6502 unShrinkler

This is a Shrinkler decompressor in 6502 assembly language.

Compilation

Use MADS or xasm.

The routine is compiled conditionally to support one of two Shrinkler variants:

• unshrinkler_PARITY=0 is the recommended "no parity context" variant that is almost sure to perform better with the data you have on an 8-bit machine
• unshrinkler_PARITY=1 is the original variant for Amiga

The routine is slow. unshrinkler_FAST=1 makes it about twice faster at the cost of increased memory usage.

Three memory areas are used:

• unshrinkler - code
• unshrinkler_data - uninitialized data
• unshrinkler_zp - zero-page variables

You must select these locations at compile time. unshrinkler is defined at the current origin.

Options	Code	Uninitialized data	Page zero
unshrinkler_PARITY=0 unshrinkler_FAST=0	320 B	1.5 KB	19 B
unshrinkler_PARITY=0 unshrinkler_FAST=1	471 B	3.0 KB	27 B
unshrinkler_PARITY=1 unshrinkler_FAST=0	325 B	2.0 KB	19 B
unshrinkler_PARITY=1 unshrinkler_FAST=1	476 B	3.5 KB	27 B

unshrinkler_FAST=1 requires unshrinkler_data to be aligned on page boundary. With unshrinkler_FAST=0 it can be unaligned, at the cost of two extra code bytes and slightly degraded performance.

unShrinkler.asm uses opt ?+. If you use '?'-prefixed labels in MADS, you might want to follow the include with opt ?-.

Usage

The decompressor assumes that the compressed and the uncompressed data fit in the memory. Before calling unshrinkler, set the locations in the zero-page variables:

lda #<packed_data
sta unshrinkler_zp
lda #>packed_data
sta unshrinkler_zp+1
lda #<unpacked_data
sta unshrinkler_zp+2
lda #>unpacked_data
sta unshrinkler_zp+3
jsr unshrinkler

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.asm for an example for Atari 8-bit.

Compression

Download the compressor from https://github.com/askeksa/Shrinkler

Use as follows:

Shrinkler -d -p -9 -b INPUT_FILE OUTPUT_FILE

Include -b for unshrinkler_PARITY=0, omit it for unshrinkler_PARITY=1.

License

This code is licensed under the standard zlib license.

Copyright (C) 2021 Krzysztof 'XXL' Dudek, 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.

Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:

1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.

2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.

3. This notice may not be removed or altered from any source distribution.