Posted By
Stinaris
on 2017-01-15
11:01:07
 |  Re: VB interrupt. How do you get it to trigger?
Thank you so much for all the responses. I didn't realise the 264 series scene was so vibrant.
@MAD - Thanks for that. The PRG says $FF13 bit 2 is "Single Clock Set" and that is it. Seems like an important register!! 
You are correct about the 800 bytes. it's the first 20 rows that I am scrolling only.
For speed each 160 characters I was copying I setup zero page addressing. So $0c01 and $3400 set 1 and $3401 and $0c00 set 2.
This was then repeated for each of the 5 copy blocks. so $0c01 + #160 and $3400 + #160 set 2 etc etc.
Now when I did this I was still getting some tearing. However after some further investigation it looks like the zero page address I was using for set 3
e.g.
LDA $41
STA $10
LDA #$0d
STA $11
; $12-$13 - $3540
LDA #$40
STA $12
LDA #$35
STA $13
; ---------------------
; 3 : $24-$25 - $3541
LDA #$41
STA $24
LDA #$35
STA $25
; $26-$27 - $0d40
LDA #$40
STA $26
LDA #$0d
STA $27
Appear to used by the Kernal.
Now this may be untrue but they are certainly not what I set them to and I can see no code that would manipulate them (barring any sneaky little bugs).
So I'm going to be less worried about speed and get it working using know and tested zero page addresses.
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