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It's surprising what a night sleep and a swim does for you.
I managed to get the scrolling mostly working now after a couple of hours today, there's still a small bug with tiles that have a 0 pixel Y axis but I'm sure I'll sort that.
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This scrolling engine is taking up about 80 scanlines of a frame which is more than I would like- plenty of little tricks to be done though.
AGA programming definitely comes with complications. A side effect of AGA that I doubt I'll be able to get around is that the display will be slightly to the right, there's not much I can do unless I lose a load of hardware sprites.
The arcade display is 256x224, and a standard Amiga display is 320x192 pixels, this allows 32 pixels at either side of the main display to scroll in new tiles into the main display - handy.
Now this means I'm having to put the Amiga bit planes into 32 bits wide mode meaning the Amiga transfers long words (instead of words on an A500) from chip ram which gives a big speed improvement. To get an even bigger improvement you can set it to transfer double long words (64 bits), but then I would need 64 pixels at each side of the main display. It is possible to over scan the display to get those 64 pixels but as most ASM Amiga coders will know you then eat into the sprite DMA which knocks out the hardware sprites.
Given I'm planning on using the hardware sprites for the large vulture and rhino enemies as they are 64 pixels wide I'm sticking with this display configuration and hopefully keeping the hardware sprites.
It wasn't easy doing this scrolling, and I didn't particularly enjoy coding it. it was frustrating and I can understand why others give up on the damn thing and swallow the Red pill/Blitz/Amos - an absolute pain in the arse to be honest.
Better times ahead as I will be loading the maps and will building in the sprite engine soon, I'll then be able to see how many sprites I can get on screen.
Geezer
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