Document more of the Window's behaviour - #626
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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.
Co-authored-by: Antonio Vivace <avivace4@gmail.com>
Co-authored-by: Antonio Vivace <avivace4@gmail.com>
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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). |
This is not true. Where did this come from? |
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The previous bullet seems a bit dodgy to me, too:
There's no mention of the prerequisite of the Window enable bit also being set in order to activate the Y condition. It could also maybe use clarification (since the bullet before that says the condition is cleared on each VBlank) that it only applies during active video scanlines, not VBlank scanlines. Also, I'm fairly certain this condition is not checked only at the beginning of the scanline, based on my emudev experience and related discussions. Since the games I recall fixing using this knowledge were GBC games, I wonder if that's where the confusion is happening? |
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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.