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Atari coding BBS
| Re: Blitter Draw Loop |
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Posted by: ggn
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Oct,13.2012-13:01
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Well, this is venturing into territories I haven't thought about yet, but I'll hazard a guess:
Let's assume that you're blitting a 128x128 sprite and the blitter stops at, say, the 3rd line, 48 pixels done. I'm pretty sure that no blitter registers are being modified until you start it up again. But, there is a possibility that the x count register is reset back to its original value since you issue a stop command. So the blitter will try to copy more bytes than you'd like in that scanline. The same thing might be happening to the source and destination addresses as well.
Again, I'm saying that all this is purely theoretical and I can't vouch for them unless I do some testing. But if you're absolutely positively sure that you're not touching the blitter regs at any other point in your code, what I wrote above might be possible. If this turns out to be true I don't know what to suggest to fix this. Perhaps a mechanism at the start of the sprite drawing that checks in what scanline you are and if there's enough CPU time in the current frame to blit the sprite - otherwise wait until the next.
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