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682 lines
26 KiB
TeX
682 lines
26 KiB
TeX
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\fancyhead[C]{A version of this document appeared in PoC~\textbar\textbar~GTFO 0x18}
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%\fancyhead[CO,CE]{A version of this document appeared in PoC || GTFO 0x18}
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\begin{document}
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\title{Making an 8k Low-resolution Graphics Demo for the Apple II}
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\author{by DEATER, AKA Vincent M. Weaver}
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\date{}
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\maketitle
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\thispagestyle{firststyle}
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\section{Why would anyone do this?}
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While making an inside-joke filled game for my retro system of choice,
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the Apple~II, I needed to create a Final-Fantasy-esque
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flying-over-the-planet sequence.
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I was originally going to fake this, but why fake graphics when you
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can laboriously spend weeks implementing the effect for real?
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It turns out the Apple~II is just barely capable of generating
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the effect in real time.
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Once I got the code working I realized it would be great as part of a
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graphical demo, so off on that tangent I went.
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This turned out well, despite the fact that all I knew about the demoscene I
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had learned from a few viewings of the Future Crew {\em Second Reality} demo
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combined with dimly remembered Commodore 64 and Amiga usenet flamewars.
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% from a few decades ago.
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% This started out as some SNES style mode7 pseudo-3d graphics code
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% I came up with while working on my TF7 game. The graphics looked
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% pretty cool, so I started developing a demo around it.
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%To make thins even better, the code ended up being roughly around 8kB so a
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%lot of time was wasted fitting it under that arbitrary size limitation.
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While I hope you enjoy the description of the demo and the work that
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went into it, I suspect this whole enterprise is primarily of note
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due to the dearth of demos for the Apple~II platform.
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%So in the end this ends up being impressive mostly because so few people
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%have bothered to write demos for this particular platform.
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If you are truly interested in seeing impressive Apple~II demos,
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I would like to make a shout out to FrenchTouch whose works
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put this one to shame.
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% The codesize ended up being roughly around 8kB, so I thought I'd
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% make it into an 8k demo. There aren't many out there for the Apple II.
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% and a Mockingboard sound card.
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% The demo tries to hit the lowest common denominator for Apple II systems,
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% so in theory you could have run this on an Apple II in 1977 if you
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% were rich enough to afford 48k of RAM. The Mockingboard sound wasn't
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% available until 1981, but still this all predates the Commodore 64.
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%I was writing a game for the Apple II and realized I had come up with
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%some clever Super-Nintendo (SNES) style graphics routines that were just
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%crying to be turned into a demo-scene style demo.
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%The Apple II was the first computer I had access too, and I grew up in an odd
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%neighborhood where it was all Apples and not a Commodore to be seen.
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%My family long ago got rid of our machine, but I rescued an Apple IIe platinum
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%from the dumpster one day and have dragged it from state to state ever since.
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%I find 6502 assembly to be oddly therapeutic, and will code in it when other
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%projects become too stressful. Especially when Linux up and hangs on me
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%because firefox tried to do something stupid in javascript. I then pine for
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%the days when you could do something useful in 64k of RAM, and not have your
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%machine fall over because somehow 4GB is not enough.
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%Background:
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%The Apple II was the first computer I programmed on, lo many years ago.
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%Mostly in Applesoft BASIC (which ended up being the only Microsoft product
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%I ever liked) but I was starting to get into assembly language about the
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%time my family got a 386 system.
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%I've revisited over the years, with some 6502 programming to show I could.
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%My skills were not that great, I had one of my size-optimization projects
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%crowd re-optimized. For a while I had a side-gig re-optimizing modern games
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%in BASIC, before getting sidetracked into going full in on 6502 assembly
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%again.
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%Introduced in 1977.
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%The Apple II runs at 1.XX check Megahertz. 6502, which can easily
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%address 64 kB of RAM (more with bank switching). Shipped with as little
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%as 4kB of RAM. Three registers, (A,X,Y) but a large ``zero page'' which
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%gives you register-like actions on the first 256 bytes of RAM.
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%
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%DOS3.3 operating system with 140k floppies. Amazing programming by Wozniak,
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%allowing all kinds of floppy protection shenanigans (cite 4am, previous
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%article).
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\section{The Hardware}
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The Apple~II was introduced in 1977.
