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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN"><!-- This collection of hypertext pages is Copyright 1995-2005 by Steve Summit. --><!-- Content from the book "C Programming FAQs: Frequently Asked Questions" --><!-- (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by --><!-- permission of the author and the publisher as a service to the community. --><!-- It is intended to complement the use of the published text --><!-- and is protected by international copyright laws. --><!-- The on-line content may be accessed freely for personal use --><!-- but may not be published or retransmitted without explicit permission. --><!-- --><!-- this page built Sat Dec 24 21:47:47 2005 by faqproc version 2.7 --><!-- from source file misctriv.sgml dated Sat Jul 3 17:26:19 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/misc/duff.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:05 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 20.35</title><link href="quine.html" rev=precedes><link href="ioccc.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="quine.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="ioccc.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../index-2.html"><img src="../images/buttontop.gif" alt="top/contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><hr><p><!-- qbegin --><h1>comp.lang.c FAQ list<font color=blue>·</font><!-- qtag -->Question 20.35</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>What is ``Duff's Device''?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>It'sa devastatinglydevious way ofunrolling a loop,devised by Tom Duff while he was at Lucasfilm.In its ``classic''form,it was used to copy bytes,andlookedlike this:<pre> register n = (count + 7) / 8; /* count > 0 assumed */ switch (count % 8) { case 0: do { *to = *from++; case 7: *to = *from++; case 6: *to = *from++; case 5: *to = *from++; case 4: *to = *from++; case 3: *to = *from++; case 2: *to = *from++; case 1: *to = *from++; } while (--n > 0); }</pre>where <TT>count</TT> bytesare to be copiedfrom the array pointed to by <TT>from</TT>to the memory location pointed to by <TT>to</TT>(which is a memory-mapped device output register,which is why <TT>to</TT> isn't incremented).It solves the problem of handling the leftover bytes(when <TT>count</TT> isn't a multiple of 8)by interleaving a<TT>switch</TT> statementwith the loopwhich copies bytes 8 at a time.(Believe it or not, it <em>is</em>legalto have <TT>case</TT> labels buriedwithin blocksnested in a <TT>switch</TT> statementlike this.In his announcement of the technique to C's developers and the world,Duff noted that C's <TT>switch</TT> syntax,in particular its``fall through''behavior,had long been controversial,and that``This code forms some sort of argument in that debate,but I'm not sure whether it's for or against.'')</p><p>Additional links:<a href="duffexpln.html">longer explanation</a></p><!-- aend --><p><hr><a href="quine.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="ioccc.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../questions.html"><img src="../images/buttontop.gif" alt="contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><br><!-- lastfooter --><a href="../about.html">about this FAQ list</a> <a href="../eskimo.html">about eskimo</a> <a href="../search.html">search</a> <a href="../feedback.html">feedback</a> <a href="copyright.html">copyright</a><p>Hosted by<a href="http://www.eskimo.com/"><img src="../../www.eskimo.com/img/link/eskitiny.gif" alt="Eskimo North"></a></body><!-- Mirrored from c-faq.com/misc/duff.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:06 GMT --></html>⌨️ 快捷键说明
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