Posted By
Crown
on 2005-09-04
10:00:15
|  Re: Writing to the borders of the screen
You can work around the memory wrap by a couple of ways.
1. Change the screen memory address before the DMA kicks in for the 26th line. The 26th line would be still wrapped of course in this case, and would need special code handling for its updates.
2. Use the other method writing to the border. Instead of extending the main screen area, you create a second (or third in some cases) screen area. Those are starting form $0000 char positions and much easier to work with. The only drawback is that you will need to have a couple of border lines beetween the two windows, so the effect is not completely continous. It might be possible to enhance this so that it doesn't need the border in beetween, but I haven't experimented with that.
3. Start a DMA mid line before the DMA for the 26th line happens, this could increment the screen offset with the number of characters the DMA is doing and not by 40 so it could offset the wrap to $0000. I don't know if this effect ever has been used on the Plus4, but theoretically it should be possible.
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