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Posted By

siz
on 2024-10-20
03:55:30

Re: smooth scrolling - strange behaviour

$ff07 underflows and turns 256 characters mode on by turning bit 7 on. In 256 characters mode the character set must be aligned at 2kB boundaries. So it effectively switches your charset address to $d000.

Posted By

Fuzzweed
on 2024-10-19
19:56:49

Re: smooth scrolling - strange behaviour

OK I figured out the basic mistake. It should be

and #$F0
ora #$07

which explains the columns going wrong - but how was it affecting the chr set? Thats a different address completely

Posted By

Fuzzweed
on 2024-10-19
19:31:45

smooth scrolling - strange behaviour

If I do:
lda $FF07 ;horizontal_scroll
and #$F7 ;shrink screen, set scroll pos. to 7
sta $FF07

lda #$D4 ;set up/low chr rom
sta $FF13 ;chr bank select

Then the correct chr set and 38 columns is correct

BUT if I then add:

ldx #$07
--
dec $FF07 ;decrease scroll position
[DELAY]
dex
bne --

the smooth scrolling works ok, but my chr set is wrong (upper/grx) and the screen goes to 40 column. What has gone wrong?