This is a Upkr unpacker in 6502 assembly language.
Three memory areas are used:
unupkr- 218 bytes of code, not modified (can be put in ROM)unupkr_probs- 319 bytes of uninitialized data (place on page boundary for smallest and fastest code)unupkr_zp- 15 bytes of zero-page variables
unupkr.asx uses opt ?+. If you use ?-prefixed
labels in MADS, you might want to follow the include with opt ?-.
The unpacked assumes that the compressed and the uncompressed data fit in the memory.
Before calling unupkr, set the locations in the zero-page variables:
lda #<packed_data
sta unupkr_zp
lda #>packed_data
sta unupkr_zp+1
lda #<unpacked_data
sta unupkr_zp+2
lda #>unpacked_data
sta unupkr_zp+3
jsr unupkr
As the compressed data is read sequentially and only once, it is possible to overlap the compressed and uncompressed data. That is, the data being uncompressed can be stored in place of some compressed data which has been already read.
See test.asx for an example for Atari 8-bit.
The routine requires 8-bit by 8-bit multiplication.
For small, portable and slow code, define unupkr_mul as zero.
The multiplication will be performed with a standard 6502 loop.
For faster execution, point unupkr_mul to a temporary page-aligned 2 KB area.
The code will grow to 296 bytes, become self-modifying, but execute about 50% faster.
As a bonus, you can use the lookup tables for fast multiplication in your code
after the routine returns.
Atari Lynx,
SNES
and X65
all have hardware multiply. Define unupkr_mul to a magic value to take advantage of it.
| Platform | unupkr_mul |
Multiplication |
|---|---|---|
| any 6502 | 0 | slow |
| any 6502 | $hh00 | with a 2 KB lookup table |
| Atari Lynx | -1989 | Suzy |
| SNES | -$5a22 | Ricoh 5A22 |
| X65 | -65 | RIA |
Download the packer from https://github.com/exoticorn/upkr
Use as follows:
upkr -9 --big-endian-bitstream --invert-new-offset-bit --invert-continue-value-bit --simplified-prob-update INPUT_FILE OUTPUT_FILE
If OUTPUT_FILE is skipped, it defaults to INPUT_FILE.upk.
This code is licensed under the standard zlib license.
Copyright (C) 2024 Piotr '0xF' Fusik
This software is provided 'as-is', without any express or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software.
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