363 lines
14 KiB
Plaintext
363 lines
14 KiB
Plaintext
Challenges found writing an 8k Lores Apple II Demo
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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by DEATER (Vince Weaver, vince@deater.net)
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http://www.deater.net/weave/vmwprod/mode7_demo/
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====================================================
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19 March 2018
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GOAL:
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~~~~~
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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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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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USING:
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~~~~~~
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Boot disk on a real system, or emulator with Mockingboard support.
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Applewin works fine (even under Wine on Linux).
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MESS does too, it's harder to setup (ROMs) but the audio sounds clearer.
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If you have no emulator you can try one of the online javascript ones.
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https://www.scullinsteel.com/apple2/
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Hardware:
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~~~~~~~~~
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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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The most common disk drive was the Disk II which typically held
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140k of data (single-sided).
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The only sound available was a bit-banged speaker. No timer,
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if you wanted music you had to cycle-count via the CPU.
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Later some sound cards were available. This demo uses the
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Mockingboard which has dual AY-3-8910 sound chips. Each
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chip provides 3 channels of square waves, with noise and
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envelope effects available.
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GRAPHICS
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~~~~~~~~
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The Apple II had nice graphics for its time, with this time being
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around 1977. Otherwise it is quite limited.
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Hardware Sprites? No
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Linear framebuffer? 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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Hardware scrolling? No
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Hardware page flip? Yes
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The hi-res graphics mode was a complex mess of NTSC hacks by Woz.
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You got 280x192 graphics, with 6 colors available. However the colors
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were from NTSC artifacts and there were limitations on which colors
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could be next to each other (in blocks of 3.5 pixels) as well as
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fringing. Also the addresses were interleaved, so not a linear
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framebuffer. Hi-res page0 is at $2000 and page1 at $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. It is 40x48 blocks
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(40x40 if 4 lines of text are displayed at the bottom).
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15 colors are available, though there is fringing at the edges.
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Again the addresses are interleaved. Lo-res page0 is at $400
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and page1 is at $800.
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========================================
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DETAILED STEP-BY-STEP REVIEW OF THE DEMO
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========================================
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BOOTLOADER
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~~~~~~~~~~
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A BASIC "HELLO" program loads the binary.
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This just makes things auto-boot at startup, this doesn't count
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towards the executable size, you could manually BRUN the 8k program
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if you wanted.
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The binary is loaded at $2000 (hi-res page0) and BASIC kicks into
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HIRES mode before loading so you can watch as the memory is loaded
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from disk in a seemingly random pattern.
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Since this is an 8k demo, the entirety of the program is shown on
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the screen (or would be if we POKEd the right address to turn off
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the 4 lines of text on the bottom of the screen).
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Execution starts at address $2000
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DECOMPRESSER
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~~~~~~~~~~~~
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The binary is LZ4 encoded. The decompresser flips to HGR page 1 so
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we can watch memory as the program is decompressed.
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The LZ4 decompression code was written by qkumba (Peter Ferrie).
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http://pferrie.host22.com/misc/appleii.htm
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The actual program/data decompresses to around 22k starting at $4000.
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It over-writes parts of DOS3.3, but since we won't be using the disk
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anymore this isn't an issue.
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At the top left corner of the screen you'll see the VMW triangles logo
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as it decompresses. To do this I had to put the proper bit pattern
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at $4000, $4400, $4800, and $4C00. I mean to have some words too
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but ran out of disk space. The bit pattern at $4000 is executable
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and is run as code.
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Optimizing for code size inside of a compressed binary is a pain.
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Removing instructions sometimes made the binary larger as it no longer
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compressed as well. Long runs of values (such as 0 padding) are
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essentially free. This was a difficult challenge.
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FADE EFFECT
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~~~~~~~~~~~
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The title screen fades in from black.
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This is a software hack, with a lookup table copying from an off-screen
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buffer. The Apple II doesn't have any palette support.
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TITLE SCREEN
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~~~~~~~~~~~~
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Once things are decompressed, we jump to $4000. We switch to low-res
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mode for the rest of the DEMO.
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A background image is loaded from disk. This is RLE encoded (probably
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unnecessary when being further LZ4 encoded).
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Why not just load the program at $400 and load the graphics image for
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free? Well, remember the graphics are 40x48 (shared with the text).
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Really it's 40x24, with each text char mapping to 4-bits top/bottom
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for color. Do the math, we have 1k reserved for this mode but 40x24
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is only 960 bytes. It turns out there are "holes" in the address range
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that aren't displayed, and various pieces of hardware use these holes
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as scratchpad memory. So if you just blindly uncompress graphics data
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there you can corrupt the scratchpad. So you have to be careful
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when uncompressing to skip the holes.
