Nine – seemingly impossible C64 demo

252 points
1/21/1970
12 days ago
by appleorchard46

Comments


nonane

Here is a video where Martin Piper uses a C64 debug tools to figure out how Nine was created: https://m.youtube.com/watch?v=Ik1vsMM2EuY .

12 days ago

iamevn

This is probably a really basic question but why does the wizard need to stop being drawn with characters when the sprites move out past the left/right borders? Is this a hardware restriction where those lines aren't able to have text on them when there are sprites on the edges?

11 days ago

FLT8

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.

10 days ago

phire

It's not just that the 'badlines' steal 40 cycles. They steal a solid block of 40 cycles that cover most of the screen, from end of hblank until just before the right border starts. This blog post [1] has a nice interactive demo showing badline timings.

During a badline it's simply impossible write to the VIC-II's registers during the left border. Though, this seems to indicate it's still possible to open the right border during a bad line, but it's a 1 cycle window (maybe 2).

[1] https://nurpax.github.io/posts/2018-06-19-bintris-on-c64-par...

10 days ago

aqueueaqueue

Rabbit holed to this https://linusakesson.net/music/withering-bytes/index.php

Thia is .... Something else!

12 days ago

noman-land

This dude's work is incredible. His entire YouTube channel is amazing. My personal recent favorite, The Commodordion: "A fully functional 8-bit accordion with bellows made out of floppy disks." Legend.

https://www.youtube.com/watch?v=EBCYvoC4muc

12 days ago

junon

I want a Savant remix of that tune. It was so good.

12 days ago

relistan

Wow

12 days ago

TheRealPomax

Can we take a moment to appreciate that the music starts in 10/8, too? Because that's just the cherry on top.

12 days ago

yuchi

Posted it few days ago but didn’t get any traction. All Linus’ work on retro computing is fenomenal. Please have a look at its channel, a lot of awesome content there.

12 days ago

[deleted]
11 days ago

donohoe

I assume it is 8 sprites but the "6" is also used to represent the "9" and jumped (and rotated) from position to position?

12 days ago

badc0ffee

The limit on the C64 is, nominally, 8 sprites. Widely known tricks exist to increase that to 8 per raster line, by IIRC repositioning them during horizontal retrace.

This demo actually shows 9 "sprites" on the same raster line using some clever tricks like low-res stretched sprites combined with custom characters. It all makes sense once you know the tricks, but the really impressive thing is how smooth it is. Which I guess is why it features a magician.

12 days ago

jll29

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.)

11 days ago

badc0ffee

> they can't do anything if WiFi is down for an hour, which worries me.

I worked with a younger guy on a pre-existing, large project written in C. He saw me use the man command and asked why I would do that - was I expecting to be offline? But I don't think of man pages that way, I still think of them as primary documentation. They're also specific to the software versions on the system I'm logged into, unlike whatever comes up when I Google `man 2 unshare`.

11 days ago

pwrrr

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).

11 days ago

Luc

> Linus has an article on sprite-crunching (mind-blowingly complex).

https://www.linusakesson.net/scene/lunatico/misc.php

Great article.

11 days ago

pwrrr

Indeed :)

11 days ago

JKCalhoun

That was my guess — but I'm not familiar with the C-64 and its sprite limitations.

12 days ago

siltcakes

As soon as I saw who made this, I cranked my speakers and wasn't disappointed! Linus is amazing. I recommend his cover of The Moon from DuckTales:

https://linusakesson.net/chipophone/ducktalesmoon.php

12 days ago

jetbalsa

I'm a huge fan of his A Mind is born demo 256 byte demo he did awhile back, freaking amazing

https://linusakesson.net/scene/a-mind-is-born/index.php

12 days ago

userbinator

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.

12 days ago

thih9

Is there a text explanation of the trick - for those that cannot watch a video?

12 days ago

masswerk

There are some basic debugging notes by Martin Piper: https://github.com/martinpiper/DebuggingDetails/blob/main/Ni...

For better understanding: The demo starts with a magician character drawn in the center of the screen, announcing the trick in text (some of this is big text achieved by overlaying sprites). Then, 9 sprites, each in the form of a colored number from 1 to 9, exit the magician's hat to rotate around him (in the notes screen #1), then, the screen gradually widens horizontally beyond the usual borders and the circle of the 9 apparent sprites spreads out over the entire screen (screen #2). (At this point, the display of the magician, which was clearly drawn by PETSCII characters, before, shouldn't be viable anymore.) The swirling numbers transition to a narrow ellipse near the top of the screen (screen #3), which then transition to the very top of the screen, above the normal display area, a zone, which is available for sprites only, showing the sprites in a linear rotation (screen #4). Meaning, these must be sprites and these 9 apparent sprites are drawn at the same scan-lines. (So there is no way this could have been achieved by conventional, vertical multiplexing. This is the highlight, the big trick of the magician's show.) At the end, the sprites return to their initial circling motion and vanish, one by one, into the magician's hat.

