mode7: Update doc
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Vince Weaver
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@ -2,7 +2,7 @@
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\usepackage{graphicx}
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\usepackage{url}
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\usepackage{hyperref}
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\usepackage{fancyvrb}
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\begin{document}
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@ -105,6 +105,7 @@ 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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@ -208,6 +209,49 @@ the actual Apple II (an Apple IIe platinum edition).
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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.jpg}
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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.jpg}
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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.jpg}
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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.jpg}
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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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@ -230,13 +274,6 @@ Upon loading, execution starts at address \$2000
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\subsection{DECOMPRESSER}
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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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The binary is encoded with the LZ4 algorithm.
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We flip to hi-res Page 2 and decompress there so the user continues to get
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a show of random noise.
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@ -274,12 +311,6 @@ copy in the faded versions on the fly.
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\subsection{TITLE SCREEN}
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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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Once decompression is done, execution continues at address \$4000.
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We switch to low-res mode for the rest of the demo.
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@ -450,13 +481,6 @@ We managed to take this algorithm and speed it up in the following ways:
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\subsection{BOUNCING BALL ON CHECKERBOARD}
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\begin{figure}
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\begin{center}
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\includegraphics[width=\columnwidth]{figures/m7_screen1.jpg}
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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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The first scence starts out viewing an infinite checkerboard.
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Any demo would be incomplete without some sort of bouncing geometric solid,
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in our case a pink sphere.
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@ -472,90 +496,72 @@ This gives some sound for those viewing the demo without a Mockingboard.
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\subsection{TFV SPACESHIP FLYING}
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\begin{figure}[tb]
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\begin{center}
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\includegraphics[width=\columnwidth]{figures/m7_screen4.jpg}
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\end{center}
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\caption{Spaceship flying over an island.\label{fig:tb1}}
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\end{figure}
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This next scene has a spaceship flying over an island.
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The spaceship, water splash, and shadows are all sprites.
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They are all drawn in software as the Apple II has no sprite hardware.
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This is the TFV game engine flying-spaceship code, with the keyboard
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routines replaced to read from memory instead (sort of like a script
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of what to do when).
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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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\begin{figure}[tb]
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\begin{center}
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\includegraphics[width=\columnwidth]{figures/m7_screen3.jpg}
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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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The spaceship takes to the stars.
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This is typical starfield code.
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Only 16 stars are modeled, and the movement code re-uses the
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same fast-multiply routine described previously.
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The starfield is your typical starfield code. Only 16 stars are modeled.
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It re-uses the fast-multiply code from the mode7 graphics.
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Random number generation is not fast on the 6502, so we cheat.
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Originally we had a 256-byte blob of "random" values generated earlier.
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This wasted space, so now instead we just treat the executable code
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at \$5000 as if it were a block of random numbers. This was arbitrarily
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chosen, I tried different areas of memory until I got one where the
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stars seemed to move in a pleasing pattern.
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A simple state machine controls if the stars move or not, whether the
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background is cleared or not (the streak effect) and what color the
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background is (for the blue flash).
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The ship moving to the distance is just done with different sized sprites.
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The star positions require random number generation, but this is not
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fast on the 6502.
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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 the lower right quadrant has fewer
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starts than one could desire.
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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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\begin{figure}[tb]
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\begin{center}
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\includegraphics[width=\columnwidth]{figures/m7_screen2.jpg}
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\end{center}
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\caption{Rasterbars, stars, and credits.\label{fig:credits}}
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\end{figure}
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Once the ship has departed, it is time for the credits as the stars
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continue to run.
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The text is written to the bottom 4 lines of the screen and appears
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to be surrounded by low-res graphics blocks.
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Mixed graphics/text would 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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I was lazy and instead used inverse-mode space characters which appear the same
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as white graphics blocks.
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The credits happen with the starfield continuing to run.
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The rasterbar effect is not really rasterbars, it's 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, so this
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was optimized using inlining and a few other methods.
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The text is written in the bottom 4 lines of the screen. Some inverse-mode
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space characters are used to try to make it look like graphics are surrounding
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the text. It's actually possible with careful cycle counting to switch
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modes fast enough to have actual mixed graphics/text (See the FrenchTouch
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demos) but I was too lazy to attempt that here.
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The rasterbar effect isn't really rasterbars, it's just a rainbow assortment
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of lines being drawn with a SINEWAVE lookup table.
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It's the same rasterbar code from my chiptune player demo. I ended up
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optimizing it a lot via inlining and a few other ways because it turned
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out just drawing a horizontal line can take a very long time.
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The rotating text is just taking the output string and rapidly rotating the
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character values through the ASCII table.
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The annoying clicking noise is the same speaker effect caused by hitting
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\$C030.
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Choosing who to thank ended up being extremely critical to fitting in 8kB,
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as unique text strings do not compress well. I'm also still not satisfied
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with how the centering looks.
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The rotating text is done by just rapidly rotating the output string through
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the ASCII table, with the clicking effect again by hitting the speaker
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at address \$C030.
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The list of people to thank ended up being extremely critical to fitting in 8kB,
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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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\begin{table}
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\begin{center}
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\begin{verbatim}
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%\section{Appendix: Memory Map}
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\begin{figure}
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\begin{center}
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\begin{scriptsize}
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\begin{BVerbatim}
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------------- $ffff
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| ROM/IO |
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------------- $c000
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@ -587,10 +593,12 @@ More details, disk image, and full source can be found at the website:
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------------- $0100
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| zero pg |
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------------- $0000
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\end{verbatim}
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\end{BVerbatim}
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\end{scriptsize}
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\end{center}
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\caption{Memory Map (not to scale)}
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\end{table}
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\caption{Memory Map (not to scale)\label{fig:map}}
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\end{figure}
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\end{document}
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