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Forums > C64 Coding > $d011 trickery – duplicated text lines
2011-04-29 21:39
Digger

Registered: Mar 2005
Posts: 339
$d011 trickery – duplicated text lines

Does any one know the $d011 trick to repeat the first char row? I am trying to cancel badlines on the 8th line of each charline (between cycles 54-57) but it doesn't work. The first line gets duplicated but only twice.
 
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2019-11-02 14:18
ChristopherJam

Registered: Aug 2004
Posts: 979
Quoting Oswald
if only 7 lines rows possible then its pointless I guess.

Eh, I used a new instance of row 0 every 5 rasters in Jam Ball 2 to keep my framebuffer size down and thought that was reasonably worthwhile :)

Alternately (assuming you can afford to lose another 60 or so cycles) you could do two badlines at the start of every group of eight and use one charset/bitmap for the first line and another for the remaining seven.
2019-11-02 20:17
Oswald

Registered: Apr 2002
Posts: 4462
what if I do the "no badline" row repeat on the last pixel row of a char row, but then manually make a badline in the next raster line ? can this be done ?:P
2019-11-02 22:44
ChristopherJam

Registered: Aug 2004
Posts: 979
Sadly not. The issue is that each “no badline” row repeat secretly updates the row pointer to the next row, so the moment you do a DMA you’d get a later row than what you want,

(Useful for other effects of course, like double buffering half res d800)
2019-11-03 07:50
Oswald

Registered: Apr 2002
Posts: 4462
what you describe sounds like the normal behaviour ? rc to 0 and next row read up.
2019-11-03 09:27
ChristopherJam

Registered: Aug 2004
Posts: 979
Yes, except it doesn't actually fetch the characters, so the internal row buffer remains unchanged.
2019-11-03 10:03
Oswald

Registered: Apr 2002
Posts: 4462
ok while we're at it can you explain last row stretch with internal counters? I'm staring at the VIC article but the more I stare at it the less sense it makes. like RC is never incremented after reaching 7 by these rules:

"In the first phase of cycle 58, the VIC checks if RC=7. If so, the video
logic goes to idle state and VCBASE is loaded from VC (VC->VCBASE). If
the video logic is in display state afterwards (this is always the case
if there is a Bad Line Condition), RC is incremented."

so RC=7, we go to idle state, thus RC is not incremented hence it is only incremented in display state.

I'm thinking of stretching the last pixel row, to get the 8 high char line I want.

ok not possible hence my last is 7th not 8th, and that stretch only works on 8th.

only straw left is repeat 1st line but that adds only a badline, urgh.
2019-11-03 10:35
Oswald

Registered: Apr 2002
Posts: 4462
ok I guess I get it, if *sometime later* there is badline then will we go back to display state and increment rc. whenever.
2019-11-03 10:53
ChristopherJam

Registered: Aug 2004
Posts: 979
Yup, exactly.

Lft's done a nice PDF of the timing, you can find it at https://www.linusakesson.net/programming/vic-timing/
2019-11-03 11:03
JackAsser

Registered: Jun 2002
Posts: 1545
Quote: Yup, exactly.

Lft's done a nice PDF of the timing, you can find it at https://www.linusakesson.net/programming/vic-timing/


Yes. Do read this. It takes a while to understand how to interpret it but once you learn it all VIC-tricks are easy to figure out.
2019-11-03 14:28
Oswald

Registered: Apr 2002
Posts: 4462
already used this pdf, but not the how rc /vc works part, that still needs a lot of thinking :)

fex, I can not explain how/why last row stretch works.
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