2022-04-11 20:15:59 +00:00
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lemm -- a Lemmings proof-of-concept for the Apple II
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by Vince `Deater` Weaver <vince@deater.net>
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http://www.deater.net/weave/vmwprod/lemm/
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Background
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~~~~~~~~~~
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Lemmings is a 1991 game by DMA Design originally for the OCS Amiga but
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ported to many other platforms
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This is a proof of concept of what it would look like on an Apple II
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from 1977
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(NOTE: while it might technically be possible to play this on an Apple II
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from 1977, you would need to upgrade it to 48k of RAM which would have been
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astronomically expensive before 1980 or so, and you'd need a Disk II drive
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with DOS3.3 which again is around 1980, and for best results you want
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64k of RAM (a language card or Apple IIe) and Mockingboard for sound which
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would push things more toward 1983.
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How was it Made
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~~~~~~~~~~~~~~~
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This was made purely from observing gameplay and taking screenshots of
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the DOS and Amiga versions. No reverse-engineering or asset decoding
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happened.
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The music files are YM5 files found on the internet that were presumably
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captured from the Atari ST version of the game.
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I developed the game in 6502 assembly language on Linux using the ca65
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assembler. Graphics were manipulated using the Gimp.
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What's missing
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~~~~~~~~~~~~~~
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This demo only has 10 levels, all single-screen. I think it might just be
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barely possible to have scrolling backgrounds like the original, but it
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would take a major re-write of the entire game engine.
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The bridge builder's bridges aren't right. Due to Apple II limitations
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it's hard to get properly sized bridges.
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You can't really play speaker sounds in the background on Apple II, so
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the sound effects are limited to "Let's Go" at the beginning.
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The one-way digging is a bit of a hack, and non-diggable surfaces are
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not supported.
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Mouse support is missing (I don't have a mouse, also mice are often
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slot#4 which would conflict with the Mockingboard). Keyboard support
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is a bit awkward, but that's due to limitations of the keyboard on
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Apple II/II+. I could probably improve this with separate code paths
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for IIe/IIc or by using the timers on the Mockingboard but not sure
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it's worth the trouble.
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Novelty
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~~~~~~~
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This actually isn't the first 8-bit port. I'd like to think it's nicer
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looking than the C64 or ZX-Spectrum ports, but those ports have scrollable
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backgrounds and a full set of levels.
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Code Layout
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~~~~~~~~~~~
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Qkumba's QBOOT fast track-at-a-time loader is used for loading data from disk.
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Raw tracks are written to disk, there's no operating system (no DOS33 or ProDos)
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involved.
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The QBOOT bootsector loads two sectors to RAM from boot at $800, then
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it loads the second-stage loader and library code to 4k at $1000.
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HGR (280x192x6 color) graphics are 8k at $2000, and HGR2 (page2, used
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for background) is another 8k at $4000. The LEMM game engine lives in 12k
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starting at $6000. Each level has to fit in 11.5k or so (qboot can't
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load up against the $C000 I/O area) starting at $9000.
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If a language card (16k RAM expansion) is available, then
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the "let's go" sound sample is stored in 4k at Language Card $D000 bank 0
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and 12k of BANK1 is used for decoded music data.
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Unlike my PT3 player which can play compact tracker music in roughly 3k,
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I could only find the lemmings music in YM format. This compresses nice
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too (and is simple to play) but to decompress the songs it takes roughly
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14 channels * 50Hz = 700 bytes per second of music, so you only get about
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10s of music fitting in the language card. To make things work the song
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is broken up in chunks and every 5s or so more music has to be decoded.
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While the current buffer is playing the next buffer is decoded so there
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are no glitches in the music, but this takes more than a video frame to
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do so there's an occasional glitch in gameplay as this happens.
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Since we are constantly playing music from the language card, we can't
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easily use any ROM routines so useful routines like WAIT and HPOSN
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are re-implemented in RAM.
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2022-04-13 03:21:51 +00:00
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Code Description
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~~~~~~~~~~~~~~~~
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TODO
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2022-04-11 20:15:59 +00:00
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Memory Map
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~~~~~~~~~~
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$0000-$00ff = zero page
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$0100-$01ff = stack
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$0300-$03c0 = disk lookup tables
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$0400-$07ff = text page 1
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$0800-$0bff = hgr lookup tables
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$0c00-$0fff = particle effects? (check this)
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$1000-$1100 = disk track loading code
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$1200-$1f00 = common library routines
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$2000-$3fff = HGR page 1
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$4000-$5fff = HGR page 2
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$6000-$8fff = LEMM game engine
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$9000-$Beff = Current Level
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$C000-$Cfff = I/O area
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$D000-$Dfff = (bank 0) sound sample for "let's go"
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$D000-$F000 = (bank 1) decoded music data
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