Loadstar 145

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Text File | 1996-01-01 | 8.8 KB | 357 lines

Heaven in the net an unedited excerpt of IRC on #c-64 by Mike Gordillo Note from Jeff: Ooops! I, er, kinda fixed a few spelling and capitalization errors -- so this piece is "mostly unedited." As a self-proclaimed "demo freak", the following transcript (largely unedited) represents one of the most interesting discussions concerning C-64 that I have ever witnessed on IRC (Internet Relay Chat) or any other venue. I present this to the reader in the hopes of encouraging further participation and patronage of IRC channel #c-64. We begin in the middle of a dissection of VIC chip internals by Firefoot. [Mike:] Can anything go on system-wise when a bad line is being "serviced" so to speak? [Firefoot:] "BAD" lines are just another way of specifying the lines where the VIC steals 40 cycles from the CPU to do a screen refresh. You can delay the bad lines, and push the whole screen down (FLD). You can force them to occur every line (FLI). You can turn them off (blanking the screen). You can move them horizontally (VSP). [Firefoot:] The CPU isn't halted, it is busy helping the VIC chip out. Any code that you are executing is "halted". [Waveform:] I never understood VSP. I thought the VIC brought the CPU off-line so it could sweep in all the data for the bitmap in those 40 cycles (for the next 8 lines). [Firefoot:] No, nothing can go on system-wise when a bad line is being serviced, not in terms of the CPU anyway. [Firefoot:] Wave - I guess that would be functionally identical. I guess it depends on whether or not you consider the bus part of the cpu. [Waveform:] The VIC reads its data independent of the CPU, but...I still never understood VSP. =) [Mike:] Yeah, but the CPU and the VIC chip can't share the bus at once. [Firefoot:] I never understood VSP well. There is something about how tricky stuff with $d011 can cause the VIC to think it has started a new scanline, when, infact, it has not. [Mike:] Well..explain FLD for me then, Firefoot. :) [Waveform:] FLD is easy but FLI is not as easy and VSP scares me. [Firefoot:] I've coded VSP, but never understood it either. However I do understand FLD. You just keep playing with $d011 (every scan line) so that the VIC keeps thinking that the [NEXT] scan line is the one where it is supposed to do the refresh of screen memory. When you stop doing this, it starts drawing the screen where it left off. [Waveform:] Exactly, FLD is very easy to understand but what about VSP? [Firefoot:] Actually, I stumbled across VSP when coding FLI. [Waveform:] FLI is essentially the same thing as FLD. Except that instead of making the VIC think the next scan line is where its supposed to draw the screen, FLI makes the VIC think the current line is the one to draw on - but you do it on every line. [Firefoot:] Well, wave, think of VSP as the same thing as FLD except instead of pushing the screen down scan lines, you push it across cycles. Also, try changing the delay at the beginning of an FLI routine, and you will see the screen shift over... voila, vsp! [Waveform:] Well, I just recently got a stable raster (double interrupt style) so I haven't actually done anything that specific (FLI for example) though FLD is very forgiving. You can do nifty FLD with virtually no timing at all. [Mike:] No timing for FLD? ok... Why am *I* having such a hard time putting sprites over FLD then! [Firefoot:] FLI can be made very forgiving as well (no stable raster needed). [Waveform:] Firefoot: It actually uses the CPU's Phi cycle as well as the VIC Phi cycle. [Waveform:] Fire: That is probably what you are remembering. [Firefoot:] Well, with FLI it *is* nice to use a stable interrupt. I use the double raster method as well, always seemed the cleanest to me. [Firefoot:] Oh, it is not easy to put sprites over FLD. You have to make sure that your delay each line is exactly correct. Very weird code... I did that and sprite over FLI with almost the same routine. ugh. [Firefoot:] Wave: That's *exactly* what I am remembering. [Waveform:] Fire: the phi thing? [Mike:] When the FLD bounces down...there -seems- to be a screen area behind it! [Firefoot:] Mike, basically what I did is constructed a section of unrolled FLI/FLD code, and played with the delay instructions each time I