Sega Mega Drive/Scrolling

The Sega Mega Drive is able to scroll the background and foreground (planes B & A) independently, both horizontally and vertically.

Horizontal scrolling

Horizontal scrolling is controlled by a table located in VRAM. The address for the table is set by VDP register $0D, while the format is set by register $0B. There are three possible settings for this register:

Type	Space in VRAM	Format	Code
Whole screen	4 bytes	FFFF BBBB 1st word controls foreground 2nd word controls background	move.w #$8B00,($c00004).l
8 pixel strips	112 ($70) bytes (NTSC) 120 ($78) bytes (PAL)	FFFF BBBB FFFF BBBB FFFF BBBB... Alternating words control foreground and background strips	move.w #$8B02,($c00004).l
Per scanline	896 ($380) bytes (NTSC) 960 ($3C0) bytes (PAL)	FFFF BBBB FFFF BBBB FFFF BBBB... Alternating words control foreground and background scanlines	move.w #$8B03,($c00004).l

Vertical scrolling

The vertical scrolling table is located in VSRAM. Its format is set by register $0B. There are only two types of vertical scrolling: whole screen and 16 pixel strips:

Type	Space in VSRAM	Format	Code
Whole screen	4 bytes	FFFF BBBB 1st word controls foreground 2nd word controls background	As above.
16 pixel strips	64 ($40) bytes (256 pixel wide mode) 80 ($50) bytes (320 pixel wide mode)	FFFF BBBB FFFF BBBB FFFF BBBB... Alternating words control foreground and background strips	with whole screen horizontal: move.w #$8B04,($c00004).l with 8 pixel horizontal: move.w #$8B06,($c00004).l with scanline horizontal: move.w #$8B07,($c00004).l

Format

One word is used to define how much each scrollable unit (whole screen, strip or scanline) is shifted:

7

6

5

4

3

2

1

0

0

0

0

0

0

0

S9-S8

7

6

5

4

3

2

1

0

S7-S0

• S9-S0: Number of pixels to shift. Between 0 and 1024 ($400).
• Nametables are at most 1024 pixels wide or tall.