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Border uses palettes 4-6 only #440

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Oct 9, 2022
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Border uses palettes 4-6 only #440

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Border uses palettes 4-6 only
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25 changes: 18 additions & 7 deletions src/SGB_Command_Border.md
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Expand Up @@ -29,7 +29,12 @@ tiles 00h-7Fh, and once for tiles 80h-FFh). Note: The BG/OBJ Bit seems
to have no effect and writes to the same VRAM addresses for both BG and
OBJ ???

TODO: explain tile format
Each tile is stored in 4-bit-per-pixel format consisting of bit planes
0 and 1 interleaved by row, followed by bit planes 2 and 3 interleaved
by row. In effect, each tile consists of two Game Boy tiles, the first
to determine bits 0 and 1 (choosing among color 0, 1, 2, or 3 within a
4-color subpalette), and the second to determine bits 2 and 3 (choosing
among colors 0-3, 4-7, 8-11, or 12-15).

## SGB Command $14 — PCT_TRN

Expand All @@ -48,16 +53,16 @@ The map data is sent by VRAM-Transfer (4 KBytes).
```
000-6FF BG Map 32x28 Entries of 16 bits each (1792 bytes)
700-7FF Not used, don't care
800-87F BG Palette Data (Palettes 4-7, each 16 colors of 16 bits each)
880-FFF Not used, don't care
800-85F BG Palette Data (Palettes 4-6, each 16 colors of 16 bits each)
860-FFF Not used, don't care
```

Each BG Map Entry consists of a 16-bit value as such:
`VH01 PP00 NNNN NNNN```

```
Bit 0-9 - Character Number (use only 00h-FFh, upper 2 bits zero)
Bit 10-12 - Palette Number (use only 4-7, officially use only 4-6)
Bit 10-12 - Palette Number (use only 4-6)
Bit 13 - BG Priority (use only 0)
Bit 14 - X-Flip (0=Normal, 1=Mirror horizontally)
Bit 15 - Y-Flip (0=Normal, 1=Mirror vertically)
Expand All @@ -77,9 +82,15 @@ border defining all unique tiles would have to define this many tiles:
- (256\*224-160\*144)/64+1 = 537 tiles in fullscreen border
- (256\*176-160\*144)/64+2 = 346 tiles in widescreen border

But the CHR RAM allocated by SGB for border holds only 256 tiles. This
means a fullscreen border must repeat at least 281 tiles and a
widescreen border at least 90.
Because the CHR RAM allocated by SGB for border holds only 256 tiles,
a fullscreen border must repeat at least 281 tiles and a widescreen
border at least 90.

The Super NES supports 8 background palettes. The SGB system software
(when run in Mesen-S) has been observed to use background palette 0
for the GB screen, palettes 1, 2, 3, and 7 for the menus, and palettes
4, 5, and 6 for the border. Thus a border can use three 15-color
palettes.

## SGB Command $18 — OBJ_TRN

Expand Down