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131 lines
3.9 KiB
Plaintext
131 lines
3.9 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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As can be seen from the memory map, if we assume our player can fit in 4k
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we have roughly from $2000 to $9600 for memory. That's $7600 (29.5k).
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If we could have single buffered, we could have had 256*3*14 (10.5k) for
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decompress and 19k for file size which would let us play most of the
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reasonable sized songs on our play list (KRW(3) in table at end).
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If we need to double buffer, then we need 256*2*14*2 (14k) for decompress
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and 15.5k for file size which still works, at least if the move to KRW(2)
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sized files doesn't bloat things too much.
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Proposed plan
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Decompress 3, but in the room of 4?
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1234 in memory
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ABCC decode as ABC, then copy C to 4
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when playing C, play from 4, bring in next 3
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DEFF
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This lets us have 14k of buffer, allowing 15.5k of compressed file.
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Do we have the spare cycles for this?
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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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Disk
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time ym5 KRW(3) KRW(2) Blocks(3)
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~~~~ ~~~ ~~~~~~ ~~~~~~ ~~~~~~
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KORO.KRW 0:54 ? 2740 3039 12 12
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FIGHTING.KRW 1:40 ? 3086 3316 14 14
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CAMOUFLAGE.KRW 1:32 1162 4054 4972 17 17
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DEMO4.KRW 2:05 1393 4061 6336 17 17
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SDEMO.KRW 2:12 1635 5266 7598 22 22
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CHRISTMAS.KRW 1:32 1751 4975 5811 21 21
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SPUTNIK.KRW 2:05 2164 8422 10779 34 34
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DEATH2.KRW 2:27 2560 8064 10295 33 33
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CRMOROS.KRW 1:29 2566 8045 9565 33 33
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TECHNO.KRW 2:23 2630 8934 11126 36 36
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WAVE.KRW 2:52 2655 8368 11318 34 34
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LYRA2.KRW 3:04 2870 9826 14418 40 40
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INTRO2.KRW 2:59 3217 9214 9294 37 37
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ROBOT.KRW 1:26 3448 7724 8337 32 32
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UNIVERSE.KRW 1:49 4320 9990 11225 41 41
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NEURO.KRW 3:47 8681 22376 25168 89
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AXELF.KRW 10:55 9692 47989 54420 189
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----- -----
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423 30:29
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Notes: my home-made songs don't have ym5 sizes as I don't have a
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working LHA encoder to make a real size.
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Apple II disk file sizes: uses 256 byte blocks. Needs an extra
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for the catalog entry (and an additional for every X blocks used)
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The Disk II / DOS3.3 can in theory hold 140k, but first 3 tracks
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are reserved for DOS (12k) and the Catalog track (4k) and the
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Hello program (512 bytes) and our chiptune player (4k), totalling
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24.5k of overhead, with 115.5k free (462 blocks)
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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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