Misc/abrash-black-book
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James Gregory 2013-12-30 12:25:34 +11:00
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@ -41,7 +41,7 @@
</P>
<P>Horizontal smooth panning works just fine, although I&#146;ve always harbored some doubts that any one horizontal-smooth-panning approach works properly on all display board clones. (More on this later.) There&#146;s a catch when using horizontal smooth panning with the split screen up, though, and it&#146;s a serious catch: You can&#146;t byte-pan the split screen (which always starts at offset zero, no matter what the setting of the start address registers)&#151;but you <I>can</I> pel-pan the split screen.</P>
<P>Put another way, when the normal portion of the screen is horizontally smooth-panned, the split screen portion moves a pixel at a time until it&#146;s time to move to the next byte, then jumps back to the start of the current byte. As the top part of the screen moves smoothly about, the split screen will move and jump, move and jump, over and over. Believe me, it&#146;s not a pretty sight.</P>
<TABLE WIDTH="100%"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/30-03i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>What&#146;s to be done? On the EGA, nothing. Unless you&#146;re willing to have your users&#146; eyes doing the jitterbug, don&#146;t use horizontal smooth scrolling while the split screen is up. Byte panning is fine&#151;just don&#146;t change the Pel Panning register from its default setting.</I></SMALL>
<TABLE WIDTH="100%"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>What&#146;s to be done? On the EGA, nothing. Unless you&#146;re willing to have your users&#146; eyes doing the jitterbug, don&#146;t use horizontal smooth scrolling while the split screen is up. Byte panning is fine&#151;just don&#146;t change the Pel Panning register from its default setting.</I></SMALL>
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<P>On the VGA, there is recourse. A VGA-only bit, bit 5 of the AC Mode Control register (AC register 10H), turns off pel panning in the split screen. In other words, when this bit is set to 1, pel panning is reset to zero before the first line of the split screen, and remains zero until the end of the frame. This doesn&#146;t allow you to pan the split screen horizontally, mind you&#151;there&#146;s no way to do that&#151;but it does let you pan the normal screen while the split screen stays rock-solid. This can be used to produce an attractive &#147;streaming tape&#148; effect in the normal screen while the split screen is used to display non-moving information.
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