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randyrossi / vicii-kawari

Commodore 64 VIC-II 6567/6569 Replacement Project
GNU General Public License v3.0
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Glitches on Higher Level (1992) demo by Lower Level #8

Closed randyrossi closed 2 years ago

randyrossi commented 2 years ago

reported by machete on discord

"In a demo I notice some glitches you can try Higher Level [1992] by Lower Level. You will see what I mean during the intro of the demo and then later on on a scroll scene. I was running under PAL mode and tried both oscillator modes, but made no difference."

https://discord.com/channels/934158426714550282/934158427834441730/1018545490956124253

randyrossi commented 2 years ago

Okay, so believe it or not this is actually due to the KERNAL version. ROM versions kernal.901227-01 and kernal.901227-02 will glitch out as you reported but kernal.901227-03 does not. It has to do with the way the screen clear routine works. (I don't remember the exact details but it has to do with setting the color cells to the background color instead of the foreground).

So this demo is probably expecting the behavior of rev 3 . If you run VICE with versions 1 & 2, you will see the exact same glitches you reported. By default, VICE gets rev 3 so that's why I didn't see the glitches. Those stray pixels are there even in rev 3 but they are black so you don't see them.

I've been caught by this difference before. The Galencia game disk has a trainer that prompts you to answer 3-4 Y/N questions. But on the older kernel revisions, you get a black screen. It looks like it crashed but it's actually just waiting for you to answer questions you can't see. But on rev 3, you see the questions.

I tried the demo on Kawari on a REV3 kernel and it renders properly.