Difference between revisions of "Sega Mega Drive/Scrolling"

Revision as of 15:56, 16 June 2018

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

@@ Line 1: / Line 1: @@
-The [[Sega Mega Drive]] is able to '''scroll''' the background and foreground (planes B & A) independently, both horizontally and vertically.
+The [[Sega Mega Drive]] is able to '''scroll''' the background and foreground ([[Sega Mega Drive/Planes|planes]] B & A) independently, both horizontally and vertically.
 ==Horizontal scrolling==
@@ Line 28: / Line 28: @@
 ==Vertical scrolling==
+The vertical scrolling table is located in VSRAM. Its format is set by [[Sega Mega Drive/VDP registers#0B|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
+| <tt>FFFF BBBB</tt>
+* 1st word controls foreground
+* 2nd word controls background
+| As above.
+|-
+| 16 pixel strips
+| 64 ($40) bytes (256 pixel wide mode)<br>80 ($50) bytes (320 pixel wide mode)
+| <tt>FFFF BBBB FFFF BBBB FFFF BBBB...</tt>
+* Alternating words control foreground and background strips
+| with whole screen horizontal:<pre>move.w #$8B04,($c00004).l</pre>
+with 8 pixel horizontal:<pre>move.w #$8B06,($c00004).l</pre>
+with scanline horizontal:<pre>move.w #$8B07,($c00004).l</pre>
+|}
 [[Category:Sega Mega Drive|Scrolling]]
 [[Category:Unofficial documentation]]

Difference between revisions of "Sega Mega Drive/Scrolling"

From Sega Retro

Revision as of 15:56, 16 June 2018

Horizontal scrolling

Vertical scrolling