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Forums > C64 Coding > Screen with no Badlines
2004-06-14 08:49
Oswald

Registered: Apr 2002
Posts: 5127
Screen with no Badlines

Hello there!

I know it's possible to have a screenmode with no bad lines at all, graphics are not displayable, but sprites are visible in this one. Once I've coded it but now I totally forgot how it was done.. some d011 fiddling at the start of the bottom border.

Can anyone tell me how to achive this ? Whats the theory ?
 
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2004-06-23 18:07
SIDWAVE
Account closed

Registered: Apr 2002
Posts: 2238
Interresting stuff!

I'd sure like to see that EGMOS thing
2004-07-07 17:17
Barfly

Registered: Jan 2002
Posts: 5
Actually you can do this trick even without opening the upper/lower borders...

.Barfly
2004-07-09 11:12
bOOZElEE

Registered: Dec 2002
Posts: 36
@ rambones: scpu owner?
2004-07-09 12:11
WVL

Registered: Mar 2002
Posts: 924
Quote: Actually you can do this trick even without opening the upper/lower borders...

.Barfly


how? you have to avoid the VIC starting drawing the border, else the whole screen will flush!
2004-07-09 13:00
Krill

Registered: Apr 2002
Posts: 3098
yeah. maybe barfly meant just a full-screen fld to scroll out all badlines =) but that would mean d011 writes at least every 6 lines.
2004-07-10 11:57
VIC

Registered: Aug 2003
Posts: 75
Quote: how? you have to avoid the VIC starting drawing the border, else the whole screen will flush!


*draw, draw, draw*
2004-07-11 18:11
Ninja

Registered: Jan 2002
Posts: 418
I hope I am not spoiling Barfly's fun, but sharing knowledge rules ;)

Badlines can just occur if "Display Enable" was set in line $30 for at least one cycle. So, make sure that it is cleared during line $30, and the it is set before $33, so that the border-flip-flops will behave like having a "normal" screen.
2004-07-11 18:23
Ninja

Registered: Jan 2002
Posts: 418
Example code:

SEI
loop3:
LDA #$2F
loop1:
CMP $D012
BNE loop1
BIT $D011
BMI loop1
LDA #$0B
STA $D011
LDA #$31
loop2:
CMP $D012
BNE loop2
LDA #$1B
STA $D011
BNE loop3
2004-07-13 11:49
WVL

Registered: Mar 2002
Posts: 924
hmmm..... okay, this should work too :) but basically the problem is the same. Whether you enable the screen before line $33 or open the border around $fa, it does not matter...

timing-wise they are both equally simple [or hard if you want a flexible irq over it], plus you get a free open upper+lower border :)
2004-07-13 15:44
Ninja

Registered: Jan 2002
Posts: 418
Of course, they are pretty similar. It was just the answer to the question "how?" :)
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