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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="ch05.css"><TITLE> 5.4 The verilog simulation reference model</TITLE></HEAD><BODY BGCOLOR="#ffffff"><DIV><HR><P><A HREF="ch05.htm">Chapter start</A> <A HREF="ch05.3.htm">Previous page</A> <A HREF="ch05.5.htm">Next page</A></P></DIV><H1 CLASS="Section"><A NAME="pgfId=131"> </A>5.4 The verilog simulation reference model</H1><P CLASS="Body"><A NAME="pgfId=132"> </A>In all the examples which follow, T refers to the current simulation time, and all events are held in the event queue, ordered by simulation time.</P><P CLASS="Body"><A NAME="pgfId=92"> </A><CODE CLASS="code"> while (there are events) {<BR> if (no active events) { <BR> if (there are inactive events) { <BR> activate all inactive events; <BR> } else if (there are non-blocking assign update events) {<BR> activate all non-blocking assign update events; <BR> } else if (there are monitor events) {<BR> activate all monitor events; <BR> } else { <BR> advance T to the next event time; <BR> activate all inactive events for time T;<BR> } <BR> }<BR> E = any active event;<BR> if (E is an update event) { <BR> update the modified object; <BR> add evaluation events for sensitive processes to event queue; <BR> } else { /* must be an evaluation event */ <BR> evaluate the process; <BR> add update events to the event queue; <BR> } <BR> }</CODE></P><P CLASS="SubSection"><A NAME="pgfId=135"> </A>Determinism</P><P CLASS="Body"><A NAME="pgfId=136"> </A>The Verilog simulation reference standard guarantees certain scheduling order.</P><OL><P CLASS="NumberedList1"><A NAME="pgfId=137"> </A>1) Statements within a <CODE CLASS="code">begin-end</CODE> block shall be executed in the order in which they appear in that <CODE CLASS="code">begin-end</CODE> block. Execution of statements in a particular <CODE CLASS="code">begin-end</CODE> block can be suspended in favor of other processes in the model; however, in no case shall the statements in a <CODE CLASS="code">begin-end</CODE> block be executed in any order other than that they appear in the source.</P><P CLASS="NumberedList2"><A NAME="pgfId=138"> </A>2) Non-blocking assignments shall be performed in the order the statements were executed. Consider the following example:<BR><BR><BR>When this block is executed, there will be two events added to the non-blocking assign update queue. The previous rule requires that they be entered on the queue in source order; this rule requires that they be taken from the queue and performed in source order as well. Hence at the end of time step <CODE CLASS="code">1</CODE>, the variable <CODE CLASS="code">a</CODE> will be assigned <CODE CLASS="code">0</CODE>, and then <CODE CLASS="code">1</CODE>.</P></OL><DIV><IMG SRC="ch05-1.gif"></DIV><P CLASS="SubSection"><A NAME="pgfId=89"> </A>Non-determinism</P><P CLASS="Body"><A NAME="pgfId=114"> </A>One source of non-determinism is the fact that active events can be taken off the queue and processed in any order. Another source of non-determinism is that statements without time-control constructs in behavioral blocks do not have to be executed as one event. Time control statements are the # expression and @ expression constructs (<A HREF="/Humuhumu/Files/Prof_Smith/Academic/ASICs/Web/ASICs/HTML/Verilog/LRM/HTML/05/ch09.htm#39185" CLASS="XRef"></A>). At any time while evaluating a behavioral statement, the simulator may suspend execution and place the partially completed event as a pending active event on the event queue. The effect of this is to allow the interleaving of process execution. Note that the order of interleaved execution is non-deterministic, and not under control of the user.</P><HR><P><A HREF="ch05.htm">Chapter start</A> <A HREF="ch05.3.htm">Previous page</A> <A HREF="ch05.5.htm">Next page</A></P></BODY></HTML>?? 快捷鍵說明
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