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<HTML><HEAD> <META NAME="GENERATOR" CONTENT="Adobe PageMill 2.0 Mac"> <LINK REL="STYLESHEET" HREF="ch03.css"> <TITLE> 3.8 Memories </TITLE></HEAD><BODY BGCOLOR="#ffffff"><P><A NAME="pgfId=536"></A><HR ALIGN=LEFT></P><P><A HREF="ch03.htm">Chapter start</A> <A HREF="ch03.7.htm">Previous page</A> <A HREF="ch03.9.htm">Next page</A></P><H1>3.8 Memories</H1><P><P CLASS="Body"><A NAME="pgfId=538"></A>An array of registers can beused to model read-only memories (ROMs), random access memories (RAMs),and register files. Each register in the array is known as an element orword and is addressed by a single array index. There shall be no arrayswith multiple dimensions.</P><P><P CLASS="Body"><A NAME="pgfId=596"></A>Memories shall be declared inregister declaration statements by specifying the element address rangeafter the declared identifier. See 3.2.2. The expressions that specify theindices of the array shall be constant expressions. The value of the constantexpression can be a positive integer, a negative integer, or zero.</P><P><P CLASS="Body"><A NAME="pgfId=373"></A>One declaration statement canbe used for declaring both registers and memories This makes it convenientto declare both a memory and some registers that will hold data to be readfrom and written to the memory in the same declaration statement.</P><P><P CLASS="Body"><A NAME="pgfId=549"></A>An n-bit register can be assigneda value in a single assignment, but a complete memory cannot. To assigna value to a memory element, an index shall be specified. The index canbe an expression. This option provides a mechanism to reference differentmemory elements, depending on the value of other registers and nets in thecircuit. For example, a program counter register could be used to indexinto a RAM.</P><P><P CLASS="Body"><A NAME="pgfId=637"></A>Examples:</P><P><P CLASS="Body"><A NAME="pgfId=544"></A>1. Memory declaration:</P><PRE><A NAME="pgfId=1369"></A> <B>reg</B> [7:0] mema[0:255]; // declares a memory <CODE>mema</CODE> of 256 eight-bit// registers. The indices are <CODE>0</CODE> to <CODE>255</CODE><B>parameter</B> // parameters are run-time constants - see 3.10wordsize = 16,memsize = 256;// Declare 256 words of 16-bit memory plus two regs<B>reg</B> [wordsize-1:0] writereg, // equivalent to [15:0]readreg,mem [memsize-1:0];// equivalent to [255:0]</PRE><P><P CLASS="Body"><A NAME="pgfId=638"></A>2. A memory of <CODE>n</CODE>1-bit registers is different from an <CODE>n</CODE> -bit vector register</P><PRE><A NAME="pgfId=1381"></A> <B>reg</B> [1:n] rega; // An n-bit register is not the same<B>reg</B> mema [1:n]; // as a memory of n 1-bit registers</PRE><P><P CLASS="Body"><A NAME="pgfId=510"></A>3. Assignment to memory elements</P><PRE><A NAME="pgfId=1391"></A> rega = 0; // Legal Syntaxmema = 0; // Illegal Syntaxmema[1] = 0; // Assigns 0 to the first element of mema</PRE><P><P CLASS="Note"><A NAME="pgfId=636"></A>NOTE--Implementations may limitthe maximum size of a register array, but will at least be 16777216 (224).</P><P><HR ALIGN=LEFT></P><P><A HREF="ch03.htm">Chapter start</A> <A HREF="ch03.7.htm">Previous page</A> <A HREF="ch03.9.htm">Next page</A></BODY></HTML>⌨️ 快捷键说明
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