Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!wuarchive!brutus.cs.uiuc.edu!jarthur!kfink
From: kfink@jarthur.Claremont.EDU (Kevin Fink)
Newsgroups: comp.sys.cbm
Subject: Another Smooth-Scrolling ML question
Message-ID: <3408@jarthur.Claremont.EDU>
Date: 4 Dec 89 19:30:20 GMT
Reply-To: kfink@jarthur.Claremont.EDU (Kevin Fink)
Followup-To: comp.sys.cbm
Organization: Harvey Mudd College, Claremont, CA
Lines: 70

My brother has another question.

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I am still having problems getting smooth scrolling to work. I am using
double-buffering but it is still flickering. I think I know why.

Try turning on the computer and typing this BASIC program:
10 Y=53265
20 X=X-1:IF X=-1 THEN X=7
30 POKE Y,(PEEK(Y) AND 248) + X : GOTO 20

Make sure you have plenty of garbage on the screen when you run this,
especially near the bottom. When I ran it, (1) a black line flickered up the
screen, and (2) the letters flickered. The area the letters flickered
'rolled' up the screen. The black line can be taken care of by turning the
screen black. I can't figure out why the letters are flickering, since I'm
only using the VIC-II chip's built-in scrolling. They don't only flicker
when jumping the seven bits, either.

I also modified line 20 to read:
20 X=X+1:IF X=8 THEN X=0

This scrolled the screen down instead of up. The screen flickered much less
this time.

I decided this was because of the position of the raster, so I wrote the
following assembly language program:
[ed. note: I think he's using the LADS assembler from some book or something.]

10 *=49152
11 .S
12 .O
20 X = 820
30 WEDGE SEI:LDA #<INT:STA $314:LDA #>INT:STA $315:LDA $D011:AND #$7F:STA
$D011
40 LDA #251:STA $D012:CLI:RTS
50 INT LDA $D011:AND #248:ORA X:STA $D011:JMP $EA31

That program scrolls the screen based on the contents of the memory location
820, but only when the raster is at the set value. The hexadecimal machine
code for the program is:
C000: 78 A9 1A 8D 14 03 A9 C0
C008: 8D 15 03 AD 11 D0 29 7F
C010: 8D 11 D0 A9 FB 8D 12 D0
C018: 58 60 AD 11 D0 29 F8 0D
C020: 34 03 8D 11 D0 4C 31 EA

To use this program, enter it into memory, then write the following BASIC
program:
1 X=PEEK(820):X=X-1:IF X=-1 THEN X=7
2 POKE 820,X:GOTO 1

This didn't help at all. By adjusting the raster position which caused the
interrupt, I could make the flickering get worse, but I couldn't improve it
much.

The demonstration program my reference guide lists scrolls up a line of
HELLOs, which don't flicker at all because when one flickers a 'shadow
image' down, it is indistinguishable from the HELLO below it. Is there some
way to eliminate the flickering, or will I have to use the same method?

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Once again I have no idea what he is talking about, but he'd appreciate a
response.

Thanks.

Kevin Fink