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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="ch09.css"><TITLE> 9.1 Behavioral model overview </TITLE></HEAD><BODY BGCOLOR="#ffffff"><DIV><HR><P><A HREF="ch09.htm">Chapter start</A> <A HREF="ch09.htm">Previous page</A> <A HREF="ch09.2.htm">Next page</A></P></DIV><H1 CLASS="Section"><A NAME="pgfId=516"> </A>9.1 Behavioral model overview </H1><P CLASS="Body"><A NAME="pgfId=517"> </A>Verilog <I CLASS="Emphasis">behavioral models</I> contain <I CLASS="Emphasis">procedural statements</I><A NAME="marker=13"> </A> that control the simulation and manipulate variables of the data types previously described. These statements are contained within procedures. Each procedure has an activity flow associated with it.</P><P CLASS="Body"><A NAME="pgfId=518"> </A>The activity starts at the control constructs <B CLASS="Keyword">initial</B><A NAME="marker=14"> </A> and <B CLASS="Keyword">always</B><A NAME="marker=15"> </A>. Each <I CLASS="Emphasis">initial</I> construct and each <I CLASS="Emphasis">always</I> construct starts a separate activity flow. All of the activity flows are <A NAME="marker=16"> </A>concurrent to model the inherent concurrence of hardware. These constructs are formally described in <A HREF="ch09.9.htm#19982" CLASS="XRef">See Structured procedures</A>.</P><P CLASS="Body"><A NAME="pgfId=519"> </A>The following example shows a complete Verilog <A NAME="marker=19"> </A>behavioral model.</P><P CLASS="Body"><A NAME="pgfId=520"> </A></P><DIV><IMG SRC="ch09-1.gif"></DIV><P CLASS="Body"><A NAME="pgfId=522"> </A>During simulation of this model, all of the flows defined by the <A NAME="marker=22"> </A>initial and <A NAME="marker=23"> </A>always constructs start together at simulation time zero. The initial constructs execute once, and the always constructs execute repetitively.</P><P CLASS="Body"><A NAME="pgfId=523"> </A>In this model, the register variables <CODE CLASS="code">a</CODE> and b initialize to 1 and 0 respectively at simulation time zero. The initial construct is then complete and does not execute again during this simulation run. This initial construct contains a begin-end block (also called a <I CLASS="Emphasis">sequential block</I>) of statements. In this begin-end block <CODE CLASS="code">a</CODE> is initialized first, followed by <CODE CLASS="code">b</CODE>.</P><P CLASS="Body"><A NAME="pgfId=524"> </A>The always constructs also start at time zero, but the values of the variables do not change until the times specified by the delay controls (introduced by #) have elapsed. Thus, register <CODE CLASS="code">a</CODE> inverts after 50 time units, and register <CODE CLASS="code">b</CODE> inverts after 100 time units. Since the always constructs repeat, this model will produce two square waves. The register <CODE CLASS="code">a</CODE> toggles with a period of 100 time units, and register <CODE CLASS="code">b</CODE> toggles with a period of 200 time units. The two always constructs proceed concurrently throughout the entire simulation run.</P><HR><P><A HREF="ch09.htm">Chapter start</A> <A HREF="ch09.htm">Previous page</A> <A HREF="ch09.2.htm">Next page</A></P></BODY></HTML>?? 快捷鍵說明
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