From 40 Raster Lines to 1

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By Kodiak

If you've read my Commodore 64 assembly tech article, Bank-Switched Scrolling, you will remember how I had to find a way to compress Parallaxian's scroll code for the distant mountains in the parallax landscape down from the original approximately 40 raster lines, and ended up getting it squeezed into a much more compact 9.

That was achieved using double-buffering, a staple Commodore 64 coding trick, to spread the time-consuming software (char) scrolling over several screen frames of hardware scrolling (a full explanation is in the article).

Bank-Switched Leapfrog Scroll Method

However, after musing upon comments from another coder on my recent article, Wild Wood Deconstructed, I thought it would be possible to have Parallaxian's main loop do the heavy lifting of the double-buffer scrolling when the interrupts are idle.

Sure, it's not an ideal solution for every layer of the parallaxed landscape, and yes, the main loop is filling up with more and more tasks, but for now there is enough cumulative interrupt idle time spread across the frames consumed between char scroll events for that slowest scrolling layer to allow the char-shifting components of its double-buffering to be removed from the interrupt schema altogether.

Ultra compressed layer 1 scroll

This was done, of course, with one eye on a possible NTSC version of the game, most likely to manifest in the form of the planned native Commodore 128 version, which may come in both NTSC and PAL flavours... subscribe to my 100% free NEWSLETTER to be kept up to date on this.


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...and get special perks!

Progress on Parallaxian has slowed down since summer 2021 for several reasons, one of which has been the very low level of support from the C64 scene which has made it difficult to continue justifying to my family the long hours of hard work a project as complex as this requires.

Now, I understand these are difficult times and I admit I am not entitled to any support at all, but it really does encourage me to continue developing this sensational game when you make a Paypal donation.

And as a special thank you, all who do this can enjoy the following perks:

  • Your name credited in the game (unless you opt out of it if you have the same kind of incognito hermit tendencies I do).
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  • The finished game on media (e.g. cartridge) for FREE one week before its release.