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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: Video Est Omnis Divisa</title><!-- HEADER -->
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<title>Michael Abrash's Graphics Programming Black Book Special Edition: Video Est Omnis Divisa</title>
<meta name="chapter" content="30" />
<meta name="pages" content="584-585" />
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<p><br /></p>
<h3><a id="Heading11"></a>Split Screens in Other Modes</h3>
<h3 id="Heading11">Split Screens in Other Modes</h3>
<p>So far we&rsquo;ve only discussed the split screen in mode 10H. What about other modes? Generally, the split screen works in any mode; the basic rule is that when a scan line on the screen matches the split screen scan line, the internal display memory pointer is reset to zero. I&rsquo;ve found this to be true even in oddball modes, such as line-doubled CGA modes and the 320x200 256-color mode (which is really a 320x400 mode with each line repeated. For split-screen purposes, the VGA and EGA seem to count purely in scan lines, not in rows or doubled scan lines or the like. However, I have run into small anomalies in those modes on clones, and I haven&rsquo;t tested all modes (nor, lord knows, all clones!) so be careful when using the split screen in modes other than modes 0DH-12H, and test your code on a variety of hardware.</p>
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<p>What of the split screen in text mode? It works fine; in fact, it not only resets the internal memory pointer to zero, but also resets the text scan line counter&mdash;which marks which line within the font you&rsquo;re on&mdash;to zero, so the split screen starts out with a full row of text. There&rsquo;s only one trick with text mode: When split screen pel panning suppression is on, the pel panning setting is forced to 0 for the rest of the frame. Unfortunately, 0 is <i>not</i> the &ldquo;no-panning&rdquo; setting for 9-dot-wide text; 8 is. The result is that when you turn on split screen pel panning suppression, the text in the split screen won&rsquo;t pan with the normal screen, as intended, but will also display the undesirable characteristic of moving one pixel to the left. Whether this causes any noticeable on-screen effects depends on the text displayed by a particular application; for example, there should be no problem if the split screen has a border of blanks on the left side.</p>
<h3><a id="Heading12"></a>How Safe?</h3>
<h3 id="Heading12">How Safe?</h3>
<p>So, how safe <i>is</i> it to use the split screen? My opinion is that it&rsquo;s perfectly safe, although I&rsquo;d welcome input from people with extensive split screen experience&mdash;and the effects are striking enough that the split screen is well worth using in certain applications.</p>
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<div align="center">
Graphics Programming Black Book &copy; 2001 Michael Abrash
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