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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">
<!DOCTYPE html>
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<table border="1">
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<td><a href="24-02.html">Previous</a></td>
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<a href="24-02.html">Previous</a>
</td>
<td><a href="index.html">Table of Contents</a></td>
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<p><br /></p>
<p>Logical function 0, which writes the CPU data unmodified, is the standard mode of operation of the ALUs. In this mode, the CPU data is combined with the latched data by ignoring the latched data entirely. Expressed as a logical function, this could be considered CPU data ANDed with 1 (or ORed with 0). This is the mode to use whenever you want to place CPU data into display memory, replacing the previous contents entirely. It may occur to you that there is no need to latch display memory at all when the data unmodified function is selected. In the sample program, that is true, but if the bit mask is being used, the latches must be loaded even for the data unmodified function, as I&rsquo;ll discuss in the next chapter.</p>
<p>Logical functions 1 through 3 cause the CPU data to be ANDed, ORed, and XORed with the latched data, respectively. Of these, XOR is the most useful, since exclusive-ORing is a traditional way to perform animation. The uses of the AND and OR logical functions are less obvious. AND can be used to mask a blank area into display memory, or to mask off those portions of a drawing operation that don&rsquo;t overlap an existing display memory image. OR could conceivably be used to force an image into display memory over an existing image. To be honest, I haven&rsquo;t encountered any particularly valuable applications for AND and OR, but they&rsquo;re the sort of building-block features that could come in handy in just the right context, so keep them in mind.</p>
@ -69,16 +72,20 @@ MOV DX,(VALUE2 SHL 8) OR VALUE1
<p>The latches and ALUs are central to high-performance VGA code, since they allow programs to process across all four memory planes without a series of <b>OUT</b>s and read/write operations. It is not always easy to arrange a program to exploit this power, however, because the ALUs are far more limited than a CPU. In many instances, however, additional hardware in the VGA, including the bit mask, the set/reset features, and the barrel shifter, can assist the ALUs in controlling data, as we&rsquo;ll see in the next few chapters.</p>
<p><br /></p>
<center>
<table border="1">
<tr>
<td><a href="24-02.html">Previous</a></td>
<td>
<a href="24-02.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="25-01.html">Next</a></td>
<td>
<a href="25-01.html">Next</a>
</td>
</tr>
</table>
</center>