moved the sprites so that the timing was always correct. [Firefoot:] Wave, yes, the phi thing. From the appendix about the vic chip. It was very informative. [Mike:] Ok...I've heard these $xfff addies pop up over and over and I guess that explains why the some of my pics have those lines behind the FLD! [Firefoot:] Mike, the area behind the FLD is taken from the last byte of the video bank ($3fff, $7fff, $bfff, or $ffff). It is also what you see when you open up the borders. [Mike:] Gawd, those annoying lines... I couldn't figure out where they were coming from. [Waveform:] Make VICBASE+$3fff equal to 0... or is it $FF?... and the lines will vanish. [Mike:] I also ran a few old demos..and someone (tfo?) was blabbing about how this border was opened up via FLD and the other one wasn't..etc., etc? How does FLD open up a border? [Style:] Is it true it becomes +$x9ff when extended color mode is on? [Firefoot:] Style, I have never heard that, but I have never used extended color mode while doing any of those. [Firefoot:] Mike, the "fld opening up the side borders" thing basically uses the stable raster created with the fld to open the side borders. Easier than other methods, like the double raster method, but sloppy, in my opinion. besides, you can't get text or gfx.... [Mike:] Firefoot, that is all well and good..but "How do you open up the nice sideborders?...period"... Apparently, you are telling me that the FLD is used as an "index" in this case. [Firefoot:] Oh... well... Do you know how to open the top/bottom borders (the theory behind it)? [Waveform:] Well, if you time it right, you make the screen 38 columns instead of 40 right at the last cycle on the raster. [Mike:] Only thing I know how to do is 38/40 column it... :D [Waveform:] Then on the next cycle, the vic thinks it has already started displaying the border... so it doesn't start. [Style:] Which location is the gfx behind an open side border??? [CRoth:] Waveform: I've tried to do that, failed miserably. [Waveform:] Style: VICBASE + $3fff [Waveform:] Anytime you open a border or open screen space and the vic doesn't have normal info to fill it will it takes the last byte in its address space and sticks it in there. [Firefoot:] Croth: Did you make a stable raster first? Without that you will fail miserably. [CRoth:] Firefoot: I couldn't figure it out. Had a friend of mine explain it to me, he was a genius when it came to that. :) [Mike:] What about TWO FLD's? I could swear I have seen one FLD bounce and then another bounce the screen behind it! Is this lunacy on my part? [Waveform:] No it isn't, you stop one FLD...The VIC starts drawing...then later on, you start another FLD... You get two open spaces on your screen! [Style:] And if you can be bothered, you can make a FLD on every line & slice the picture up :D [Waveform:] FLD make the whole screen data move down and up. You can achieve similar effects with sprites... just make all the Y coords go up and down. [Mike:] Ok..can you limit how WIDE the FLD is ? Or is it always full-screen? [Waveform:] You can't FLD on every line... can you? doesn't that pooch your pic? I've seen every eight lines... but every line?!?! I thought that if you FLD after the VIC draws on the next line, you get a STRETCH, don't you? [Style:] Wave, check out "Finely Sliced" by Christopher Jam. [Firefoot:] Mike, I think the FLD is always fullscreen wide. I can't see how it wouldn't be. [Firefoot:] Same here Wave, I've only seen every 8 lines as well. Perhaps you could do some sort of weird cross between FLI and FLD to get every line. Of course, the timing would be most annoying. [Firefoot:] Wave, I've seen that, but it seems to "stretch" every two lines? Like that one arson demo, forget the name, and the one foe demo, and a few others as well. [Firefoot:] [Style], is "Finely Sliced" NTSC fixed? [Waveform:] I think I accidentally did a stretch when trying to code FLD and it seemed like it did every two lines as well...weird. [Mike:] Indulge me, what is a "stretch" ? [Style:] Maybe it is every two lines.. Its been a while since I've seen it. I think Albion made a demo called ache! That did pretty much the same thing. [Style:] Fire: I dont know if it is NTSC fixed... [Firefoot:] Mike, a stretch is an FLD with some slight modifications so that it "stretches" the screen data. [---end of transcript---]