Fix layer
The fix layer is a non-scrollable, tile-based layer with transparency which has the highest priority on the display (it always overlaps sprites). It is used to display text or HUDs like scores and health bars.
As only 12 bits are reserved to address fix tiles, tilesets can only contain a maximum of 4096 tiles (128KiB) without any kind of special bankswitching (see below).
Graphics format
Fix tiles are 8*8 pixels, they're stored in the S ROMs, or in FIX files before getting loaded to PSRAM in CD systems. In MVS systems, there is an embedded fix tileset stored in the SFIX ROM which can be swapped with the cartridge one.
The tiles graphics are 4BPP like sprites tiles (15 colors, index 0 is transparent), each can be individually mapped to one of the first 16 palettes (4bit palette number).
See Fix graphics format for more details.
Fix map
The map is in VRAM starting at address $7000. It is 40*32 words in size, but:
- Only 40x28 tiles are drawn in NTSC mode (2 hidden lines on top and bottom)
- Only 40x30 tiles are drawn in PAL mode (1 hidden line on top and bottom)
Some games also don't use the leftmost and rightmost columns, as some misaligned CRT screens can cut them off. SNK recomends using only a 38x28 area.
Each tile in VRAM uses a 16bit word:
| Bit | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Def | Palette number | Tile number | ||||||||||||||
Tiles are mapped from top to bottom, and left to right (see layout). The VRAM_MOD ($3C0004) register can be set to 32 if tiles need to be updated in lines. See Displaying text.
Bankswitching
Games using the NEO-CMC chips are able to use more than 4096 fix tiles by using VRAM space $7500+. See Fix bankswitching.