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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Duff's device</title><link rel="stylesheet" href="../../jargon.css" type="text/css"/><meta name="generator" content="DocBook XSL Stylesheets V1.61.0"/><link rel="home" href="../index.html" title="The Jargon File"/><link rel="up" href="../D.html" title="D"/><link rel="previous" href="dub-dub-dub.html" title="dub dub dub"/><link rel="next" href="dumb-terminal.html" title="dumb terminal"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Duff's device</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="dub-dub-dub.html">Prev</a> </td><th width="60%" align="center">D</th><td width="20%" align="right"> <a accesskey="n" href="dumb-terminal.html">Next</a></td></tr></table><hr/></div><dt><a id="Duffs-device"/><dt xmlns="" id="Duffs-device"><b>Duff's device</b>: <span xmlns="http://www.w3.org/1999/xhtml" class="grammar">n.</span></dt></dt><dd><p> The most dramatic use yet seen of
<a href="../F/fall-through.html"><i class="glossterm">fall through</i></a> in C, invented by Tom Duff when he was at Lucasfilm.
Trying to optimize all the instructions he could out of an inner loop that
copied data serially onto an output port, he decided to unroll it. He then
realized that the unrolled version could be implemented by
<span class="emphasis"><em>interlacing</em></span> the structures of a switch and a
loop:</p><table border="0" bgcolor="#E0E0E0"><tr><td><pre class="programlisting">
register n = (count + 7) / 8; /* count &gt; 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 &gt; 0);
}
</pre></td></tr></table><p>Shocking though it appears to all who encounter it for the first
time, the device is actually perfectly valid, legal C. C's default
<a href="../F/fall-through.html"><i class="glossterm">fall through</i></a> in case statements has long been its
most controversial single feature; Duff observed that &#8220;<span class="quote">This code
forms some sort of argument in that debate, but I'm not sure whether it's
for or against.</span>&#8221; Duff has discussed the device in detail at <a href="http://www.lysator.liu.se/c/duffs-device.html" target="_top">http://www.lysator.liu.se/c/duffs-device.html</a>.
Note that the omission of postfix <b class="command">++</b> from
<b class="command">*to</b> was intentional (though confusing).
Duff's device can be used to implement memory copy, but the original aim
was to copy values serially into a magic IO register.</p><p>[For maximal obscurity, the outermost pair of braces above could
actually be removed &#8212; GLS]</p></dd><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="dub-dub-dub.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="../D.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="dumb-terminal.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">dub dub dub </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> dumb terminal</td></tr></table></div></body></html>