Nine – seemingly impossible C64 demo

I think it's probably about the timing. I'm not an expert, but I believe that opening the side borders requires cycle exact timing at both edges of the screen, and when you have characters enabled on the screen, the VIC-II graphics chip ends up 'stealing' memory cycles from the CPU every 8 scanlines to fetch character and font data from RAM. These lines are known as 'badlines'.

Across an entire scanline I think you normally get something like 63 6510 cpu cycles to do 'work' in, but only 23 if you hit a badline - keeping in mind that some instructions take multiple cycles to execute. This probably makes the timing difficult or impossible to manage with the characters turned on.

You can't change the sprites every rasterline. Sprites are 21 pixels in height in unexpanded mode. So you can only change the sprite after displaying all 21 lines, unless you use sprite-crunching, where the limit is still several lines. Linus has an article on sprite-crunching (mind-blowingly complex).

The video demonstrates a second common trick: making use of the black frame "outside" the screen buffer. Once the first demos showed it was possible it quickly spread and nearly everyone used it.

It's been a while, but I recall that you had to do some fancy interrupt timing stuff for that. To find where that was in the code, the same procedure applied that we used to find the handler for background in the code of games: search for 0x78 (SEI), which temporarily blocked interrupts so background stuff could be installed ("pseudo-multitasking").

We used to purchase a tape or 5 1/4" floppy disk with a game per month and instead of playing the game, we competed who could remove the music from the game the fastest (so that it could play in the background alone, e.g. while writing code). In the end that was a matter of just seconds for previously unseen machine code using a hex monitor/disassembler. The advantage of such a "teenage sports" is you will never forget that "169 = A9 = load accumulator" and the rest of the MOS 6510 opcode table, even after not using it for 40 years.

I wonder what today's kids will remember? (They seem to have to Google each Python keyword, and they can't do anything if WiFi is down for an hour, which worries me.)

Discussion of it showed up on HN too: https://news.ycombinator.com/item?id=26511266

I like how the title really invites you to pun it as "a mind is blown", because mine sure was when I first watched it.

Or he's somewhere where enabling audio is not an option.

Some people have sight problems, or are even totally blind. But they still use the Internet.

I don't know about the GP, but for me it meant "I have ten seconds to understand this curiosity, but not 32 minutes. If someone wanted to spend ten seconds explaining to the rest of us, I'd be grateful".

The music is very, very good too.

Video of the explanation, at the timestamp showing 8 sprites that look like 9 sprites: https://youtu.be/Ik1vsMM2EuY?t=1509

> the last two Sprites there are multicolored mode; (...) we have a different number of of high res mode Sprites and multicolored Sprites and expanded Sprites; and those high res and multicolor and some horizontally expanded Sprites are all munged together and then the apparent view with animation of the Sprite data as well the Sprite frames (...) gives an apparent view of having nine independent Sprites; but we know it's not, we know that it's a great big blob of eight Sprites all grouped together horizontally (...)

via youtubetranscript: https://youtubetranscript.com/?v=Ik1vsMM2EuY

Thanks!!

By putting all the sprites on one horizontal line in the top “border”, the demo also proves it’s not using any of the standard tricks to fake extra sprites. The only thing that the C64 will draw up there are real sprites (not character/bitmap graphics), and you can’t multiplex sprites in the same horizontal line of pixels.

Not really the point: that is a tongue-in-cheeck trick, as we know how to solve that problem - but it takes using different stripes of the screen, whereas in the demo they are before 2:00 all put in the same stripe...

At that point we go "Ah."

(It's very normal in illusionism: the practitioner states something that puts you in the relaxed mood of participation to a game you know - then actually surprises you.)

Yeah, you can x and/or y expand a sprite, then change it to make it look like it contains more data by cleverly drawing the sprite because 1 pixel becomes 2 pixels wide in expanded mode.

I don't know the trick, but take a look at the "7". It looks like it's made from the vertical line in "4" and a portion of another number.

That stops working when you open the borders and put sprites outside the area the video chip will render characters/bitmaps.

But in this case, that (alone) can't be it because all 9 sprites clip (smoothly) in and out of the border. So they've really gotten cheeky with this one.

This is an extremely common type of comment on articles like this. The commenter has to ensure everyone knows their credentials before offering any sort of praise to the creator. I also despise this type of self-serving comment.

9 sprites on screen at the same time is trivial (this is known as sprite multiplexing), 9 sprites on the same raster line is the actual 'aha effect' because the C64 can only do 8 sprites per line.