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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML EXPERIMENTAL 970324//EN"><HTML><HEAD><META NAME="GENERATOR" CONTENT="Adobe FrameMaker 5.5/HTML Export Filter"><LINK REL="STYLESHEET" HREF="ch16.css"><TITLE> 16.4 `ifdef, `else, `endif</TITLE></HEAD><BODY BGCOLOR="#ffffff"><DIV><HR><P><A HREF="ch16.htm">Chapter start</A> <A HREF="ch16.3.htm">Previous page</A> <A HREF="ch16.5.htm">Next page</A></P></DIV><H1 CLASS="Section"><A NAME="pgfId=144"> </A>16.4 <A NAME="80302"> </A>`ifdef, `else, `endif</H1><P CLASS="Body"><A NAME="pgfId=156"> </A>These conditional compilation compiler directives are used to optionally include lines of a Verilog HDL source description during compilation. The <B CLASS="Keyword">`ifdef</B> compiler directive checks for the definition of a variable name. If the variable name is defined then the lines following the <B CLASS="Keyword">`ifdef</B> directive are included. If the variable name is not defined and an <B CLASS="Keyword">`else</B> directive exists then this source is compiled.</P><P CLASS="Body"><A NAME="pgfId=204"> </A>These directives may appear anywhere in the source description.</P><P CLASS="Body"><A NAME="pgfId=205"> </A>Situations where the <B CLASS="Keyword">`ifdef</B>, <B CLASS="Keyword">`else</B>, and <B CLASS="Keyword">`endif</B> compiler directives may be useful include:</P><UL><LI CLASS="DashedList"><A NAME="pgfId=206"> </A>selecting different representations of a module such as behavioral, structural, or switch level</LI><LI CLASS="DashedList"><A NAME="pgfId=207"> </A>choosing different timing or structural information</LI><LI CLASS="DashedList"><A NAME="pgfId=208"> </A>selecting different stimulus for a given run of a tool</LI></UL><P CLASS="Body"><A NAME="pgfId=209"> </A>The <B CLASS="Keyword">`ifdef</B>, <B CLASS="Keyword">`else</B>, and <B CLASS="Keyword">`endif</B> compiler directives have the following syntax:</P><PRE CLASS="BNFDef"><A NAME="pgfId=210"> </A><CODE CLASS="code"> </CODE><B CLASS="Keyword">`ifdef</B><CODE CLASS="code"> text_macro_name first_group_of_lines</CODE>[<B CLASS="Keyword"> `else</B><CODE CLASS="code">second_group_of_lines</CODE><B CLASS="Keyword">`endif</B> ] </PRE><P CLASS="Body"><A NAME="pgfId=213"> </A>The text macro name is a Verilog HDL identifier. The first group of lines and the second group of lines are any parts of a Verilog HDL source description. The<B CLASS="Keyword"> `else</B> compiler directive and the second group of lines are optional. </P><P CLASS="Body"><A NAME="pgfId=214"> </A>The <B CLASS="Keyword">`ifdef</B>, <B CLASS="Keyword">`else</B>, and <B CLASS="Keyword">`endif</B> compiler directives work in the following manner:</P><UL><LI CLASS="DashedList"><A NAME="pgfId=215"> </A>When an <B CLASS="Keyword">`ifdef</B> is encountered, the text macro name is tested to see if it is defined as a text macro name using <B CLASS="Keyword">`define</B> within the Verilog HDL source description.</LI><LI CLASS="DashedList"><A NAME="pgfId=216"> </A>If the text macro name is defined, the first group_of_lines is compiled as part of the description. If there is an <B CLASS="Keyword">`else</B> compiler directive, the second group of lines is ignored.</LI><LI CLASS="DashedList"><A NAME="pgfId=218"> </A>If the text macro name has not been defined, the first group of lines is ignored. If there is an <B CLASS="Keyword">`else</B> compiler directive the second group of lines is compiled.</LI></UL><P CLASS="Note"><A NAME="pgfId=211"> </A>NOTES</P><OL><P CLASS="NumberedNote1"><A NAME="pgfId=256"> </A>1) --Any group of lines that the compiler ignores still must follow the Verilog HDL lexical conventions for white space, comments, numbers, strings, identifiers, keywords, and operators.</P><P CLASS="NumberedNote2"><A NAME="pgfId=212"> </A>2) --These compiler directives<CODE CLASS="code"> </CODE>may be nested.</P></OL><P CLASS="Body"><A NAME="pgfId=220"> </A>Examples:</P><P CLASS="Body"><A NAME="pgfId=303"> </A>1. The example below shows a simple usage of `ifdef directive for conditional copilation. If the identifier <CODE CLASS="code">behavioral</CODE> is defined, a continuous net assignment will be compiled in, else an <B CLASS="Keyword">and</B> gate will be instantiated.</P><P CLASS="Body"><A NAME="pgfId=236"> </A></P><DIV><IMG SRC="ch16-4.gif"></DIV><P CLASS="Body"><A NAME="pgfId=267"> </A>2. The following example shows usage of nested conditional compilation directives.</P><P CLASS="Body"><A NAME="pgfId=308"> </A></P><DIV><IMG SRC="ch16-5.gif"></DIV><HR><P><A HREF="ch16.htm">Chapter start</A> <A HREF="ch16.3.htm">Previous page</A> <A HREF="ch16.5.htm">Next page</A></P></BODY></HTML>?? 快捷鍵說明
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