I have this horizontal smooth scrolling text on line 1 on the screen. the smooth scrolling effect is made using the $d016 hardware scrolling effect by iterating on the 7 lowest bits of $d016). The scroller runs on line 1 of the screen. I have set up two raster interrupts.
The "noScroller" interrupt is the part of the screen that should not be scrolled - which is whole screen except line 1.
The "scroller" is the interrupt that happens on line 1. I have set this interrupt to #50 even though I think it would make sense to set it to #0 since the scroll should only happen on line 1, but if I do set it to #0 then the scrolling text jumps around.
The "noscroller" interrupt is set to happen on line #66 - if I set it to #58 which seems to be the place where line 1 happens then the scrolling text starts to jump around strangely.
My problem is that I don't know what is wrong with my 2 interrupts. I would like to have the $d016 smooth scrolling only happen on line 1 but I have to make a larger area of the screen scroll than just line 1 otherwise the text will jump around. Here is my working code (with a too large scrolling screen area):
*=$c000
sei
lda #$7f
sta $dc0d
sta $dd0d
and $d011
sta $d011
ldy #50
sty $d012
lda #<scroller
ldx #>scroller
sta $0314
stx $0315
lda #$01
sta $d01a
cli
rts
noScroller lda $d016
and #$f8
sta $d016
ldy #50
sty $d012
lda #<scroller
ldx #>scroller
sta $0314
stx $0315
inc $d019
jmp $ea31
scroller lda $d016
and #$f8
adc offset
sta $d016
dec offset
bpl continue
lda #07
sta offset
shiftrow ldx #$00
lda $0401,x
sta $0400,x
inx
cpx #39
bne shiftrow+2
fetchnewchar ldx nextchar
lda message,x
sta $0427
inx
lda message,x
cmp #255
bne continue-3
ldx #00
stx nextchar
continue ldx #66
stx $d012
lda #<noScroller
ldy #>noScroller
sta $0314
sty $0315
inc $d019
jmp $ea31
offset byte 07
nextchar byte 00
message byte 011, 009, 012, 018, 015, 025, 032, 023, 001, 019, 032, 006, 009, 014, 001, 012, 012, 025, 032, 008, 005, 018, 005, 032, 032, 032, 032, 032, 032, 255
It has been a long time ;-) I remember that doing real work in interrupts is sometimes problematic because the computer is busy and you will not get the next interrupt in time. You do update the $0400 area while you are in that area, that will flicker. Maybe that is the reason you need to increase the window of scan lines.
I recommend you try separating the change of register $d016 from storing the text at $0400. Move the copying of text into the second interrupt noScroller after resetting $d016 because there you have all the time you need. The change will not be visible until you hit the top scan line again. Then experiment with the scan lines $d012 again if you can make the area exactly as small as needed.
During debugging you can change the background colour of the screen at the begin of your interrupt and reset it at the end. You should get a short coloured line on your screen that is wobbling a bit around. That will show you "where" your interrupt is happening. If you see that every 8th interrupt is taking too long, try unrolling the loop shiftrow with 39 times LDA/STA, which is faster.