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51 lines
2.5 KiB
Markdown
51 lines
2.5 KiB
Markdown
# Palette RAM (PALRAM)
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* **Address Span:** `0x500_0000` to `0x500_03FF` (1k)
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Palette RAM has a 16-bit bus, which isn't really a problem because it
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conceptually just holds `u16` values. There's no automatic wait state, but if
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you try to access the same location that the display controller is accessing you
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get bumped by 1 cycle. Since the display controller can use the palette ram any
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number of times per scanline it's basically impossible to predict if you'll have
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to do a wait or not during VDraw. During VBlank you won't have any wait of
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course.
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PALRAM is among the memory where there's weirdness if you try to write just one
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byte: if you try to write just 1 byte, it writes that byte into _both_ parts of
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the larger 16-bit location. This doesn't really affect us much with PALRAM,
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because palette values are all supposed to be `u16` anyway.
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The palette memory actually contains not one, but _two_ sets of palettes. First
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there's 256 entries for the background palette data (starting at `0x500_0000`),
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and then there's 256 entries for object palette data (starting at `0x500_0200`).
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The GBA also has two modes for palette access: 8-bits-per-pixel (8bpp) and
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4-bits-per-pixel (4bpp).
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* In 8bpp mode an 8-bit palette index value within a background or sprite
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simply indexes directly into the 256 slots for that type of thing.
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* In 4bpp mode a 4-bit palette index value within a background or sprite
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specifies an index within a particular "palbank" (16 palette entries each),
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and then a _separate_ setting outside of the graphical data determines which
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palbank is to be used for that background or object (the screen entry data for
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backgrounds, and the object attributes for objects).
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### Transparency
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When a pixel within a background or object specifies index 0 as its palette
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entry it is treated as a transparent pixel. This means that in 8bpp mode there's
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only 255 actual color options (0 being transparent), and in 4bpp mode there's
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only 15 actual color options available within each palbank (the 0th entry of
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_each_ palbank is transparent).
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Individual backgrounds, and individual objects, each determine if they're 4bpp
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or 8bpp separately, so a given overall palette slot might map to a used color in
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8bpp and an unused/transparent color in 4bpp. If you're a palette wizard.
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Palette slot 0 of the overall background palette is used to determine the
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"backdrop" color. That's the color you see if no background or object ends up
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being rendered within a given pixel.
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Since display mode 3 and display mode 5 don't use the palette, they cannot
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benefit from transparency.
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