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
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<center>
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<td><a href="25-02.html">Previous</a></td>
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<a href="25-02.html">Previous</a>
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<td><a href="index.html">Table of Contents</a></td>
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<a href="index.html">Table of Contents</a>
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<td><a href="25-04.html">Next</a></td>
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<p><br /></p>
<p>It&rsquo;s worth pointing out again that the bit mask operates on the data in the latches, not on the data in display memory. This makes the bit mask a flexible resource that with a little imagination can be used for some interesting purposes. For example, you could fill the latches with a solid background color (by writing the color somewhere in display memory, then reading that location to load the latches), and then use the Bit Mask register (or write mode 3, as we&rsquo;ll see later) as a mask through which to draw a foreground color stencilled into the background color <i>without</i> reading display memory first. This only works for writing whole bytes at a time (clipped bytes require the use of the bit mask; unfortunately, we&rsquo;re already using it for stencilling in this case), but it completely eliminates reading display memory and does foreground-plus-background drawing in one blurry-fast pass.</p>
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<p>The program in Listing 25.2 illustrates this problem. A green pattern (plane 1 set to 1, planes 0, 2, and 3 set to 0) is first written to display memory. Display memory is then filled with blue (only plane 0 set to 1), with a Map Mask setting of 01H. Where the blue crosses the green, cyan is produced, rather than blue, because the Map Mask register setting of 01H that produces blue leaves the green plane (plane 1) unchanged. In order to generate blue unconditionally, it would be necessary to set the Map Mask register to 0FH, clear memory, and then set the Map Mask register to 01H and fill with blue.</p>
<p><br /></p>
<center>
<table border="1">
<tr>
<td><a href="25-02.html">Previous</a></td>
<td>
<a href="25-02.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="25-04.html">Next</a></td>
<td>
<a href="25-04.html">Next</a>
</td>
</tr>
</table>
</center>