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In theory this demo will run on hardware that old, although I do
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not have access to a system of that vintage.
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I like to troll Commodore fans by noting this predates the Commodore 64 by
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five years.
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\vspace{1ex}
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\noindent
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{\bf CPU, RAM and Storage:}
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The Apple II has a 6502 processor running at roughly 1.023MHz.
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Early models only shipped with 4k of RAM, but later 48k, 64k, and 128k
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systems were common.
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While the demo itself fits in 8k, it decompresses to a larger size and uses
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a full 48k of RAM;
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this would have been very expensive in 1977.
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See Figure~\ref{fig:map} for a diagram of the memory map.
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Also in 1977 you would probably be loading this from cassette tape.
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It would be another year before Woz's single-sided
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$5\frac{1}{4}$" Disk II came about (eventually offering 140k of
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storage per side with the release of Apple DOS3.3 in 1980).
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\vspace{1ex}
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\noindent
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{\bf Sound:}
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The only sound available in a stock Apple II is a bit-banged speaker.
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There was no timer interrupt; if you wanted music you had to cycle-count
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via the CPU to get the waveforms you needed.
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The demo uses a Mockingboard soundcard which was introduced in 1981.
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This board contains dual AY-3-8910 sound generation chips connected via
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6522 I/O chips.
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Each sound chip provides 3 channels of square waves as well as noise and
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envelope effects.
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\vspace{1ex}
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\noindent
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{\bf Graphics:}
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It is hard to imagine now, but the Apple II had nice graphics for its time.
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Compared to later competitors, however, it had some limitations.
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\begin{center}
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\begin{tabular}{|c|c|}
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\hline
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Hardware Sprites & No \\
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User-defined charset & No \\
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Blanking interrupts & No \\
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Palette selection & No \\
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Linear framebuffer & No \\
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Hardware scrolling & No \\
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Hardware page flip & Yes \\
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\hline
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\end{tabular}
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\end{center}
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The hi-res graphics mode is a complex mess of NTSC hacks by Woz.
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You get approximately 280x192 resolution, with 6 colors available.
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The colors are NTSC artifacts with limitations
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on which colors can be next to each other (in blocks of 3.5 pixels).
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There is plenty of fringing on edges, and colors change depending on
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whether they are drawn at odd or even locations.
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To add to the madness, the framebuffer is interleaved in a complex way,
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and pixels are drawn least-significant-bit first (all of this to get
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DRAM refresh for free and to shave a few 7400 series logic chips from
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the design).
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You do get two pages of graphics, Page 1 is at
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{\tt \$2000}\footnote{On 6502 systems hexadecimal values are
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traditionally indicated by a dollar sign}
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and Page 2 at {\tt \$4000}.
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Optionally 4 lines of text can be shown at the bottom of the
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screen instead of graphics.
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The lo-res mode is a bit easier to use.
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It provides 40x48 blocks, reusing the same memory as the 40x24 text mode.
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(As with hi-res you can switch to a 40x40 mode with four lines of
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text displayed at the bottom).
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Fifteen colors are available (there are two greys which are indistinguishable).
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Again the addresses are interleaved in a non-linear fashion.
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Lo-res Page 1 is at {\tt \$400} and Page 2 is at {\tt \$800}.
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Some amazing effects can be achieved by cycle counting, reading
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the floating bus, and racing the beam while toggling graphics
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modes on the fly.
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Unfortunately for you this demo does not do any of those things
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so you will not be reading about that today.
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%Later models added double low-res (80x48) and double hi-res (x y in
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%NTSC 15 color) but didn't appear until 198x, and only on later IIe, IIc
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%models.
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%Apple also came out with the IIgs which arguably was much more advanced
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%and cheaper than the Mac, but Apple cancelled the II line much to the
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%sadness of the users (Apple II forever).
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\section{Development Setup}
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I do all of my coding under Linux, using the nano text editor.
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I use the ca65 assembler from the cc65 project, which I find to be a reasonable
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tool although many ``real'' Apple II programmers look down on it for some
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reason.
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I cross-compile the code, constructing Apple DOS3.3 disk images using
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custom tools I have written.
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I test using emulators:
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AppleWin (run under the wine emulator) is the easiest to use, but
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until recently MESS/MAME had cleaner sound.
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Once the code appears to work, I put it on a USB stick and transfer
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to actual hardware using a CFFA3000 disk emulator installed in
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the actual Apple II (an Apple IIe platinum edition).
