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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27-05.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="27-04.html">Previous</a></td>
<td>
<a href="27-04.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="28-01.html">Next</a></td>
<td>
<a href="28-01.html">Next</a>
</td>
</tr>
</table>
</center>
<p><br /></p>
<p>Another interesting point about flipping from graphics to text and back is that the standard mode 3 character/attribute map doesn&rsquo;t actually take up every byte of the first 4000 bytes of planes 0 and 1. The standard mode 3 character/attribute map actually only takes up every even byte of the first 4000 in each plane; the odd bytes are left untouched. This means that only about 12K bytes actually have to be saved when going to text mode. The code in Listing 27.3 flips from graphics mode to text mode and back, saving only those 12K bytes that actually have to be saved. This code saves and restores the first 8K of plane 2 (the font area) while in graphics mode, but performs the save and restore of the 4000 bytes used for the character/attribute map while in text mode, because the characters and attributes, which are actually stored in the even bytes of planes 0 and 1, respectively, appear to be contiguous bytes in memory in text mode and so are easily saved as a single block.</p>
<p>Explaining why only every other byte of planes 0 and 1 is used in text mode and why characters and attributes appear to be contiguous bytes when they are actually in different planes is a large part of the explanation I&rsquo;m not going to go into now. One bit of fallout from this, however, is that if you flip to text mode and preserve the graphics bitmap using the mechanism illustrated in Listing 27.3, you shouldn&rsquo;t write to any text page other than page 0 (that is, don&rsquo;t write to any offset in display memory above 3999 in text mode) or alter the Page Select bit in the Miscellaneous Output register (3C2H) while in text mode. In order to allow completely unfettered access to text pages, it would be necessary to save every byte in the first 32K of each of planes 0 and 1. (On the other hand, this <i>would</i> allow up to 16 text screens to be stored simultaneously, with any one displayable instantly.) Moreover, if any fonts other than the default font are loaded, the portions of plane 2 that those particular fonts are loaded into would have to be saved, up to a maximum of all 64K of plane 2. In the worst case, a full 128K would have to be saved in order to preserve all the memory potentially used by text mode.</p>
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end Start
</pre>
<p><br /></p>
<center>
<table border="1">
<tr>
<td><a href="27-04.html">Previous</a></td>
<td>
<a href="27-04.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="28-01.html">Next</a></td>
<td>
<a href="28-01.html">Next</a>
</td>
</tr>
</table>
</center>