ch01.3.htm

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<HTML><HEAD>  <META NAME="GENERATOR" CONTENT="Adobe PageMill 2.0 Mac">  <TITLE>1.3 Syntactic description</TITLE></HEAD><BODY><P><A NAME="anchor640956"></A><HR ALIGN=LEFT></P><P><A HREF="ch01.htm">Chapter&nbsp;&nbsp;start</A>&nbsp;&nbsp;&nbsp;<A HREF="ch01.2.htm">Previous&nbsp;&nbsp;page</A>&nbsp;&nbsp;<A HREF="ch01.4.htm">Next&nbsp;&nbsp;page</A></P><H2>1.3 Syntactic description</H2><P><P CLASS="Body"><A NAME="pgfId=72"></A>The formal syntax of Verilog HDLis described using Backus-Naur Form (BNF). The following conventions areused:</P><OL>  <P><P CLASS="NumberedLista"><A NAME="pgfId=84"></A>a) Lower case words,  some containing embedded underscores, are used to denote syntactic categories,  for example:</OL><PRE><A NAME="pgfId=85"></A> module_declaration</PRE><OL>  <P><P CLASS="NumberedListb"><A NAME="pgfId=86"></A>b) Bold face words are  used to denote reserved keywords, operators and punctuation marks as required  part of the syntax. These words appear in larger font for distinction.  For example:</OL><PRE><A NAME="pgfId=87"></A> <B>module</B>      <B>=&gt;</B>       <B>;      </B></PRE><OL>  <P><P CLASS="NumberedListb"><A NAME="pgfId=67"></A>c) A vertical bar separates  alternative items unless it appears in bold face, in which case it stands  for itself. For example:</OL><PRE><A NAME="pgfId=68"></A> port_expression ::=  &nbsp;port_reference | <B>{</B> port_reference  {<B> ,</B> port_reference } <B>}</B><A NAME="pgfId=73"></A> unary_operator ::=    <B>+ </B>| <B> -  </B>| <B> ! </B>| <B> ~ </B>| <B> &amp; </B>| <B>  ~&amp; </B>| <B>  |  </B>| <B> ~|  </B>| <B> ^  </B>| <B> ~^ </B>| <B> ^~</B></PRE><OL>  <P><P CLASS="NumberedListb"><A NAME="pgfId=90"></A>d) Square brackets enclose  optional items. For example:</OL><PRE><A NAME="pgfId=91"></A> input_declaration ::= <B>input</B> [range]  list_of_variables <EM>;</EM></PRE><OL>  <P><P CLASS="NumberedListb"><A NAME="pgfId=92"></A>e) Braces enclose a  repeated item. The item may appear zero or more times; the repetitions  occur from left to right as with an equivalent left-recursive rule. Thus,  the following two rules are equivalent:</OL><PRE><A NAME="pgfId=111"></A> list_of_param_assignments ::= param_assignment<B>  </B>{ <B>,</B><EM> </EM>param_assignment }<A NAME="pgfId=117"></A> list_of_param_assignments ::=  &nbsp;param_assignment| param_assignment <B>,</B> list_of_param_assignment</PRE><OL>  <P><P CLASS="NumberedListb"><A NAME="pgfId=103"></A>f) If the name of any  category starts with an italicized part, it is equivalent to the category  name without the italicized part. The italicized part is intended to convey  some semantic information. For example, <I>msb_</I> constant_expression  and <I>lsb_</I> constant_expression are equivalent to constant_expression.</OL><P><P CLASS="Body"><A NAME="pgfId=80"></A>The main text uses <I>italicized</I>font when a term is being defined, and <CODE>constant-width</CODE> fontfor examples, file names, and while referring to constants, especially <CODE>0</CODE>, <CODE>1</CODE> , <CODE>x</CODE> , and <CODE>z</CODE> values.</P><P><A HREF="ch01.htm">Chapter&nbsp;&nbsp;start</A>&nbsp;&nbsp;&nbsp;<A HREF="ch01.2.htm">Previous&nbsp;&nbsp;page</A>&nbsp;&nbsp;<A HREF="ch01.4.htm">Next&nbsp;&nbsp;page</A></BODY></HTML>

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