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
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28-04.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="28-03.html">Previous</a></td>
<td>
<a href="28-03.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="28-05.html">Next</a></td>
<td>
<a href="28-05.html">Next</a>
</td>
</tr>
</table>
</center>
<p><br /></p>
<p><b>LISTING 28.2 L28-2.ASM</b></p>
<pre>
; Program to illustrate use of read mode 1 (color compare mode)
@ -209,16 +212,20 @@ end Start
<p>Why? Well, as I&rsquo;ve said, when all planes are &ldquo;don&rsquo;t care&rdquo; planes, read mode 1 reads always return 0FFH. Now, when you AND any value with 0FFH, the value remains unchanged, and that can be awfully handy when you&rsquo;re using the bit mask to modify selected pixels in VGA memory. Recall that you must always read VGA memory to load the latches before writing to VGA memory when you&rsquo;re using the bit mask. Traditionally, two separate instructions&mdash;a read followed by a write&mdash;are used to perform this task. The code in Listing 28.2 uses this approach. Suppose, however, that you&rsquo;ve set the VGA to read mode 1, with the Color Don&rsquo;t Care register set to 0 (meaning all reads of VGA memory will return 0FFH). Under these circumstances, you can use a single <b>AND</b> instruction to both read and write VGA memory, since ANDing any value with 0FFH leaves that value unchanged.</p>
<p><br /></p>
<center>
<table border="1">
<tr>
<td><a href="28-03.html">Previous</a></td>
<td>
<a href="28-03.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="28-05.html">Next</a></td>
<td>
<a href="28-05.html">Next</a>
</td>
</tr>
</table>
</center>