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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="07-04.html">Previous</a></td>
<td>
<a href="07-04.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="08-01.html">Next</a></td>
<td>
<a href="08-01.html">Next</a>
</td>
</tr>
</table>
</center>
<p><br /></p>
<h4 align="left" id="Heading8">Rotating and Shifting with Tables</h4>
<h4 id="Heading8">Rotating and Shifting with Tables</h4>
<p>As another example of local optimization, consider the matter of rotating or shifting a mask into position. First, let&rsquo;s look at the simple task of setting bit N of AX to 1.</p>
@ -80,7 +83,7 @@ BIT_PATTERN=BIT_PATTERN SHL 1
</tr>
</table>
<h4 align="left" id="Heading9">NOT Flips Bits&mdash;Not Flags</h4>
<h4 id="Heading9">NOT Flips Bits&mdash;Not Flags</h4>
<p>The <b>NOT</b> instruction flips all the bits in the operand, from 0 to 1 or from 1 to 0. That&rsquo;s as simple as could be, but <b>NOT</b> nonetheless has a minor but interesting talent: It doesn&rsquo;t affect the flags. That can be irritating; I once spent a good hour tracking down a bug caused by my unconscious assumption that <b>NOT</b> does set the flags. After all, every other arithmetic and logical instruction sets the flags; why not <b>NOT</b>? Probably because <b>NOT</b> isn&rsquo;t considered to be an arithmetic or logical instruction at all; rather, it&rsquo;s a data manipulation instruction, like <b>MOV</b> and the various rotates. (These are <b>RCR, RCL, ROR,</b> and <b>ROL,</b> which affect only the Carry and Overflow flags.) NOT is often used for tasks, such as flipping masks, where there&rsquo;s no reason to test the state of the result, and in that context it can be handy to keep the flags unmodified for later testing.</p>
@ -94,7 +97,7 @@ BIT_PATTERN=BIT_PATTERN SHL 1
<p>The x86 instruction set offers many ways to accomplish almost any task. Understanding the subtle distinctions between the instructions&mdash;whether and which flags are set, for example&mdash;can be critical when you&rsquo;re trying to optimize a code sequence and you&rsquo;re running out of registers, or when you&rsquo;re trying to minimize branching.</p>
<h4 align="left" id="Heading10">Incrementing with and without Carry</h4>
<h4 id="Heading10">Incrementing with and without Carry</h4>
<p>Another case in which there are two slightly different ways to perform a task involves adding 1 to an operand. You can do this with <b>INC,</b> as in <b>INC AX,</b> or you can do it with <b>ADD,</b> as in <b>ADD AX,1.</b> What&rsquo;s the difference? The obvious difference is that <b>INC</b> is usually a byte or two shorter (the exception being <b>ADD AL,1,</b> which at two bytes is the same length as <b>INC AL</b>), and is faster on some processors. Less obvious, but no less important, is that <b>ADD</b> sets the Carry flag while <b>INC</b> leaves the Carry flag untouched.</p>
@ -158,16 +161,20 @@ ADC DX,0
<p>As always, pay attention!</p>
<p><br /></p>
<center>
<table border="1">
<tr>
<td><a href="07-04.html">Previous</a></td>
<td>
<a href="07-04.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="08-01.html">Next</a></td>
<td>
<a href="08-01.html">Next</a>
</td>
</tr>
</table>
</center>