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raster interrupt gets weird when I hold down a key

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So, I have a raster interrupt that uses one sprite for two clouds, horizontally

independent of each other, with fixed vertical positions. It looks good until I hold down a key on the keyboard, then the interrupt doesn't work at all!

 

The sprite is a set of two clouds. After the first half of the sprite is drawn (the first cloud), the X position is changed, and the second half is drawn. But when I hold down a key, the second cloud is not repositioned (it reverts to the same horizontal position as the first cloud), so it seems to me that the interrupt is not taking place.

 

I'm a noob, so (as usual) excuse me if this is a silly question. :P

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Hard to say without seeing code but it sounds like you're using $ea31 to exit an interrupt at some point not far above the the split that's changing the sprite X position. An easy way to tell is to leave the runtime doing an inc $d020 / jmp *-$03 and you'll be able to see where the CPU is busy and what happens to that use when you press a key. $ea31 only really needs to be called once a frame so if every interrupt is exiting with it that will be using a lot more CPU grunt than is needed as well.

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