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%\section{Related Work}
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%
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%See anything by the group FrenchTouch, whose Apple II demos outclass
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%mine by a lot.
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% http://www.deater.net/weave/vmwprod/mode7_demo/
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\begin{figure}[tb]
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\begin{center}
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\includegraphics[width=2in]{figures/hidden_vmw.png}
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\end{center}
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\caption{VMW logo hidden in the executable data.\label{fig:vmw}}
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\end{figure}
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\begin{figure}[tb]
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\begin{center}
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\includegraphics[width=\columnwidth]{figures/mode7_demo_title.png}
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\end{center}
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\caption{The title screen.\label{fig:title}}
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\end{figure}
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\begin{figure}[tb]
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\begin{center}
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\includegraphics[width=\columnwidth]{figures/m7_screen1.png}
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\caption{Bouncing ball on infinite checkerboard.\label{fig:ball}}
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\end{center}
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\end{figure}
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\begin{figure}[tb]
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\begin{center}
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\includegraphics[width=\columnwidth]{figures/m7_screen4.png}
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\caption{Spaceship flying over an island.\label{fig:tb1}}
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\end{center}
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\end{figure}
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\begin{figure}[tb]
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\begin{center}
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\includegraphics[width=\columnwidth]{figures/m7_screen3.png}
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\end{center}
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\caption{Spaceship with starfield.\label{fig:stars}}
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\end{figure}
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\begin{figure}[tb]
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\begin{center}
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\includegraphics[width=\columnwidth]{figures/m7_screen2.png}
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\end{center}
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\caption{Rasterbars, stars, and credits. Stealth Susie was a particularly
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well-traveled guinea pig.
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\label{fig:credits}}
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\end{figure}
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\section{The Demo}
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\subsection{BOOTLOADER}
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An Applesoft BASIC ``HELLO'' program loads the binary automatically at bootup.
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This does not count towards the executable size, as you could manually BRUN
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the 8k machine-language program if you wanted.
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To make the loading time slightly more interesting the HELLO program enables
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graphics mode and loads the program to address {\tt \$2000} (hi-res page1).
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This causes the display to filled with the colorful pattern corresponding
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to the compressed image.
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This conveniently fills all 8k of the display RAM, or would have
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if we had POKEd the right soft-switch to turn off
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the bottom 4 lines of text.
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Upon loading, execution starts at address {\tt \$2000}.
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\subsection{DECOMPRESSION}
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The binary is encoded with the LZ4 algorithm.
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We flip to hi-res Page 2 and decompress to this region so the display
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now shows the executable code.
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The 6502 size-optimized LZ4 decompression code was written by qkumba
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(Peter Ferrie).
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% http://pferrie.host22.com/misc/appleii.htm
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The program and data decompress to around 22k starting at {\tt \$4000}.
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This over-writes parts of DOS3.3, but since we are done with the disk
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this is not an issue.
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If you look carefully at the upper left corner of the screen during
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decompress you will see my triangular logo, which is supposed to evoke
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my VMW initials (see Figure~\ref{fig:vmw}).
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To do this I had to put the proper bit pattern inside the code
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at the interleaved addresses of {\tt \$4000}, {\tt \$4400}, {\tt \$4800},
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and {\tt \$4C00}.
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The image data at {\tt \$4000} maps to (mostly)
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harmless code so it is left in place and executed.
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Making this work turned out to be more trouble than it was worth, especially
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as the logo is not visible in the youtube capture of the demo (the video
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compression does not handle screens full of seemingly random noise well).
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The demo was optimized to fit in 8k.
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Optimizing code inside of a compressed image is much more complicated than
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regular size optimization.
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Removing instructions sometimes makes the binary {\em larger} as it no longer
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compresses as well.
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Long runs of values (such as 0 padding) are essentially free.
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This mostly turned into an exercise of guess-and-check until everything fit.
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\subsection{TITLE SCREEN}
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Once decompression is done, execution continues at address {\tt \$4000}.
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We switch to low-res mode for the rest of the demo.
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\noindent
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{\bf FADE EFFECT}:
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The title screen fades in from black.
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This is a software hack as the Apple II does not have palette support.
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The image is loaded to an off-screen buffer and a lookup table is used to
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copy in the faded versions on the fly.
