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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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@ -1,5 +1,4 @@
<!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">
<head>
@ -19,18 +18,22 @@
<center>
<table border="1">
<tr>
<td><a href="09-06.html">Previous</a></td>
<td>
<a href="09-06.html">Previous</a>
</td>
<td><a href="index.html">Table of Contents</a></td>
<td>
<a href="index.html">Table of Contents</a>
</td>
<td><a href="10-01.html">Next</a></td>
<td>
<a href="10-01.html">Next</a>
</td>
</tr>
</table>
</center>
<p><br /></p>
<h4 align="left" id="Heading10">Hard-Core Cycle Counting</h4>
<h4 id="Heading10">Hard-Core Cycle Counting</h4>
<p>Next, we come to an item that cycle counters will love, especially since it involves apparently incorrect documentation on Intel&rsquo;s part. According to Intel&rsquo;s documents, all <b>RCR</b> and <b>RCL</b> instructions, which perform rotations through the Carry flag, as shown in Figure 9.4, take 9 cycles on the 386 when working with a register operand. My measurements indicate that the 9-cycle execution time almost holds true for <i>multibit</i> rotate-through-carries, which I&rsquo;ve timed at 8 cycles apiece; for example, <b>RCR AX,CL</b> takes 8 cycles on <i>my</i> 386, as does <b>RCL DX,2</b>. Contrast that with <b>ROR</b> and <b>ROL</b>, which can rotate the contents of a register any number of bits in just 3 cycles.</p>
@ -43,7 +46,7 @@
<p>No great lesson here, just a caution to be leery of multibit <b>RCR</b> and <b>RCL</b> when performance matters&mdash;and to take cycle-time documentation with a grain of salt.</p>
<h4 align="left" id="Heading11">Hardwired Far Jumps</h4>
<h4 id="Heading11">Hardwired Far Jumps</h4>
<p>Did you ever wonder how to code a far jump to an absolute address in assembly language? Probably not, but if you ever do, you&rsquo;re going to be glad for this next item, because the obvious solution doesn&rsquo;t work. You might think all it would take to jump to, say, 1000:5 would be <b>JMP FAR PTR 1000:5</b>, but you&rsquo;d be wrong. That won&rsquo;t even assemble. You might then think to construct in memory a far pointer containing 1000:5, as in the following:</p>
<pre>
@ -84,7 +87,7 @@ start:
<p>If the obvious doesn&rsquo;t work (and it usually doesn&rsquo;t), just try everything you can think of, no matter how ridiculous, until you find something that does&mdash;a rule with plenty of history on its side.</p>
<h4 align="left" id="Heading12">Setting 32-Bit Registers: Time versus Space</h4>
<h4 id="Heading12">Setting 32-Bit Registers: Time versus Space</h4>
<p>To finish up this chapter, consider these two items. First, in 32-bit protected mode,</p>
<pre>
@ -111,16 +114,20 @@ move bx,-1
<p>Be warned, though, that I&rsquo;ve found <b>OR, AND, ADD</b>, and the like to be a cycle slower than <b>MOV</b> when working with immediate operands on the 386 under some circumstances, for reasons that thus far escape me. This just reinforces the first rule of optimization: Measure your code in action, and place not your trust in documented cycle times.</p>
<p><br /></p>
<center>
<table border="1">
<tr>
<td><a href="09-06.html">Previous</a></td>
<td>
<a href="09-06.html">Previous</a>
</td>
<td><a href="index.html">Table of Contents</a></td>
<td>
<a href="index.html">Table of Contents</a>
</td>
<td><a href="10-01.html">Next</a></td>
<td>
<a href="10-01.html">Next</a>
</td>
</tr>
</table>
</center>