Sega Mega Drive/Planes

In addition to sprites, the Sega Mega Drive uses three planes to display graphics. These planes consist of two grids of tiles - a foreground (plane A) and background (plane B) - which can be moved independently, as well as a static window plane. The content of the planes is determined by nametables, which are stored in VRAM. Planes A & B can be moved around by modifying the horizontal scroll table (in VRAM) and vertical scroll table (in VSRAM).

Nametables

Location

Nametables for all three planes are stored in VRAM, and their location is set by VDP registers 02, 03 and 04. Nametables can't be placed anywhere in VRAM, rather they must be aligned to particular multiples.

Nametable	Valid locations	Code
Foreground (Plane A)	0, $2000, $4000, $6000, $8000, $A000, $C000, $E000	move.w #$8200+($xxxx>>10),($c00004).l
Background (Plane B)	0, $2000, $4000, $6000, $8000, $A000, $C000, $E000	move.w #$8400+($xxxx>>13),($c00004).l
Window (320 pixel wide mode)	0, $1000, $2000, $3000, $4000, $5000, $6000, $7000, $8000, $9000, $A000, $B000, $C000, $D000, $E000, $F000	move.w #$8300+($xxxx>>10),($c00004).l
Window (256 pixel wide mode)	0, $800, $1000, $1800, $2000, $2800, $3000, $3800, $4000, $4800, $5000, $5800, $6000, $6800, $7000, $7800, $8000, $8800, $9000, $9800, $A000, $A800, $B000, $B800, $C000, $C800, $D000, $D800, $E000, $E800, $F000, $F800	move.w #$8300+($xxxx>>10),($c00004).l

Format

A nametable consists of one word per tile, with the following format:

7

6

5

4

3

2

1

0

PR

PL

V

H

T10-T8

7

6

5

4

3

2

1

0

T7-T0

• PR: Priority. 1 = high; 0 = low.
• PL: Palette line.
• V: Vertical flip. 1 = flipped; 0 = normal.
• H: Horizontal flip. 1 = flipped; 0 = normal.
• T10-T0: Tile index. The number of the tile in VRAM to use, or its address divided by $20.