> Yes, you could probably do something like that with hardware tricks if you wanted. > But what I wanted to say: You don't have to do anything to prevent it. You have an > interrupt for screen refreshing and that's where everything is done (game logic, > gamepad input, graphics output). And that's it.
Even that wasn't true for long. Tricks like parallax scrolling and smooth sky gradient backgrounds involved reprogramming the graphics chip per-scanline, somewhat like the Atari 2600 (MAMEdev tends to call this "raster effects"). The water effect in the original Genesis Sonic games was just changing the palette quickly when the screen got to whereever the water started, for instance.
This was so common that the SNES included dedicated hardware that you could program to screw with the video chip on arbitrary scanlines without CPU intervention (called "HDMA").
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