments <B>)</B> ] <B>;</B><A NAME="pgfId=5377"></A> | <B>$</B><I>system_function_</I>identifier<B> </B>[<B> (</B> list_of_arguments <B>)</B> ] <B>;</B><A NAME="pgfId=5379"></A> list_of_arguments ::=<A NAME="pgfId=5381"></A> argument { <B>,</B> [ argument ] }<A NAME="pgfId=5383"></A> argument ::=<A NAME="pgfId=5385"></A> expression<A NAME="pgfId=287"></A></PRE><P><P CLASS="BNFCapBody">Syntax 2-2: Syntax for system tasks and functions</P><P><P CLASS="Body"><A NAME="pgfId=174"></A>The $identifier system task orfunction may be defined in three places:</P><UL> <LI><A NAME="pgfId=183"></A>A standard set of $identifier system tasks and functions as defined in Section 14. <LI><A NAME="pgfId=187"></A>Additional $identifier system tasks and functions defined using the Programming Language Interface, as described in Section 17. <LI><A NAME="pgfId=248"></A>Additional $identifier system tasks and functions defined by software implementations.</UL><P><P CLASS="Body"><A NAME="pgfId=254"></A>Any valid identifier, includingkeywords already in use in contexts other than this construct can be usedas a system task or function name. The system tasks and functions describedin Section 14 are part of the IEEE 1364 Verilog HDL standard. Additionalsystem tasks and functions with the $identifier construct are not part ofthe Verilog HDL standard.</P><P><P CLASS="Body"><A NAME="pgfId=288"></A>Examples:</P><PRE><A NAME="pgfId=289"></A> <B>$display</B> ("display a message");<B>$finish</B>;</PRE><P><P CLASS="SubSection"><A NAME="pgfId=290"></A>The `keyword construct</P><P><P CLASS="Body"><A NAME="pgfId=292"></A>The <B>`</B> character (the ASCIIvalue 60, called open quote or accent grave) introduces a language constructused to implement compiler directives. The compiler behavior dictated bya compiler directive shall take effect as soon as the compiler reads thedirective. The directive shall remain in effect for the rest of the compilationunless a different compiler directive specifies otherwise. A compiler directivein one description file can therefore control compilation behavior in multipledescription files.</P><P><P CLASS="Body"><A NAME="pgfId=179"></A>The `identifier compiler directiveconstruct may be defined in two places:</P><UL> <LI><A NAME="pgfId=186"></A>A standard set of `identifier compiler directives defined in Section 16. <LI><A NAME="pgfId=241"></A>Additional `identifier compiler directives defined by software implementations.</UL><P><P CLASS="Body"><A NAME="pgfId=250"></A>Any valid identifier, includingkeywords already in use in contexts other than this construct can be usedas a compiler directive name. The compiler directives described in Section16 are part of the IEEE 1364 Verilog HDL standard. Additional compiler directiveswith the `identifier construct are not part of theVerilog HDL standard.</P><P><P CLASS="Body"><A NAME="pgfId=293"></A>Example:</P><PRE><A NAME="pgfId=294"></A> <B>`define</B> wordsize 8</PRE><P><P CLASS="Body"><A NAME="pgfId=305"></A> </P><P><HR ALIGN=LEFT></P><P><A HREF="ch02.htm">Chapter start</A> <A HREF="ch02.6.htm">Previous page</A></BODY></HTML>⌨️ 快捷键说明
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