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<HTML><HEAD> <META NAME="GENERATOR" CONTENT="Adobe PageMill 2.0 Mac"> <LINK REL="STYLESHEET" HREF="ch03.css"> <TITLE> 3.3 Vectors </TITLE></HEAD><BODY BGCOLOR="#ffffff"><P><A NAME="pgfId=434"></A><HR ALIGN=LEFT></P><P><A HREF="ch03.htm">Chapter start</A> <A HREF="ch03.2.htm">Previous page</A> <A HREF="ch03.4.htm">Next page</A></P><H1>3.3 Vectors</H1><P><P CLASS="Body"><A NAME="pgfId=436"></A>A net or reg declaration withouta range specification shall be considered one bit wide and is known as a<I>scalar</I> . Multiple bit net and reg data type shall be declared byspecifying a range, and is known as a <I>vector</I> .</P><P><P CLASS="SubSection"><A NAME="pgfId=438"></A>Specifying vectors</P><P><P CLASS="Body"><A NAME="pgfId=439"></A>The range specification givesaddresses to the individual bits in a multi-bit net or register. The mostsignificant bit specified by <I>msb</I> constant expression is the left-handvalue in the range and the least significant bit <I>lsb</I> constant expressionis the right-hand value in the range.</P><P><P CLASS="Body"><A NAME="pgfId=442"></A>Both msb constant expressionand lsb constant expression shall be constant expressions. The msb and lsbconstant expressions can be any value--positive, negative, or zero, andlsb constant expression can be a greater, equal, or lesser value than msbconstant expression.</P><P><P CLASS="Body"><A NAME="pgfId=444"></A>Vector nets and registers shallobey laws of arithmetic modulo 2 to the power n (2n), where n is the numberof bits in the vector. Vector nets and registers shall be treated as unsignedquantities.</P><P><P CLASS="Body"><A NAME="pgfId=429"></A>examples:</P><PRE><CODE> </CODE> <B>wand</B><CODE> w; // a scalar net of type 'wand' </CODE> <B>tri</B><CODE> [15:0] busa; // a tri-state 16-bit bus </CODE> <B>trireg</B><CODE> (</CODE> <B>small</B><CODE> ) storeit; // a charge storage node of strength small </CODE> <B>reg</B><CODE> a; // a scalar register </CODE> <B>reg</B><CODE> [3:0] v; // a 4-bit vector register made up of (from most to // least significant) v[3], v[2], v[1] and v[0] </CODE> <B>reg</B><CODE> [-1:4] b; // a 6-bit vector register </CODE> <B>wire</B><CODE> w1, w2; // declares 2 wires </CODE> <B>reg</B><CODE> [4:0] x, y, z; // declares 3 5-bit registers</CODE></PRE><P><P CLASS="Note"><A NAME="pgfId=400"></A>NOTES</P><OL> <P><P CLASS="NumberedNote1"><A NAME="pgfId=576"></A>1) --Implementations may set a limit on the maximum length of a vector, but will at least be 65536 (216) bits. <P><P CLASS="NumberedNote2"><A NAME="pgfId=443"></A>2) --Implementations do not have to detect overflow of integer operations.</OL><P><P CLASS="Body"><A NAME="pgfId=446"></A> </P><P><P CLASS="SubSection"><A NAME="pgfId=561"></A>Vector net accessibility</P><P><P CLASS="Body"><A NAME="pgfId=386"></A><I>Vectored</I> and <I>scalared</I>shall be optional advisory keywords to be used in vector net or reg declaration.If these keywords are implemented, certain operations on vectors may berestricted. If the keyword <B>vectored</B> is used, bit and part selectsand strength specifications may not be permitted, and the PLI may considerthe object <I>unexpanded</I> . If the keyword <B>scalared</B> is used, bitand part selects of the object shall be permitted and the PLI shall considerthe object <I>expanded</I> .</P><P><P CLASS="Body"><A NAME="pgfId=595"></A>Examples:</P><PRE><A NAME="pgfId=569"></A> <B>tri1</B> <B>scalared</B> [63:0] bus64; // a bus that will be expanded<B>tri</B> <B>vectored</B> [31:0] data; //a bus that will not be expanded</PRE><P><HR ALIGN=LEFT></P><P><A HREF="ch03.htm">Chapter start</A> <A HREF="ch03.2.htm">Previous page</A> <A HREF="ch03.4.htm">Next page</A></BODY></HTML>?? 快捷鍵說明
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