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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> 9.8 Block statements </TITLE></HEAD><BODY BGCOLOR="#ffffff"><DIV><HR><P><A HREF="ch09.htm">Chapter start</A> <A HREF="ch09.7.htm">Previous page</A> <A HREF="ch09.9.htm">Next page</A></P></DIV><H1 CLASS="Section"><A NAME="pgfId=747"> </A>9.8 <A NAME="16349"> </A><A NAME="marker=247"> </A>Block statements </H1><P CLASS="Body"><A NAME="pgfId=748"> </A>The <I CLASS="Emphasis">block statements</I><A NAME="marker=248"> </A> are a means of grouping two or more statements together so that they act syntactically like a single statement. There are two types of blocks in Verilog HDL:</P><UL><LI CLASS="DashedList"><A NAME="pgfId=609"> </A><I CLASS="Emphasis">sequential block</I>, also called <I CLASS="Emphasis">begin-end block</I></LI><LI CLASS="DashedList"><A NAME="pgfId=614"> </A><I CLASS="Emphasis">parallel block</I>, also call<A NAME="marker=52"> </A>ed <I CLASS="Emphasis">fork-join block</I><A NAME="marker=50"> </A></LI></UL><P CLASS="Body"><A NAME="pgfId=646"> </A>The sequential <A NAME="marker=249"> </A>block shall be delimited by the keywords <B CLASS="Keyword">begin</B> and <B CLASS="Keyword">end</B>. The procedural statements in sequential block shall be executed sequentially in the given order.</P><P CLASS="Body"><A NAME="pgfId=652"> </A>The parallel bl<A NAME="marker=140"> </A>ock shall be delimited by the keywords <B CLASS="Keyword">fork</B> and <B CLASS="Keyword">join</B>. The procedural statements in parallel block shall be executed concurrently.</P><P CLASS="SubSection"><A NAME="pgfId=749"> </A><A NAME="46319"> </A><A NAME="60444"> </A><A NAME="marker=252"> </A><A NAME="marker=253"> </A><A NAME="marker=254"> </A>Sequential blocks</P><P CLASS="Body"><A NAME="pgfId=750"> </A>A <I CLASS="Emphasis">sequential block</I> shall have the following characteristics:</P><UL><LI CLASS="DashedList"><A NAME="pgfId=751"> </A>statements shall be executed in sequence, one after another</LI><LI CLASS="DashedList"><A NAME="pgfId=752"> </A>delay values for each statement shall be treated relative to the simulation time of the execution of the previous statement</LI><LI CLASS="DashedList"><A NAME="pgfId=753"> </A>control shall pass out of the block after the last statement executes</LI></UL><P CLASS="Body"><A NAME="pgfId=754"> </A>The following is the formal <A NAME="marker=255"> </A>syntax for a <A NAME="marker=256"> </A>sequential block:</P><P CLASS="Body"><A NAME="pgfId=755"> </A></P><DIV><IMG SRC="ch09-39.gif"></DIV><P CLASS="BNFCapBody"><A NAME="pgfId=756"> </A>Syntax 9-13: Syntax for the sequential block</P><DIV><H2 CLASS="Example"><A NAME="pgfId=757"> </A></H2><P CLASS="Body"><A NAME="pgfId=394"> </A>1. A sequential block enables the following two assignments to have a deterministic result:<A NAME="marker=258"> </A></P><P CLASS="Body"><A NAME="pgfId=630"> </A></P><DIV><IMG SRC="ch09-40.gif"></DIV><P CLASS="Body"><A NAME="pgfId=758"> </A>The first assignment is performed and <CODE CLASS="code">areg</CODE> is updated before control passes to the second assignment.</P><P CLASS="Body"><A NAME="pgfId=654"> </A>2. Delay control can be used in a sequential block to separate the two assignments in time.</P><P CLASS="Body"><A NAME="pgfId=673"> </A></P><DIV><IMG SRC="ch09-41.gif"></DIV><P CLASS="Body"><A NAME="pgfId=759"> </A>3. The following example shows how the combination of the sequential block and delay control can be used to specify a <A NAME="marker=259"> </A>time-sequenced waveform.</P><P CLASS="Body"><A NAME="pgfId=760"> </A></P><DIV><IMG SRC="ch09-42.gif"></DIV><P CLASS="SubSection"><A NAME="pgfId=761"> </A><A NAME="47887"> </A><A NAME="53916"> </A>Parallel blocks</P><P CLASS="Body"><A NAME="pgfId=763"> </A>A <I CLASS="Emphasis">parallel block</I> shall have the following characteristics:</P><UL><LI CLASS="DashedList"><A NAME="pgfId=764"> </A>statements shall execute concurrently</LI><LI CLASS="DashedList"><A NAME="pgfId=765"> </A>delay values for each statement shall be considered relative to the simulation time of entering the block</LI><LI CLASS="DashedList"><A NAME="pgfId=766"> </A>delay control can be used to provide time-ordering for assignments</LI><LI CLASS="DashedList"><A NAME="pgfId=767"> </A>control shall pass out of the block when the last time-ordered statement executes</LI></UL><P CLASS="Body"><A NAME="pgfId=768"> </A><A HREF="ch09.8.htm#74596" CLASS="XRef">See : Syntax for the parallel block</A> gives the formal <A NAME="marker=266"> </A>syntax for a <A NAME="marker=267"> </A>parallel block.