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gba/book/src-bak/05-palram.md
Lokathor 09b4c8804c YABU: Yet Another Book Update
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Palette RAM (PALRAM)

  • Address Span: 0x500_0000 to 0x500_03FF (1k)

Palette RAM has a 16-bit bus, which isn't really a problem because it conceptually just holds u16 values. There's no automatic wait state, but if you try to access the same location that the display controller is accessing you get bumped by 1 cycle. Since the display controller can use the palette ram any number of times per scanline it's basically impossible to predict if you'll have to do a wait or not during VDraw. During VBlank you won't have any wait of course.

PALRAM is among the memory where there's weirdness if you try to write just one byte: if you try to write just 1 byte, it writes that byte into both parts of the larger 16-bit location. This doesn't really affect us much with PALRAM, because palette values are all supposed to be u16 anyway.

The palette memory actually contains not one, but two sets of palettes. First there's 256 entries for the background palette data (starting at 0x500_0000), and then there's 256 entries for object palette data (starting at 0x500_0200).

The GBA also has two modes for palette access: 8-bits-per-pixel (8bpp) and 4-bits-per-pixel (4bpp).

  • In 8bpp mode an 8-bit palette index value within a background or sprite simply indexes directly into the 256 slots for that type of thing.
  • In 4bpp mode a 4-bit palette index value within a background or sprite specifies an index within a particular "palbank" (16 palette entries each), and then a separate setting outside of the graphical data determines which palbank is to be used for that background or object (the screen entry data for backgrounds, and the object attributes for objects).

Transparency

When a pixel within a background or object specifies index 0 as its palette entry it is treated as a transparent pixel. This means that in 8bpp mode there's only 255 actual color options (0 being transparent), and in 4bpp mode there's only 15 actual color options available within each palbank (the 0th entry of each palbank is transparent).

Individual backgrounds, and individual objects, each determine if they're 4bpp or 8bpp separately, so a given overall palette slot might map to a used color in 8bpp and an unused/transparent color in 4bpp. If you're a palette wizard.

Palette slot 0 of the overall background palette is used to determine the "backdrop" color. That's the color you see if no background or object ends up being rendered within a given pixel.

Since display mode 3 and display mode 5 don't use the palette, they cannot benefit from transparency.