The VIC-20's VIC chip provides a simple yet flexible video display and one of the features that can be quite useful is the ability to alter the picture origin on the screen. This feature allows us to move the position that the screen displays on the TV up and down or left and right by changing either of two memory locations.
TV Picture Origin
Two memory locations define the TV picture origin.
$9000 (36864)
Horizontal TV picture origin / interlace bit
- Bit 7
- The interlace bit, which we'll ignore for this article. Default: 0.
- Bits 6-0
- The horizontal origin. To move the picture origin to the left reduce the number and to move it to the right increase the number. Every change moves the picture by 4 pixels. Default: 5 on NTSC systems, 12 on PAL systems.
$9001 (36865)
Vertical TV Picture Origin
To move the picture origin up reduce the number and to move it down increase the number. Every change moves the picture by 2 pixels. Default: 25 on NTSC systems, 38 on PAL systems.
Simple Vertical Scroll Program
The programs below simply scrolls the screen from a position off the bottom up to the default origin at the top. Both programs store the default location and therefore will work on PAL or NTSC systems.
You may want to run each program with the normal colour scheme as well as with alternative colour schemes which make the scrolling look better. For example to change to a black background and border with yellow text:
POKE 36879,8:POKE 646,7
Basic
10 D=PEEK(36865)
20 FOR X=190 TO D STEP -1
30 POKE 36865, X
40 FOR Y=1 TO 3:NEXT
50 NEXT
Assembler
Below is a simple assembler program to do the same as the Basic program. It could easily be entered using Vicmon, please see Beginning Assembly Programming on the Commodore VIC-20 for more details on how to use it. The program is only small so it could fit in the tape buffer at location 828 ($33C).
JMP MAIN
DFLT NOP ; VAR: Default vertical picture origin minus one
MAIN LDX $9001
DEX ; Decrement picture origin to make loop simpler
STX DFLT ; Store default vertical picture origin minus one
LDA #$BE ; Begin with picture origin off screen
LOOP STA $9001 ; Change vertical picture origin
LDX #$09
WAIT LDY #$FF
WAI2 DEY
BNE WAI2
DEX
BNE WAIT
SBC #$01 ; Move picture origin up one
CMP DFLT ; Is vertical picture origin back at default?
BNE LOOP
BRK
Simple Horizontal Scroll Program
The program below simply scrolls the screen from a position off the right of the screen to the default origin on the left. The program stores the default location and therefore will work on PAL or NTSC systems. As with the vertical scroll program above it is worth experimenting with different screen colours.
Basic
10 D=PEEK(36864)
20 FOR X=60 TO D STEP -1
30 POKE 36864, X
40 FOR Y=1 TO 6:NEXT
50 NEXT
Video of Simple Scrollers
You can see the scrollers being written in Basic and Assembler in the video below:
