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  • Why does the colour map m

    From Dennis Katsonis@21:1/5 to Lawrence Woodman on Sat Dec 12 13:23:00 2020
    Lawrence Woodman wrote to All <=-

    Hello,

    Does anyone know why the colour map moves between $9400 and $9600 on
    the Vic? Is there a reason it couldn't stay at a single location such
    as $9400?

    I've wondered about this for a while and there is probably an obvious reason that I'm missing but I would love to know. My interest in this
    was renewed recently when I was creating a video (links below) which
    shows the colour map being used as a location to store data and now it seems I really should find out. I've posted about this on vcfed.org
    but there's been no answer yet so am hoping someone here will know.

    Run Code and Store Data in Display Memory on the VIC-20
    Video: https://www.youtube.com/watch?v=EkfBijC8MSU
    Article: https://techtinkering.com/articles/code-and-data-in-display-memory-on-th e-vic-20/


    Best wishes

    I think it had to do with how the VIC chip addressed memory internally. It was only able to "see" a portion of the Vic 20 memory map, and if you moved the screen memory, then the color RAM would also have to move so it would map correctly.

    http://tinyvga.com/6561

    ... Dennis Katsonis
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  • From Lawrence Woodman@21:1/5 to Dennis Katsonis on Fri Dec 18 12:41:52 2020
    On Sat, 12 Dec 2020 13:23:00 +1300, Dennis Katsonis wrote:

    Lawrence Woodman wrote to All <=-
    Does anyone know why the colour map moves between $9400 and $9600 on
    the Vic? Is there a reason it couldn't stay at a single location such
    as $9400?

    I think it had to do with how the VIC chip addressed memory internally. It was
    only able to "see" a portion of the Vic 20 memory map, and if you moved the screen memory, then the color RAM would also have to move so it would map correctly.

    http://tinyvga.com/6561

    Unfortunately, I don't think it really solves the mystery as while I
    understand that the 14-bit address used by the VIC restricts where the
    screen map can be, I don't think this alone explains why the colour map
    changes between $9400 and $9600 as it uses the same number of bit
    positions either way.

    Having said that, the link you gave has some excellent information and I
    love how it is presented. Thanks for posting it.

    Lorry

    ---
    Run Code and Store Data in Display Memory on the Commodore VIC-20 https://techtinkering.com/articles/code-and-data-in-display-memory-on-the-vic-20/

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