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Forums > C64 Coding > asymmetric 38 column border?
2017-05-29 14:32
Paulko64

Registered: Jul 2010
Posts: 24
asymmetric 38 column border?

Is there a particular reason why, if I switch to 38 column mode, the side border is not centered anymore? I.e. the left border comes in 7 pixels, and the right border 9 pixels! Why not 8 pixels on both sides?
If I want to center the screen in 38 column mode, I first have to copy everything one character to the left, and then shift the X fine scroll register to it's max value...
 
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2017-05-29 19:05
Jammer

Registered: Nov 2002
Posts: 670
Hah, I came across that coding my thing ;)
2017-05-29 20:57
lft

Registered: Jul 2007
Posts: 303
I'm puzzled by this too. If they hade used 8 pixels on both sides, they could have used 1 MHz logic for the entire border unit, improving yield.

I've got a theory anyway, although it's pure speculation. The current approach creates a symmetry between yscroll and xscroll: The number written into the register is one less than the number of revealed pixels in the top row or left column respectively.

Still, if they wanted symmetry, they should have used 39 columns, and moved both borders three pixels to the right in the wide configuration.

But we also have to be careful about what we wish for: The way it's done in the VIC chip, with a 9-pixel difference in the right border position, virtually guarantees that there is a cycle in which we can open the sideborder. A 40/39-column system would only have a 4-pixel difference, so d016 might well get sampled at two instances within the same clock cycle. Then the sideborder would remain closed forever.
2017-05-29 21:05
Compyx

Registered: Jan 2005
Posts: 271
Aargh, scary. I need my open borders. Please, somebody close this thread ;)
2017-05-29 21:21
Digger

Registered: Mar 2005
Posts: 246
Wait. How about making a vertical split and close/open the masking border (bit #3) each line. Then you can have your 39 columns. For the high price but still :)
2017-05-29 21:43
JackAsser

Registered: Jun 2002
Posts: 1242
Quote: Wait. How about making a vertical split and close/open the masking border (bit #3) each line. Then you can have your 39 columns. For the high price but still :)

There is a LOOOONG thread about just that.
2017-05-30 06:23
Oswald

Registered: Apr 2002
Posts: 4090
guess its also possible to close the border per line, but its a pointless excersize :)
2017-05-30 07:19
tlr

Registered: Sep 2003
Posts: 1123
Quote: guess its also possible to close the border per line, but its a pointless excersize :)

Maybe to selectively blank out graphics to something else than black?
2017-05-30 21:45
Digger

Registered: Mar 2005
Posts: 246
Is there a thread on splitting $d011? I can't find it.
2017-05-30 21:58
lft

Registered: Jul 2007
Posts: 303
I think it's this one: Left/Right Border switching in middle of rasterline
It's long, but mostly off-topic.
Also, you probably mean splitting d016, right?
2017-06-04 21:32
Digger

Registered: Mar 2005
Posts: 246
@lft: Nope, I actually meant $d011. There was a Crest demo (Crest Light) with split vertical scroll + logo moving independently, but I guess it must be using char animation. So, VSP is made with $d016, line-cruncher forces badlines but is there any other practical/possible use of splitting $d011?
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