Misc/abrash-black-book
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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
commit a96a114403
362 changed files with 8472 additions and 5040 deletions
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20-03.html
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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>
@ -19,17 +18,21 @@
<center>
<table border="1">
<tr>
<td><a href="20-02.html">Previous</a></td>
<td>
<a href="20-02.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="20-04.html">Next</a></td>
<td>
<a href="20-04.html">Next</a>
</td>
</tr>
</table>
</center>
<p><br /></p>
<h3 id="Heading5">Lockstep Execution</h3>
<p>You may wonder why anyone would bother breaking <b>ADD [MemVar],EAX</b> into three instructions, given that this instruction can go through either pipe with equal ease. The answer is that while the memory-accessing instructions other than <b>MOV, PUSH</b>, and <b>POP</b> listed in Table 20.1 (that is, <b>INC/DEC [<i>mem</i>], ADD/SUB/XOR/AND/OR/CMP/ADC/SBB <i>reg</i>,[<i>mem</i>]</b>, and <b>ADD/SUB/XOR/AND/OR/CMP/ADC/SBB [<i>mem</i>],<i>reg/immed</i></b>) can be paired, they do not provide the 100 percent overlap that we seek. If you look at Tables 20.1 and 20.2, you will see that instructions taking from 1 to 3 cycles can pair. However, any pair of instructions goes through the two pipes in lockstep. This means, for example, that if <b>ADD [EBX],EDX</b> is going through the U-pipe, and <b>INC EAX</b> is going through the V-pipe, the V-pipe will be idle for 2 of the 3 cycles that the U-pipe takes to execute its instruction, as shown in Figure 20.4. Out of the theoretical 6 cycles of work that can be done during this time, we actually get only 4 cycles of work, or 67 percent utilization. Even though these instructions pair, then, this sequence fails to make maximum use of the Pentium&rsquo;s horsepower.</p>
@ -82,16 +85,20 @@ add esi,[SourceSkip] ;U-pipe cycles 1 and 2
add edi,[DestinationSkip] ;V-pipe cycles 1 and 2
</pre>
<p><br /></p>
<center>
<table border="1">
<tr>
<td><a href="20-02.html">Previous</a></td>
<td>
<a href="20-02.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="20-04.html">Next</a></td>
<td>
<a href="20-04.html">Next</a>
</td>
</tr>
</table>
</center>