The demo is a combination of various tricks and artful timing, which enables perfect transitions between them.

11 days ago

thih9

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

11 days ago

isoprophlex

The music is very, very good too.

11 days ago

exitb

The demo shows nine separate sprites, even though C64 supports showing only eight. It starts off by separating them vertically, which is a common trick, but then moves them all to the same raster lines and overlapping locations, which is much more surprising.

11 days ago

nottorp

By "cannot" i assume you mean you watched the demo video but your life is too short to waste it on video explanations of assembly tricks that are much better done in text? :)

11 days ago

stavros

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".

11 days ago

human3047628272

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

11 days ago

nottorp

Yeah but the OP sounds like they saw the demo and are wondering how it's done now.

11 days ago

account42

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

11 days ago

[deleted]
11 days ago

[deleted]
12 days ago

nerdralph

My guess is it is chasing the beam. Change the border color from black to your sprite color, then back to black again. The change takes a minimum of 4 cycles (store absolute), which I think explains the thickness of the numbers. Some calculations involving the PAL scan line frequency could confirm if 4 cycles is quick enough.

edit for further detail: This would be used for one of the numbers, with the other 8 using sprites.

11 days ago

nerdralph

As mentioned in other posts, the trick is using multi-color sprites to look like multiple single-color sprites.

I did the scan line frequency calculations, and my though of changing the border color would not work. In one cycle the scanline advances 8 pixels, so toggling the border color would be a minimum of 32 pixels in width.

11 days ago

yumraj

I didn’t get. Can someone please explain what’s impossible about it?

12 days ago

themadturk

The problem is presented in the intro screen: The Commodore 64 only has eight independent sprites, yet this demo showed what appear to be nine sprites.

12 days ago

panic

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.

12 days ago

mdp2021

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.)

11 days ago

yumraj

Thanks!!

12 days ago

aappleby

Eight hardware sprites and one blitted to the background every frame?

12 days ago

erik

That works until you see what appears to be 9 sprites on the same scan line in the border region.

12 days ago

badc0ffee

Not quite. Drawing a full sprite-sized bitmap would not be so smooth on the C64.

12 days ago

pwrrr

They are in the border, Only sprites, ghostbyte (a repeating byte only shown in black. It is the last byte of the chosen videobank) and background color can be displayed there.

11 days ago

boltzmann-brain

VIC II doesn't have that functionality. the first commodore that could do this was the Amiga series.

12 days ago

neuroelectron

I probably don't understand it completely but from what I gathered from Martin Piper's video is that the spite locations and data are updated during the scan lines allowing the renderer to reuse the same hardware sprites before the scanline is complete.

12 days ago

pimlottc

Just guessing here, but does it have anything to do with the fact that 9 is the same as 6 rotated?

12 days ago

boltzmann-brain

no. there are 9 separately moving objects and them being visually similar or even the same doesn't mean much

12 days ago

kbelder

Neat. Having more than 8 sprites on screen is not hard, it just requires some well-timed interrupts to move a sprite from above the current raster location to below it, so that the same sprite is drawn multiple times.

But I'm not sure about the end of the demo where they are all rotating on the same line. I guess it could be done with interrupts occurring partway through the horizontal scanline, but I didn't think they could be that precise?

12 days ago

Dylan16807

I don't like how you worded this post. You know the demo's point is having all 9 go at the same time, so it's quite misleading to start off describing something that sounds like the demo's point, but is not the demo's point, and saying it's "not hard". Your second paragraph mostly corrects that false impression, but I dislike that you gave a false impression to start with.

12 days ago

flohofwoe

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.

12 days ago

Dylan16807

I'm not sure why you replied to me? Are you nitpicking that I said "go at the same time" because I think it's clear enough in context.

11 days ago

Underphil

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.

12 days ago

egypturnash

Nope, that’s not it. There’s a video out there already that dissects it and figures out the trick.

12 days ago

peterleiser

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.

12 days ago

badc0ffee

This is true, but not relevant to the trick.

12 days ago

boltzmann-brain

you can't multiplex sprites so that more than 8 appear on the same line.

the secret is to pretend you have a 9th sprite by emulating it with characters that look exactly like one of the sprites would.

12 days ago

jchw

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.

12 days ago

[deleted]
12 days ago

egypturnash

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

12 days ago

eru

Yes, but the demo is really cheeky about that.

The borders are open when you can still do the normal multiplexing, because there's lots of vertical separation between the sprites.

Later, when they put the sprites together on the same line, the borders are closed.

12 days ago

o11c

Youtube comments said something about "multiple apparent sprites per actual sprite, with stretching"?

12 days ago

pwrrr

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.

11 days ago

flohofwoe

The VIC-II reads the data for each sprite at hardwired horizontal positions outside the visible area, so this sort of 'horizontal multiplexing' shouldn't work.

12 days ago

[deleted]
12 days ago

pinoy420

All you need is a little “Just”(TM) and everything is possible.

Typical HN comment minimising the immense effort required to do %thing% due to their own arrogance.

12 days ago