</P><P CLASS="Body"><A NAME="pgfId=769"> </A></P><DIV><IMG SRC="ch09-43.gif"></DIV><P CLASS="BNFCapBody"><A NAME="pgfId=770"> </A>Syntax 9-14<A NAME="74596"> </A>: Syntax for the parallel block</P><P CLASS="Body"><A NAME="pgfId=674"> </A>The <A NAME="marker=147"> </A>timing controls in a fork-join block do not have to be ordered sequentially in time. </P></DIV><DIV><H2 CLASS="Example"><A NAME="pgfId=771"> </A></H2><P CLASS="Body"><A NAME="pgfId=392"> </A>The following example codes the waveform description shown above by using a parallel block instead of a sequential block. The waveform produced on the register is exactly the same for both implementations.</P><P CLASS="Body"><A NAME="pgfId=772"> </A></P><DIV><IMG SRC="ch09-44.gif"></DIV><P CLASS="SubSection"><A NAME="pgfId=774"> </A><A NAME="marker=271"> </A><A NAME="marker=272"> </A>Block names</P><P CLASS="Body"><A NAME="pgfId=775"> </A>Both sequential and parallel blocks can be named by adding<B CLASS="Keyword">: </B>name_of_bloc<B CLASS="Keyword">k</B> after the keywords <B CLASS="Keyword">begin</B> or <B CLASS="Keyword">fork</B>. The naming of blocks serves several purposes:</P><UL><LI CLASS="DashedList"><A NAME="pgfId=776"> </A>It allows local registers to be declared for the block.</LI><LI CLASS="DashedList"><A NAME="pgfId=777"> </A>It allows the block to be referenced in statements such as the disable statement (<A HREF="/Humuhumu/Files/Prof_Smith/Academic/ASICs/Web/ASICs/HTML/Verilog/LRM/HTML/09/ch11.htm#74699" CLASS="XRef"></A>).</LI></UL><P CLASS="Body"><A NAME="pgfId=778"> </A><EM CLASS="--">All registers shall be static--that is, a unique location exists for all registers and leaving or entering blocks shall not affect the values stored in them.</EM> </P><P CLASS="Body"><A NAME="pgfId=779"> </A><EM CLASS="-">The block names give a means of uniquely identifying all registers at any simulation time. </EM><A NAME="marker=275"> </A><A NAME="marker=276"> </A></P><P CLASS="SubSection"><A NAME="pgfId=781"> </A><A NAME="marker=277"> </A>Start and finish times</P><P CLASS="Body"><A NAME="pgfId=782"> </A>Both sequential and procedural blocks have the notion of a start and finish time. For <A NAME="marker=278"> </A><A NAME="marker=279"> </A>sequential blocks, the <A NAME="marker=280"> </A>start time is when the first statement is executed, and the <A NAME="marker=281"> </A>finish time is when the last statement has been executed. For <A NAME="marker=282"> </A><A NAME="marker=283"> </A>parallel blocks, the <A NAME="marker=284"> </A>start time is the same for all the statements, and the <A NAME="marker=285"> </A>finish time is when the last time-ordered statement has been executed. </P><P CLASS="Body"><A NAME="pgfId=659"> </A>Sequential and parallel blocks can be embedded within each other allowing complex control structures to be expressed easily and with a high degree of structure. When <A NAME="marker=286"> </A>blocks are embedded within each other, the timing of when a block starts and finishes is important. Execution shall not continue to the statement following a block until the finish time for the block has been reached--that is, until the block has completely finished executing.</P></DIV><DIV><H2 CLASS="Example"><A NAME="pgfId=521"> </A></H2><P CLASS="Body"><A NAME="pgfId=525"> </A>1. The following example shows the statements from previous example written in the reverse order and still producing the same waveform.</P><P CLASS="Body"><A NAME="pgfId=783"> </A></P><DIV><IMG SRC="ch09-45.gif"></DIV><P CLASS="Body"><A NAME="pgfId=784"> </A>2. When an assignment is to be made after two separate events have occurred, known as the <I CLASS="Emphasis">joining of events</I>, a <CODE CLASS="code">fork-join</CODE> block can be useful.</P><P CLASS="Body"><A NAME="pgfId=785"> </A></P><DIV><IMG SRC="ch09-46.gif"></DIV><P CLASS="Body"><A NAME="pgfId=786"> </A>The two events can occur in any order (or even at the same simulation time) and the <CODE CLASS="code">fork-join</CODE> block will complete and the assignment will be made. In contrast to this, if the <CODE CLASS="code">fork-join</CODE> block was a <CODE CLASS="code">begin-end</CODE> block and the <CODE CLASS="code">Bevent</CODE> occurred before the <CODE CLASS="code">Aevent</CODE>, then the block would be waiting for the next <CODE CLASS="code">Bevent</CODE>.</P><P CLASS="Body"><A NAME="pgfId=787"> </A>3. This example shows two sequential blocks, each of which will execute when its controlling event occurs. Because the event controls are within a <CODE CLASS="code">fork-join</CODE> block, they execute in parallel and the sequential blocks can therefore also execute in parallel. <A NAME="marker=289"> </A></P><P CLASS="Body"><A NAME="pgfId=788"> </A></P><DIV><IMG SRC="ch09-47.gif"></DIV></DIV><HR><P><A HREF="ch09.htm">Chapter start</A> <A HREF="ch09.7.htm">Previous page</A> <A HREF="ch09.9.htm">Next page</A></P></BODY></HTML>⌨️ 快捷键说明
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