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<HTML><HEAD> <META NAME="GENERATOR" CONTENT="Adobe PageMill 2.0 Mac"> <LINK REL="STYLESHEET" HREF="ch03.css"> <TITLE> 3.2 Nets and registers</TITLE></HEAD><BODY BGCOLOR="#ffffff"><P><A NAME="pgfId=409"></A><HR ALIGN=LEFT></P><P><A HREF="ch03.htm">Chapter start</A> <A HREF="ch03.1.htm">Previous page</A> <A HREF="ch03.3.htm">Next page</A></P><H1>3.2 Nets and registers</H1><P><P CLASS="Body"><A NAME="pgfId=410"></A>There are two main groups ofdata types: the register data types and the net data types. These two groupsdiffer in the way that they are assigned and hold values. They also representdifferent hardware structures.</P><P><P CLASS="SubSection"><A NAME="pgfId=412"></A>Nets</P><P><P CLASS="Body"><A NAME="pgfId=408"></A>The <I>net</I> data types shallrepresent physical connections between structural entities, such as gates.A net shall not store a value (except for the trireg net). Instead, itsvalue shall be determined by the value of its driver, such as a continuousassignment or a gate. See Section 6 and Section 7 for definitions of theseconstructs. If no driver is connected to a net, its value shall be high-impedance(<CODE> z</CODE> )--unless the net is a trireg, in which case, it shallhold the previously driven value.</P><P><P CLASS="Body"><A NAME="pgfId=422"></A>The syntax for net declarationsis as follows:</P><PRE><A NAME="pgfId=1333"></A> net_declaration ::= net_type [ <B>vectored</B> | <B>scalared</B> ] [range] [delay3] list_of_net_identifiers <B>;</B> | <B>trireg</B> [ <B>vectored</B> | <B>scalared</B> ] [charge_strength] [range] [delay3] list_of_net_identifiers <B>;</B>| net_type [ <B>vectored</B> | <B>scalared</B> ] [drive_strength] [range] [delay3] list_of_net_decl_assignments <B>;</B><A NAME="pgfId=1334"></A> net_type ::= <B>wire</B> | <B>tri</B> | <B>tri1</B> | <B>supply0</B> | <B>wand</B> | <B>triand</B> | <B>tri0</B> | <B>supply1</B> | <B>wor</B> | <B>trior</B><A NAME="pgfId=1335"></A> range ::= <B>[</B> msb_constant_expression <B>:</B> lsb_constant_expression <B>]</B><A NAME="pgfId=1336"></A> drive_strength ::= <B> (</B> strength0 <B>,</B> strength1 <B>)</B> | <B>(</B> strength1 <B>,</B> strength0 <B>)</B>| <B>(</B> strength0 <B>,</B> <B>highz1</B> <B>)</B>| <B>(</B> strength1 <B>,</B> <B>highz0</B> <B>)</B>| <B>(</B> <B>highz1</B> <B>,</B> strength0 <B>)</B>| <B>(</B> <B>highz0</B> <B>,</B> strength1 <B>)</B><A NAME="pgfId=1337"></A> strength0 ::= <B>supply0</B> | <B>strong0</B> | <B>pull0</B> | <B>weak0</B><A NAME="pgfId=1338"></A> strength1 ::= <B>supply1</B> | <B>strong1</B> | <B>pull1</B> | <B>weak1</B><A NAME="pgfId=1339"></A> charge_strength ::= <B>( small </B>) | <B>( medium )</B> | <B>( large )</B><A NAME="pgfId=1340"></A> delay3 ::= <B>#</B> delay_value |<B> # (</B> delay_value [ <B>,</B> delay_value [ <B>,</B> delay_value ] ] <B>)</B><A NAME="pgfId=1341"></A> delay_value ::= unsigned_number | <I>parameter</I>_identifier | constant_mintypmax_expression<A NAME="pgfId=1342"></A> list_of_net_decl_assignments ::= net_decl_assignment { <B>,</B> net_decl_assignment }<A NAME="pgfId=1343"></A> net_decl_assignment ::= net_identifier <B>=</B> expression<A NAME="pgfId=1344"></A> list_of_net_identifiers ::= net_identifier { <B>,</B> net_identifier }</PRE><P><P CLASS="BNFCapBody"><A NAME="pgfId=370"></A>Syntax 3-1: Syntaxfor net declaration</P><P><P CLASS="Body"><A NAME="pgfId=625"></A>The first two forms of net declarationare described in this section. The third form, called net assignment, isdescribed in Section 6.</P><P><P CLASS="SubSection"><A NAME="pgfId=416"></A>Registers</P><P><P CLASS="Body"><A NAME="pgfId=415"></A>A <I>register</I> is an abstractionof a data storage element. The keyword for the register data type is <B>reg</B>.A register shall store a value from one assignment to the next. An assignmentstatement in a procedure acts as a trigger that changes the value in thedata storage element. The default initialization value for a <B>reg</B>data type shall be the unknown value, <CODE>x</CODE> .</P><P><P CLASS="Body"><A NAME="pgfId=575"></A>The syntax for reg declarationsis as follows:</P><PRE><A NAME="pgfId=1352"></A> reg_declaration ::= <B>reg</B> [range] list_of_register_identifiers <B>;</B><A NAME="pgfId=1353"></A> time_declaration ::= <B>time</B> list_of_register_identifiers <B>;</B><A NAME="pgfId=1354"></A> integer_declaration ::= <B>integer</B> list_of_register_identifiers <B>;</B><A NAME="pgfId=1355"></A> real_declaration ::= <B>real</B> real_identifier { <B>,</B> real_identifier } <B>;</B><A NAME="pgfId=1356"></A> realtime_declaration ::= <B>realtime</B> real_identifier { <B>,</B> real_identifier } <B>;</B><A NAME="pgfId=1357"></A> list_of_register_identifiers ::= register_name { <B>,</B> register_name }<A NAME="pgfId=1358"></A> register_name ::= register_identifier | memory_identifier <B>[</B> upper_limit_constant_expression <B>:</B> lower_limit_constant_expression <B>]</B></PRE><P><P CLASS="BNFCapBody"><A NAME="pgfId=411"></A>Syntax 3-2: Syntaxfor reg declaration</P><P><P CLASS="Body"><A NAME="pgfId=414"></A>If a set of nets or registersshare the same characteristics, they can be declared in the same declarationstatement.</P><P><P CLASS="Caution"><A NAME="pgfId=419"></A>CAUTION</P><P><P CLASS="CautionBody"><A NAME="pgfId=420"></A>Registers can be assignedvalue shall be treated as an unsigned (positive) value. For example, a minusone (-1) in a four-bit register shall function as the number 15 if the registeris an expression operand. See 4.1.3 for more information on Numeric Conventionsin Expressions.</P><P><HR ALIGN=LEFT></P><P><A HREF="ch03.htm">Chapter start</A> <A HREF="ch03.1.htm">Previous page</A> <A HREF="ch03.3.htm">Next page</A></BODY></HTML>?? 快捷鍵說(shuō)明
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