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Retidy, removing align=left from headings and empty paragraphs

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James Gregory 2013-12-30 15:37:13 +11:00
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
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<center>
<table border="1">
<tr>
<td><a href="09-04.html">Previous</a></td>
<td>
<a href="09-04.html">Previous</a>
</td>
<td><a href="index.html">Table of Contents</a></td>
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<p><br /></p>
<p>You&rsquo;ll notice that in Listing 9.2 I didn&rsquo;t use a table of character frequencies in English text to determine the character for which to scan, but rather let the caller make that choice. Each buffer of bytes has unique characteristics, and English-letter frequency could well be inappropriate. What if the buffer is filled with French text? Cyrillic? What if it isn&rsquo;t text that&rsquo;s being searched? It might be worthwhile for an application to build a dynamic frequency table for each buffer so that the best scan character could be chosen for each search. Or perhaps not, if the search isn&rsquo;t time-critical or the buffer is small.</p>
<p>The point is that you can improve performance dramatically by understanding the nature of the data with which you work. (This is equally true for high-level language programming, by the way.) Listing 9.2 is very similar to and only slightly more complex than Listing 9.1; the difference lies not in elbow grease or cycle counting but in the organic integrating optimizer technology we all carry around in our heads.</p>
<h4 align="left" id="Heading7">Short Sorts</h4>
<h4 id="Heading7">Short Sorts</h4>
<p>David Stafford (recently of Borland and Borland Japan) who happens to be one of the best assembly language programmers I&rsquo;ve ever met, has written a C-callable routine that sorts an array of integers in ascending order. That wouldn&rsquo;t be particularly noteworthy, except that David&rsquo;s routine, shown in Listing 9.4, is exactly <i>25 bytes</i> long. Look at the code; you&rsquo;ll keep saying to yourself, &ldquo;But this doesn&rsquo;t work...oh, yes, I guess it does.&rdquo; As they say in the Prego spaghetti sauce ads, <i>it&rsquo;s in there</i>&mdash;and what a job of packing. Anyway, David says that a 24-byte sort routine eludes him, and he&rsquo;d like to know if anyone can come up with one.</p>
@ -74,7 +77,7 @@ _sort: pop dx ;get return address (entry point)
end
</pre>
<h4 align="left" id="Heading8">Full 32-Bit Division</h4>
<h4 id="Heading8">Full 32-Bit Division</h4>
<p>One of the most annoying limitations of the x86 is that while the dividend operand to the <b>DIV</b> instruction can be 32 bits in size, both the divisor and the result must be 16 bits. That&rsquo;s particularly annoying in regards to the result because sometimes you just don&rsquo;t know whether the ratio of the dividend to the divisor is greater than 64K-1 or not&mdash;and if you guess wrong, you get that godawful Divide By Zero interrupt. So, what is one to do when the result might not fit in 16 bits, or when the dividend is larger than 32 bits? Fall back to a software division approach? That will work&mdash;but oh so slowly.</p>
@ -87,16 +90,20 @@ _sort: pop dx ;get return address (entry point)
<p>As for handling signed division with arbitrarily large dividends, that can be done easily enough by remembering the signs of the dividend and divisor, dividing the absolute value of the dividend by the absolute value of the divisor, and applying the stored signs to set the proper signs for the quotient and remainder. There may be more clever ways to produce the same result, by using <b>IDIV</b>, for example; if you know of one, drop me a line c/o Coriolis Group Books.</p>
<p><br /></p>
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<tr>
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<a href="09-04.html">Previous</a>
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</center>