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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: Saving Screens and Other VGA Mysteries</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: Saving Screens and Other VGA Mysteries</title>
<meta name="chapter" content="29" />
<meta name="pages" content="550-555" />
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<p>Video function 10H is invoked by performing an <b>INT</b> 10H with AH set to 10H. If AL is 0 (subfunction 0), then BL contains the number of the palette register to set, and BH contains the value to set that register to. If AL is 1 (subfunction 1), then BH contains the value to set the overscan (border) color to. Finally, if AL is 2 (subfunction 2), then ES:DX points to a 17-byte array containing the values to set palette registers 0-15 and the overscan register to. (For completeness, although it&rsquo;s unrelated to the palette registers, there is one more subfunction of video function 10H. If AL = 3 (subfunction 3), bit 0 of BL is set to 1 to cause bit 7 of text attributes to select blinking, or set to 0 to cause bit 7 of text attributes to select highreverse video.)</p>
<p><a id="Fig3"><!-- </A><A HREF="javascript:displayWindow('images/29-03.jpg',407,152 )"> --><img src="images/29-03.jpg" /><br />
<!-- </A>
<BR><A HREF="javascript:displayWindow('images/29-03.jpg',407,152)"> --><b>Figure 29.3</b></a>&nbsp;&nbsp;<i>Bit organization within a palette register.</i></p>
<p><a id="Fig3"><img src="images/29-03.jpg" /><br />
<b>Figure 29.3</b></a>&nbsp;&nbsp;<i>Bit organization within a palette register.</i></p>
<p>Listing 29.3 uses video function 10H, subfunction 2 to step through all 64 possible colors. This is accomplished by putting up 16 color bars, one for each of the 16 possible 4-bit pixel values, then changing the mapping provided by the palette registers to select a different group of 16 colors from the set of 64 each time a key is pressed. Initially, colors 0-15 are displayed, then 1-16, then 2-17, and so on up to color 3FH wrapping around to colors 0-14, and finally back to colors 0-15. (By the way, at mode set time the 16 palette registers are not set to colors 0-15, but rather to 0H, 1H, 2H, H, 4H, 5H, 14H, 7H, 38H, 39H, 3AH, 3BH, 3CH, 3DH, 3EH, and 3FH, respectively. Bits 6, 5, and 4&mdash;secondary red, green, and blue&mdash;are all set to 1 in palette registers 8-15 in order to produce high-intensity colors. Palette register 6 is set to 14H to produce brown, rather than the yellow that the expected value of 6H would produce.)</p>
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<p>It&rsquo;s important to understand that in Listing 29.3 the contents of display memory are never changed after initialization. The only change is the mapping from the 4-bit pixel data coming out of display memory to the 6-bit data going to the monitor. For this reason, it&rsquo;s technically inaccurate to speak of bits in display memory as representing colors; more accurately, they represent attributes in the range 0-15, which are mapped to colors 0-3FH by the palette registers.</p>
<p><b>LISTING 29.3 L29-3.ASM</b></p><!-- CODE //-->
<p><b>LISTING 29.3 L29-3.ASM</b></p>
<pre>
; Program to illustrate the color mapping capabilities of the
; EGA&rsquo;s palette registers.
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end Start
</pre><!-- END CODE //-->
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<p><br /></p>
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<div align="center">
Graphics Programming Black Book &copy; 2001 Michael Abrash
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