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\noindent
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{\bf TITLE GRAPHICS}:
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The title screen is shown in Figure~\ref{fig:title}.
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The image is run-length encoded (RLE) which is
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probably unnecessary in light of it being further LZ4 encoded.
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(The LZ4 compression was a late addition to this endeavor).
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Why not save some space and just load our demo at {\tt \$400} and negate
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the need
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to copy the image in place?
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Remember the graphics are 40x48 (shared with the text display region).
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It might be easier to think of it as 40x24 characters, with the top / bottom
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4-bits of each ASCII character being interpreted as colors for a half-height
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block.
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If you do the math you will find this takes 960 bytes of space, but the memory
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map reserves 1k for this mode.
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There are ``holes'' in the address range that are not displayed, and
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various pieces of hardware can use these as scratchpad memory.
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This means just overwriting the whole 1k with data might not work out well
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unless you know what you are doing.
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To this end our RLE decompression code skips the holes just to be safe.
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\noindent
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{\bf SCROLL TEXT}:
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The title screen has scrolling text at the bottom.
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This is nothing fancy, the text is in a buffer off screen and a 40x4
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chunk of RAM is copied in every so many cycles.
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You might notice that there is tearing/jitter in the scrolling even
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though we are double-buffering the graphics.
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Sadly there is not a reliable cross-platform way to get the VBLANK info
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on Apple II machines, especially the older models.
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This is even more noticeable in the recorded video, as the capture card and
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video encoding conspire to make this look worse than things look in person.
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\subsection{MOCKINGBOARD MUSIC}
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No demo is complete without some exciting background music.
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I like chiptune music, especially the kind written
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for AY-3-8910 based systems.
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During the long time waiting for my Mockingboard hardware to arrive
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I designed and built a Raspberry Pi chiptune player that uses
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essentially the same hardware.
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This allowed me to build up some expertise with the software/hardware
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interface in advance.
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The song being played is a stripped down and re-arranged version of
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``Electric Wave'' from CC'00 by EA (Ilya Abrosimov).
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Most of my sound infrastructure involves YM5 files, a format commonly
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used by ZX Spectrum and Atari ST users.
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The YM file format is just AY-3-8910 register dumps taken at 50Hz.
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To play these back one sets up the sound card to interrupt 50 times a second
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and then writes out the 14 register values from each frame in an interrupt
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handler.
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% To program the Mockingboard, each AY-3-8910 chip has 14 sound related
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% registers that control the 3 channels. Each AY chip has a dedicated
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% VIA 6522 parallel I/O chip that handles the I/O.
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Writing out the registers quickly enough is a challenge on the Apple II.
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For each register you have to do a handshake then set both the register
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number and the value.
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It is hard to do this in less than forty 1MHz cycles for each register.
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With complex chiptune files (especially those written on an ST with much
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faster hardware) it is sometimes not possible to get exact playback
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due to the delay.
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Further slowdown happens as you want to write both AY chips (the output
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is stereo, with one AY on the left and one on the right).
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To help with latency on playback we keep track of the last frame written
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and only write to the registers that have changed.
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% I have a whole suite of code for manipulating YM sound data, in my
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% vmw-meter git repository.
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Our code detects the Mockingboard at startup; we are lazy and only support
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finding the card in Slot 4 (which is a fairly typically location).
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% The first step for getting this to work is detecting if a Mockingboard is
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%% there. This can be in any slot 1-7 on the Apple II, though typically
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% Slot 4 is standard (in this demo we only check slot 4).
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The board is initialized, and then one of the 6522 timers is set to
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interrupt at 25Hz.
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% (it has to be an on-board timer as the default
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% Apple II has no timers).
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Why 25Hz and not 50Hz? At 50Hz with 14 registers you use 700 bytes/s.
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So a 2 minute song would take 84k of RAM, which is much more than is available.
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Also the Disk II requires hard real-time response involving the full
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CPU to read from disk, so it is not possible to read more data while
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the demo is running.
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To allow the song to fit in memory (without the fancy circular buffer
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decompression routine utilized in my VMW Chiptune music-disk demo) we have
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to reduce the size.
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First the music is changed so it only needs to be updated at 25Hz.
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Then the register data is compressed from 14 bytes to 11 bytes by stripping off
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the envelope effects and packing together fields that have unused bits.
