VDP Scrolling

Horizontal Scrolling

Each Scrolls can be scrolled horizontally on different basis :

• Overall
• One CELL unit
• One line unit

This is selected via the vdp register 0xB.

Table format

The HScroll table is in the VRAM and its address is setup via the vdp register 0xD.

ScrollA and ScrollB both use this table alternately.

Address	Data	Affect
0	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollA	Overall, CELL 0, Line 0
2	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollB	Overall, CELL 0, Line 0
4	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollA	Line 1
6	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollB	Line 1
...	...	...
0x10	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollA	CELL 1, Line 8
0x12	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollB	CELL 1, Line 8
...	...	...
0x3FC	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollA	Line 511
0x3FE	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollB	Line 511
...	...	...

Vertical Scrolling

Each Scrolls can be scrolled vertically on two different basis :

• Overall
• 2-CELL unit

The VScroll table is located in the VSRAM.

ScrollA and ScrollB both use this table alternately.

Address	Data	Affect
0	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollA	Overall, CELL 0-1
2	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollB	Overall, CELL 0-1
4	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollA	CELL 2-3
6	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollB	CELL 2-3
...	...	...
0x4C	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollA	CELL 38-39
0x4E	Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Def Free Quantity of ScrollB	CELL 38-39