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The title screen has scrolling text at the bottom. This is nothing fancy,
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the text is in a buffer off screen and a 40x4 chunk of RAM is copied in
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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. This is because there is
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not a reliable cross-platform way to get the VBLANK info (especially
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on older machines) so we are having some bad luck about when we flip
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pages.
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MOCKINGBOARD MUSIC
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~~~~~~~~~~~~~~~~~~
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I like chiptune music, especially that for AY-3-8910 based systems.
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Before obtaining a Mockingboard I built a Raspberry Pi chiptune player
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that is essentially the same hardware.
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Most of my sound infrastructure involves YM5 files, which are often used
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by ZX Spectrum and ATARI ST users. These are usually register dumps
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taken typically at 50Hz. So to play them back you just have to interrupt
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50 times a second and write the registers.
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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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Doing this quickly enough is a challenge on the Apple II. For each
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register you have to do a handshake, set the register # and the value.
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This can take upwards of 40 1MHz cycles per register.
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For complex chiptune files (especially those written on an ST with much
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faster hardware) it's sometimes not possible to get exact playback
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due to the delay. Also one AY is on the left channel and one on the right
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so you have to write both if you want sound from both speakers.
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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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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 (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, much more than is available.
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For this demo I run at 25Hz, and also pack the 14 registers of the data
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into 11 (there are various fields that are not packed well, we can
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unpack at play time). Also I stripped out the envelope data as many
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songs do not use it (so this is a lossy compression method).
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Also, we keep track of the last values written last frame and only
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write out to the board if things change, which helps with the latency
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a bit.
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The sound quality suffered a bit, but it's hard to fit a catchy chiptune
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file in 8K.
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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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MODE7 BACKGROUND
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~~~~~~~~~~~~~~~~
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"MODE7" was a Super Nintendo (SNES) graphics mode that took a tiled
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background and transformed it to look as if it was squashed out to
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the horizon, giving a 3d look. The SNES did this in hardware, but
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in this demo we do this in software.
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As found on Wikipedia, the transform is of the type
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[x'] = [a b]([x]-[x0])+[x0]
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[y'] [c d]([y] [y0]) [y0]
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For our code, we managed to reduce things to a small number of additions
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and subtractions for each pixel on the screen. Of course the 6502 can't
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do floating point, so we do fixed point math. We convert as much as we
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can to table lookups that are pre-calculated. We also make liberal use
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of self-modifying code.
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Despite all of this there are still some cases where we have to do a
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16bit x 16bit = 32bit multiply, something that is *really* slow on 6502,
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around 700 cycles (for a 8.8 x 8.8 fixed point multiply).
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To make this faster we use a method described by Stephen Judd.
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The key to note is that (a+b)^2 = a^2+2ab+b^2 and (a-b)^2=a^2-2ab+b^2
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and if you add them you can simplify to:
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(a+b)^2 (a-b)^2
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a*b = --------- - -------
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4 4
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This is you have a table of squares from 0..511 (all 8-bit a+b and a-b
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will fall in this range) then you can convert a multiply into a table
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lookup plus a subtract.
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The downsize is you will need 2kB of squares lookup tables (which can
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be generated at startup). This reduces the multiply cost to the order
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of 200 to 250 cycles.
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By using the fast multiply and a lot of careful optimization you can
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generate a Mode7 background in 40x40 graphics mode at about 5 frames/second.
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The engine can be parameterized with different tilesets to use, which we
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do to provide both a black+white checkerboard background, as well as the
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island background from the TFV game.
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BOUNCING BALL ON CHECKERBOARD
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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What would a demo be without some sort of bouncing geometric shape.
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This is just done with 16 sprites. The sphere was modeled in OpenGL
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from a 2000-era game-engine that I never finished. I then took screenshots
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and then reduced the size/color to an appropriate value.
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The shadow is also just sprites.
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The clicking noise on bounce is just touching the speaker at $C030.
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It's mostly there to give some sound effects for those playing the demo
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without a mockingboard.
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TFV SPACESHIP FLYING
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~~~~~~~~~~~~~~~~~~~~
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The spaceship, water splash, and shadows are all sprites. This is all
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done in software, 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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STARFIELD
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~~~~~~~~~
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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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RASTERBARS/CREDITS
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~~~~~~~~~~~~~~~~~~
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The credits happen with the starfield continuing to run.
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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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Memory Map
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==========
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(not to scale)
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-------- $ffff
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| ROM/IO |
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-------- $c000
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| | 32k decompress
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-------- $4000
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| load | 8k
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-------- $2000
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| free |
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-------- $1c00
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| Scroll |
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| Data |
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-------- $1800
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|Multiply|
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| Tables |
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-------- $1000
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|GR pg 2 | 1k
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|-------- $0c00
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|GR pg 1 | 1k
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|-------- $0800
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|GR pg 0 | 1k
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-------- $0400
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| | 0.5
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-------- $0200
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| stack | 0.25
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-------- $0100
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|zero pg | 0.25
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------- $0000
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