View source for Sega Mega Drive/Planes

In addition to [[Sega Mega Drive/Sprites|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 [[Sega Mega Drive/Scrolling|horizontal scroll table]] (in VRAM) and [[Sega Mega Drive/Scrolling|vertical scroll table]] (in VSRAM).

==Nametables==
===Size===
The dimensions of the foreground and background planes are set by [[Sega Mega Drive/VDP registers#10|VDP register $10]]. Both planes have the same size. The following are the possible size settings:

{|
! Size || Space in VRAM (for 1 plane) || Code
|-
| 256x256 pixels (32x32 cells)
| 2048 ($800) bytes
| <tt>move.w #$9000,($c00004).l</tt>
|-
| 512x256 pixels (64x32 cells)
| 4096 ($1000) bytes
| <tt>move.w #$9001,($c00004).l</tt>
|-
| 1024x256 pixels (128x32 cells)
| 8192 ($2000) bytes
| <tt>move.w #$9003,($c00004).l</tt>
|-
| 256x512 pixels (32x64 cells)
| 4096 ($1000) bytes
| <tt>move.w #$9010,($c00004).l</tt>
|-
| 512x512 pixels (64x64 cells)
| 8192 ($2000) bytes
| <tt>move.w #$9011,($c00004).l</tt>
|-
| 256x1024 pixels (32x128 cells)
| 8196 ($2000) bytes
| <tt>move.w #$9030,($c00004).l</tt>
|}

Dimensions of 512x1024 pixels (64x128 cells) and 1024x1024 pixels (128x128 cells) are not valid, because the space used by a plane can't exceed 8196 ($2000) bytes.

===Location===
Nametables for all three planes are stored in VRAM, and their location is set by [[Sega Mega Drive/VDP registers|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
| {{nobr|<tt>move.w #$8200+($xxxx>>10),($c00004).l</tt>}}
|-
| Background (Plane B)
| 0, $2000, $4000, $6000, $8000, $A000, $C000, $E000
| <tt>move.w #$8400+($xxxx>>13),($c00004).l</tt>
|-
| Window ([[Sega Mega Drive/VDP registers#0C|320 pixel wide mode]])
| 0, $1000, $2000, $3000, $4000, $5000, $6000, $7000, $8000, $9000, $A000, $B000, $C000, $D000, $E000, $F000
| <tt>move.w #$8300+($xxxx>>10),($c00004).l</tt>
|-
| Window ([[Sega Mega Drive/VDP registers#0C|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
| <tt>move.w #$8300+($xxxx>>10),($c00004).l</tt>
|}

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

{{bitfield|PR|PL;2|V|H|T10-T8;3}}{{bitfield|T7-T0;8}}
* '''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.

==Window==

[[Category:Sega Mega Drive|Planes]]
[[Category:Unofficial documentation]]