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 almost always used to display text or HUDs.
Graphics format
It uses 8x8 pixels tiles which are stored in the S ROMs or FIX files. In MVS systems, the onboard fix tileset is stoed in the SFIX ROM.
The tiles graphics are in 4BPP (15 colors + transparency) and can be individually mapped to the first 16 palettes.
Fix map
The map is 40x32 tiles but only 40x28 tiles are drawn to the screen. Therefor, there are 2 hidden lines of tiles at the beginning and the end of the map. Some games also don't use the leftmost and rightmost columns, as some misaligned CRT screens (?) can cut them off.
Tiles are mapped from up to bottom, and left to right (see layout). The VRAM_MOD ($3C0004) register can be set to 32 instead of 1 if tile have to be updated in lines. See Displaying text.
The base address in VRAM for the map is $7000. Each tile uses one 16bit word:
| Bit | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Def | Palette number | Tile number | ||||||||||||||