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James Gregory 2013-12-30 15:11:04 +11:00
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<meta name="vspubdate" content="07/01/97" />
<meta name="vscategory" content="Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development" />
<title>Michael Abrash's Graphics Programming Black Book Special Edition: Changing Colors without Writing Pixels</title><!-- HEADER -->
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<!--ISBN=1576101746//-->
<!--TITLE=Michael Abrash's Graphics Programming Black Book Special Edition//-->
<!--AUTHOR=Michael Abrash//-->
<!--PUBLISHER=The Coriolis Group, Inc.//-->
<title>Michael Abrash's Graphics Programming Black Book Special Edition: Changing Colors without Writing Pixels</title>
<meta name="chapter" content="34" />
<meta name="pages" content="641-643" />
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<p><br /></p>
<h4 align="left"><a id="Heading5"></a>Loading the DAC via the BIOS</h4>
<h4 align="left" id="Heading5">Loading the DAC via the BIOS</h4>
<p>The DAC can be loaded either directly or through subfunctions 10H (for a single DAC register) or 12H (for a block of DAC registers) of the BIOS video service interrupt 10H, function 10H, described in Chapter 33. For cycling the contents of the entire DAC, the block-load function (invoked by executing <b>INT</b> 10H with AH = 10H and AL = 12H to load a block of CX DAC locations, starting at location BX, from the block of RGB triplets&mdash;3 bytes per triplet&mdash;starting at ES:DX into the DAC) would be the better of the two, due to the considerably greater efficiency of calling the BIOS once rather than 256 times. At any rate, we&rsquo;d like to use one or the other of the BIOS functions for color cycling, because we know that whenever possible, one should use a BIOS function in preference to accessing hardware directly, in the interests of avoiding compatibility problems. In the case of color cycling, however, it is emphatically <i>not</i> possible to use either of the BIOS functions, for they have problems. Serious problems.</p>
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<p>Which is not to say that loading the DAC directly is a picnic either, as we&rsquo;ll see next.</p>
<h4 align="left"><a id="Heading6"></a>Loading the DAC Directly</h4>
<h4 align="left" id="Heading6">Loading the DAC Directly</h4>
<p>So we must load the DAC directly in order to perform color cycling. The DAC is loaded directly by sending (with an <b>OUT</b> instruction) the number of the DAC location to be loaded to the DAC Write Index register at 3C8H and then performing three <b>OUT</b>s to write an RGB triplet to the DAC Data register at 3C9H. This approach must be repeated 256 times to load the entire DAC, requiring over a thousand <b>OUT</b>s in all.</p>
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<div align="center">
Graphics Programming Black Book &copy; 2001 Michael Abrash
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