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In the end the sound quality suffered a bit, but we were able to fit an
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acceptably catchy chiptune inside of our 8k payload.
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\subsection{MODE7 BACKGROUND}
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``Mode7'' is a Super Nintendo (SNES) graphics mode that takes a tiled
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background and transforms it by rotating and scaling.
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The most common effect squashes the background out to the horizon, giving
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a three-dimensional look.
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The SNES did these transforms in hardware, but our demo must do
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them in software.
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% As found on Wikipedia, the transform is of the type
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%
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% [x'] = [a b]([x]-[x0])+[x0]
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% [y'] [c d]([y] [y0]) [y0]
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% http://www.helixsoft.nl/articles/circle/sincos.htm
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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Our algorithm is based on code by Martijn van Iersel.
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It iterates through each horizontal line on the screen and calculates the color
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to output based on the camera height ({\em spacez}) and {\em angle} as well
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as the current x and y coordinates ({\em cx} and {\em cy}).
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First the distance {\em d} is calculated based on fixed scale and
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distance-to-horizon factors.
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Instead of a costly division we use a pre-generated lookup table for this.
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\[d = \frac{z \times yscale}{y+horizon}\]
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Next calculate the horizontal scale (distance between points on
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this line):
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\[h = \frac{d}{xscale}\]
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Then calculate delta x and delta y values between each block on the line.
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We use a pre-computed sine/cosine lookup table.
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\pagebreak
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\[dx = -sin(angle) \times h\]
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\[dy = cos(angle) \times h\]
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The leftmost position in the tile lookup is calculated:
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\[tilex = cx + (d*cos(angle) - (width/2) * dx\]
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\[tiley = cy + (d*sin(angle) - (width/2) * dy\]
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Then an inner loop happens that adds dx and dy as we lookup the color
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from the tilemap (just a wrap-around array lookup) for each block
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on the line.
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\[color = tilelookup(tilex,tiley)\]
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\[plot (x, y) \]
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\[tilex += dx, tiley+= dy\]
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\noindent
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{\bf Optimizations:}
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The 6502 processor cannot do floating point, so all of our routines use
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8.8 fixed point math.
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We eliminate all use of division, and convert as much as possible
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to table lookups (which involves limiting the heights and angles a bit).
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We also save some cycles by using self-modifying code,
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most notably hard-coding the height (z) value and modifying the code
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whenever this is changed.
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The code started out only capable of roughly 4.9fps in 40x20 resolution
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and in the end we improved this to 5.7fps in 40x40 resolution.
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Care was taken to optimize the innermost loop, as every cycle saved there
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results in 1280 cycles saved overall.
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\noindent
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{\bf Fast Multiply:}
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One of the biggest bottlenecks in the mode7 code was the multiply.
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Even our optimized algorithm calls for at least seven
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16bit x 16bit = 32bit multiplies, something that is {\em really} slow on
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the 6502.
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A typical implementation takes around 700 cycles
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for a 8.8 x 8.8 fixed point multiply.
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% Note, this is Quarter-square multiplication, apparently an ancient algorithm
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% https://en.wikipedia.org/wiki/Multiplication_algorithm#Quarter_square_multiplication
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We improved this by using the ancient quarter-square
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multiply algorithm, first described for 6502 use by Stephen Judd.
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This works by noting these factorizations:
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\[(a+b)^{2} = a^{2}+2ab+b^{2}\]
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\[(a-b)^{2}=a^{2}-2ab+b^{2}\]
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If you subtract these you can simplify to
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\[a\times b =\frac{(a+b)^{2}}{4} - \frac{(a-b)^2}{4}\]
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For 8-bit values if you create a table of squares from 0 to 511
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(all 8-bit a+b and a-b fall in this range) then you can convert a multiply
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into two table lookups and a subtraction.
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This does have the downside of requiring 2kB of lookup tables
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(which can be generated at startup) but it reduces the multiply
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cost to the order of 250 cycles or so.
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\subsection{BALL ON CHECKERBOARD}
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The first Mode7 scene transpires on an infinite checkerboard.
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A demo would be incomplete without some sort of bouncing geometric solid,
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in this case we have a pink sphere.
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The sphere is represented by 16 sprites that were captured from
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a 20 year old OpenGL game engine.
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Screenshots were taken then reduced to the proper size and color
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limitations.
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The shadows are also just sprites.
