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Split Screens in Other Modes

So far we’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’ve found this to be true even in oddball modes, such as line-doubled CGA modes and the 320×200 256-color mode (which is really a 320×400 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’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.

Come to think of it, I warn you about the hazards of running fancy VGA code on clones pretty often, don’t I? Ah, well—just one of the hazards of the diversity and competition of the PC market! It is a fact of life, though—if you’re a commercial developer and don’t test your video code on at least half a dozen VGAs, you’re living dangerously.

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—which marks which line within the font you’re on—to zero, so the split screen starts out with a full row of text. There’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 not the “no-panning” 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’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.

How Safe?

So, how safe is it to use the split screen? My opinion is that it’s perfectly safe, although I’d welcome input from people with extensive split screen experience—and the effects are striking enough that the split screen is well worth using in certain applications.

I’m a little more leery of horizontal smooth scrolling, with or without the split screen. Still, the Wilton book doesn’t advise any particular caution, and I haven’t heard any horror stories from the field lately, so the clone manufacturers must finally have gotten it right. (I vividly remember some early clones years back that didn’t quite get it right.) So, on balance, I’d say to use horizontal smooth scrolling if you really need it; on the other hand, in fast animation you can often get away with byte scrolling, which is easier, faster, and safer. (I recently saw a game that scrolled as smoothly as you could ever want. It was only by stopping it with Ctrl-NumLock that I was able to be sure that it was, in fact, byte panning, not pel panning.)

In short, use the fancy stuff—but only when you have


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Graphics Programming Black Book © 2001 Michael Abrash