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README.
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Piotr Fusik
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6502 inflate routine
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====================
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[DEFLATE](http://en.wikipedia.org/wiki/DEFLATE) is a popular compression format,
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used in ZIP, gzip, PNG and many other formats.
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In 2001 I wrote a DEFLATE decompression routine (called "inflate")
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in the [6502](http://en.wikipedia.org/wiki/6502) assembly language.
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In 2007 I optimized it so it is about 30% shorter and 10% faster than before.
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Compilation
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-----------
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The routine uses three memory areas:
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* `inflate` - code and initialized data (509 bytes)
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* `inflate_data` - uninitialized data (764 bytes)
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* `inflate_zp` - variables on zero page
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You must select these locations at compile time, for example:
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xasm -d inflate="$b700" -d inflate_data="$b900" -d inflate_zp="$f0" inflate.asx
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Source code uses [xasm](https://github.com/pfusik/xasm) syntax.
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This cross-assembler includes many original syntax extensions.
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Usage
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-----
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The `inflate` routine assumes that the compressed and uncompressed data fit
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in the memory. Before calling `inflate` set the locations of compressed
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and uncompressed data in zero-page variables:
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mwa #compressedData inflate_zp
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mwa #uncompressedData inflate_zp+2
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jsr inflate
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As the compressed data is read sequentially and only once, it is possible
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to overlap the compressed and uncompressed data. That is, the data being
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uncompressed can be stored in place of some compressed data which has been
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already read.
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It is also possible to get the compressed data from any forward-only stream.
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In this case, modify the `getBit` routine to look like this:
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getBit
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lsr getBit_buffer
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bne getBit_return
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pha
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stx getBit_buffer
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jsr readByte
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ldx getBit_buffer
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ldy #0
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sec
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ror @
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sta getBit_buffer
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pla
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getBit_return
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rts
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Compression
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-----------
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There are several ways to get DEFLATE compressed data.
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Originally I wrote a command-line utility called `deflater`
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which used the standard [zlib library](http://www.zlib.net/).
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However, better compression can be obtained with [7-Zip](http://7-zip.org/).
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It supports the gzip format which is a thin layer on top of DEFLATE.
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My program `gzip2deflate` extracts the DEFLATE stream from a gzip file.
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It reads gzip on standard input and writer DEFLATE to standard output.
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I recommend using it like this:
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7z a -tgzip -mx=9 -so dummy INPUT_FILE | gzip2deflate >OUTPUT_FILE.dfl
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If you don't have 7-Zip, use:
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gzip -c -9 INPUT_FILE | gzip2deflate >OUTPUT_FILE.dfl
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If you are looking for maximum compression, [KZIP](http://advsys.net/ken/utils.htm)
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is also worth a try. It creates ZIP files, so I wrote `zip2deflate` to extract
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DEFLATE data from ZIP.
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Windows binaries of `gzip2deflate` and `zip2deflate` are
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[available for download](http://pfusik.github.io/zlib6502/2deflate.zip).
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For other platforms you will need to compile these programs yourself.
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zlib?
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-----
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This project is named zlib6502, but only supports DEFLATE decompression.
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Someday I'm going to include more functions, including compression.
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Meanwhile, you may look at [cc65](https://github.com/oliverschmidt/cc65) `zlib.h`.
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This is my old code, which includes an old version of `inflate`
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plus zlib-compatible `uncompress`, `adler32` and `crc32`.
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License
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-------
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This code is licensed under the standard zlib license.
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Copyright (C) 2001-2013 Piotr Fusik
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This software is provided 'as-is', without any express or implied
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warranty. In no event will the authors be held liable for any damages
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arising from the use of this software.
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Permission is granted to anyone to use this software for any purpose,
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including commercial applications, and to alter it and redistribute it
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freely, subject to the following restrictions:
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1. The origin of this software must not be misrepresented; you must not
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claim that you wrote the original software. If you use this software
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in a product, an acknowledgment in the product documentation would be
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appreciated but is not required.
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2. Altered source versions must be plainly marked as such, and must not be
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misrepresented as being the original software.
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3. This notice may not be removed or altered from any source distribution.
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