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Note that the Apple II has no dedicated sprite hardware, so these
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are drawn completely in software.
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The clicking noise on bounce is generated by accessing the speaker port
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at address {\tt \$C030}.
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This gives some sound for those viewing the demo without the benefit
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of a Mockingboard.
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\subsection{TFV SPACESHIP FLYING}
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This next scene has a spaceship flying over an island.
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The Mode7 graphics code is generic enough that only one copy of the code
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is needed to generate both the checkerboard and island scenes.
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The spaceship, water splash, and shadows are all sprites.
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The path the ship takes is pre-recorded; this is adapted from the
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Talbot Fantasy~7 game engine with the keyboard code replaced by a hard-coded
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script of actions to take.
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\subsection{STARFIELD}
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The spaceship now takes to the stars.
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This is typical starfield code, where on each iteration the x and y
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values are changed by
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\[dx=\frac{x}{z}, dy=\frac{y}{z}\]
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In order to get a good frame rate and not clutter the lo-res screen
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only 16 stars are modeled.
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To avoid having to divide, the reciprocal of all possible z values
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are stored in a table, and the fast-multiply routine described
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previously is used.
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The star positions require random number generation, but there is no
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easy way to quickly get random data on the Apple II.
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Originally we had a 256-byte blob of pre-generated ``random'' values
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included in the code.
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This wasted space, so now instead we just use our code at address
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at \$5000 as if it were a block of random numbers.
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This was arbitrarily chosen, and it is not as random as it could be
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as seen when the ship enters hyperspace and the lower-right quadrant
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is distressingly star-free.
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A simple state machine controls star speed, ship movement, hyperspace,
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background color (for the blue flash) and the eventual sequence of sprites
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as the ship vanishes into the distance.
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\subsection{RASTERBARS/CREDITS}
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Once the ship has departed, it is time to run the credits as the stars
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continue to fly by.
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The text is written to the bottom four lines of the screen, seemingly
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surrounded by graphics blocks.
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Mixed graphics/text is generally not be possible on the Apple II, although
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with careful cycle counting and mode switching groups such as FrenchTouch
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have achieved this effect.
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What we see in this demo is the use of inverse-mode (inverted color)
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space characters which appear the same as white graphics blocks.
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The rasterbar effect is not really rasterbars, just a colorful assortment
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of horizontal lines drawn at a location determined with a sine lookup table.
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Horizontal lines can take a surprising amount of time to draw, but these
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were optimized using inlining and a few other tricks.
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The spinning text is done by just rapidly rotating the output string through
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the ASCII table, with the clicking effect again generated
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by hitting the speaker at address {\tt \$C030}.
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The list of people to thank ended up being the primary limitation to
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fitting in 8kB, as unique text strings do not compress well.
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I apologize to everyone whose moniker got compressed beyond recognition,
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and I am still not totally happy with the centering of the text.
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\section{Obtaining the Code}
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More details, disk image, and full source can be found at the website:
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\url{http://www.deater.net/weave/vmwprod/mode7_demo/}
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%\section{Appendix: Memory Map}
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|
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\begin{figure}
|
|
\begin{center}
|
|
\begin{scriptsize}
|
|
\begin{BVerbatim}
|
|
------------- $ffff
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| ROM/IO |
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------------- $c000
|
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| |
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|
| Uncompressed|
|
|
| Code/Data |
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| |
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------------- $4000
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| Compressed |
|
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| Code |
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------------- $2000
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| free |
|
|
------------- $1c00
|
|
| Scroll |
|
|
| Data |
|
|
------------- $1800
|
|
| Multiply |
|
|
| Tables |
|
|
------------- $1000
|
|
| LORES pg 3 |
|
|
------------- $0c00
|
|
| LORES pg 2 |
|
|
------------- $0800
|
|
| LORES pg 1 |
|
|
------------- $0400
|
|
|free/vectors |
|
|
------------- $0200
|
|
| stack |
|
|
------------- $0100
|
|
| zero pg |
|
|
------------- $0000
|
|
\end{BVerbatim}
|
|
\end{scriptsize}
|
|
\end{center}
|
|
\caption{Memory Map (not to scale)\label{fig:map}}
|
|
\end{figure}
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|
|
\appendix
|
|
|
|
\input{dram_notes}
|
|
|
|
|
|
\end{document}
|