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96 lines
2.3 KiB
Plaintext
96 lines
2.3 KiB
Plaintext
The Challenges of an Apple II chiptune player.
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The goal is to design a chiptune player that can play large
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(150k+ uncompressed) chiptune files on an Apple II with 48k of RAM
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and a Mockingboard sound card.
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An interrupt routine wakes at 50Hz to write the registers and a few other
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houskeeping things.
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Not enough RAM to hold full raw ym5 sound data (one byte for each of 14
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registers, every 50Hz). This compresses amazingly. Using LZ4 by at
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least a factor of 10. But it won't fit all in RAM so we have to load
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the full file from disk (no way to do disk I/O, disk I/O disables interrupts)
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then decompress in chunks. So we need room for both the compressed file
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plus uncompressed data.
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The problem is decompression also takes a while, longer than the 50Hz.
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So if we just decompress the next chunk when needed the sound will noticibly
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pause for a fraction of a second.
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One solution to that is to have two decompress areas and flip between them,
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decompressing in the background to one while the other is playing. The problem
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is splitting the decompressed data into smaller chunks like this is that
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it doesn't compress as well so it takes up more disk/memory space
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for the raw file.
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Memory Map
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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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|DOS3.3 |
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-------| $9600
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| FREE |
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|------- $0c00
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|GR pg 1|
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|------- $0800
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|GR pg 0|
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------- $0400
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------- $0200
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|stack |
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------- $0100
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|zero pg|
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------- $0000
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Sizes
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time ym5 KRW(3) KRW(2)
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~~~~ ~~~ ~~~~~~ ~~~~~~
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KORO.KRW 0:54 ? 2740
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FIGHTING.KRW 1:40 ? 3086
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CAMOUFLAGE.KRW 1:32 1162 4054
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DEMO4.KRW 2:05 1393 4061
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SDEMO.KRW 2:12 1635 5266
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CHRISTMAS.KRW 1:32 1751 4975
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SPUTNIK.KRW 2:05 2164 8422
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DEATH2.KRW 2:27 2560 8064
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CRMOROS.KRW 1:29 2566 8045
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TECHNO.KRW 2:23 2630 8934
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WAVE.KRW 2:52 2655 8368
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LYRA2.KRW 3:04 2870 9826
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INTRO2.KRW 2:59 3217 9214
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ROBOT.KRW 1:26 3448 7724
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UNIVERSE.KRW 1:49 4320 9990
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NEURO.KRW 3:47 8681 22376
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AXELF.KRW 10:55 9692 47989
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Interesting bugs that were hard to debug:
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+ Bug in qkumba's LZ4 decoder, only happened when a copy-block size was
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exactly a multiple of 256, in which case it would copy
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an extra time.
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+ Bug where the box-drawing was starting at 0 rather than at Y.
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Turns out I was padding the filename buffer with A0 but going
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one too far and it was writing A0 to the first byte of the
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hlin routine, and A0 is a LDY # instruction.
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