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"City Trouble" by Den Kat Games
http://forums.nesdev.com/viewtopic.php?f=22&t=15642
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Author:  thefox [ Mon Sep 18, 2017 8:08 am ]
Post subject:  Re: "City Trouble" by Den Kat Games

DRW wrote:
But shouldn't the sprite overflow bit still be triggered at nine sprites, independent from how many sprites are actually capable to be shown on screen?

I think at least Nestopia does not trigger it at 9+ sprites (which I always lamented) if extra sprites are turned on, if you want something to test with.

Author:  DRW [ Mon Sep 18, 2017 8:21 am ]
Post subject:  Re: "City Trouble" by Den Kat Games

Thanks for that information.

And yes, this actually was the reason why the game freezes on some AVSes.

Author:  tokumaru [ Mon Sep 18, 2017 9:01 am ]
Post subject:  Re: "City Trouble" by Den Kat Games

Weird that they would change the behavior of the flag when configured to display more sprites, since that could break existing logic.

IMO this should be a visual change only, specially when the sprite limit is eliminated, not increased. But even changing the behavior so the flag only gets set when the increased limit is broken doesn't really help.

Author:  tepples [ Mon Sep 18, 2017 9:04 am ]
Post subject:  Re: "City Trouble" by Den Kat Games

I imagine it would have been very tricky to have the OAM evaluator process both the normal OAM fetch pattern, which is required for increasing the sprite limit, and the pathological diagonal fetch pattern, which is required for accurate sprite overflow flag behavior. I guess the firmware could have treated "9th secondary OAM slot filled" as an approximation of the result of the pathological diagonal fetch pattern when the sprite limit is increased.

Author:  tokumaru [ Mon Sep 18, 2017 9:11 am ]
Post subject:  Re: "City Trouble" by Den Kat Games

Yeah, that's true. You can't easily "break" the evaluation process like the original PPU does and still fetch extra valid sprites.

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