Document more of the Window's behaviour #626
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avivace
requested changes
Sep 23, 2025
LIJI32
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Sep 23, 2025
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| If the WY condition has already been triggered and at the start of a row the window enable bit was set, | ||
| then resetting that bit before the WX condition gets triggered on that row yields a nice window glitch pixel where the window would have been activated. | ||
| The scroll registers are re-read on each [tile fetch](<#Get Tile>), except for the low 3 bits of `SCX`, which are only read at the beginning of the scanline (for the initial shifting of pixels). |
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You'd think the low bits of SCX would only be read once per line, because that's the logical way to do so, but unfortunately this isn't true. The fetched tile's X position is calculated (roughly) by adding SCX to LX and taking the 5 topmost bits, rather than adding the 5 topmost bits of SCX and LX.
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I see. Do you know of a test ROM that checks this?
Also, it seems like it'd be a better fit for a separate PR, so I'll open one when I have some info to pen down.
avivace
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Oct 1, 2025
Co-authored-by: Antonio Vivace <avivace4@gmail.com>
Co-authored-by: Antonio Vivace <avivace4@gmail.com>
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Fixes #376.
cc @LIJI32 for review
In particular, I have a TODO for linking to a test ROM, as I believe mealybug-tearoom has one for the “trigger Window multiple times in a single scanline” behaviour, but I couldn't figure out which one.