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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>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
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<center>
<table border="1">
<tr>
<td><a href="32-05.html">Previous</a></td>
<td>
<a href="32-05.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="33-02.html">Next</a></td>
<td>
<a href="33-02.html">Next</a>
</td>
</tr>
</table>
</center>
<p><br /></p>
<h2 id="Heading1">Chapter 33<br />
Yogi Bear and Eurythmics Confront VGA Colors</h2>
@ -49,13 +52,13 @@
<p>Briefly put, the VGA color translation circuitry takes in one 4- or 8-bit pixel value at a time and translates it into three 6-bit values, one each of red, green, and blue, that are converted to corresponding analog levels and sent to the monitor. Seems simple enough, doesn&rsquo;t it? Unfortunately, nothing is ever that simple on the VGA, and color translation is no exception.</p>
<h4 align="left" id="Heading4">The Palette RAM</h4>
<h4 id="Heading4">The Palette RAM</h4>
<p>The color path in the VGA involves two stages, as shown in Figure 33.1. The first stage fetches a 4-bit pixel from display memory and feeds it into the EGA-compatible palette RAM (so called because it is functionally equivalent to the palette RAM color translation circuitry of the EGA), which translates it into a 6-bit value and sends it on to the DAC. The translation involves nothing more complex than the 4-bit value of a pixel being used as the address of one of the 16 palette RAM registers; a pixel value of 0 selects the contents of palette RAM register 0, a pixel value of 1 selects register 1, and so on. Each palette RAM register stores 6 bits, so each time a palette RAM register is selected by an incoming 4-bit pixel value, 6 bits of information are sent out by the palette RAM. (The operation of the palette RAM was described back in Chapter 29.)</p>
<p>The process is much the same in text mode, except that in text mode each 4-bit pixel value is generated based on the character&rsquo;s font pattern and attribute. In 256-color mode, which we&rsquo;ll get to eventually, the palette RAM is not a factor from the programmer&rsquo;s perspective and should be left alone.</p>
<h4 align="left" id="Heading5">The DAC</h4>
<h4 id="Heading5">The DAC</h4>
<p>Once the EGA-compatible palette RAM has fulfilled its karma and performed 4-bit to 6-bit translation on a pixel, the resulting value is sent to the DAC (Digital/Analog Converter). The DAC performs an 8-bit to 18-bit conversion in much the same manner as the palette RAM, converts the 18-bit result to analog red, green, and blue signals (6 bits for each signal), and sends the three analog signals to the monitor. The DAC is a separate chip, external to the VGA chip, but it&rsquo;s an integral part of the VGA standard and is present on every VGA.</p>
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<p>The DAC contains 256 18-bit storage registers, used to translate one of 256 possible 8-bit values into one of 256K (262,144, to be precise) 18-bit values. The 18-bit values are actually composed of three 6-bit values, one each for red, green, and blue; for each color component, the higher the number, the brighter the color, with 0 turning that color off in the pixel and 63 (3FH) making that color maximum brightness. Got all that?</p>
<p><br /></p>
<center>
<table border="1">
<tr>
<td><a href="32-05.html">Previous</a></td>
<td>
<a href="32-05.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="33-02.html">Next</a></td>
<td>
<a href="33-02.html">Next</a>
</td>
</tr>
</table>
</center>