6502 inflate routine

DEFLATE is a popular compression format, used in ZIP, gzip, PNG and many other formats.

In 2000 I wrote a DEFLATE decompression routine (called "inflate") in the 6502 assembly language. In 2007 I optimized it so it is about 30% shorter and 10% faster than before.

Compilation

Use xasm.

The routine uses three memory areas:

• inflate - code and initialized data (508 bytes)
• inflate_data - uninitialized data (764 bytes)
• inflate_zp - variables on zero page

You must select these locations at compile time, for example:

xasm -d inflate=$b700 -d inflate_data=$b900 -d inflate_zp=$f0 inflate.asx

(escape the dollars if in Unix shell or Makefile).

Usage

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

mwa #compressedData inflate_zp
mwa #uncompressedData inflate_zp+2
jsr inflate

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.

It is also possible to get the compressed data from any forward-only stream. In this case, modify the getBit routine to use your readByte:

getBit
    lsr getBit_buffer
    bne getBit_return
    pha
    stx getBit_buffer
    jsr readByte
    ldx getBit_buffer
    ldy #0
    sec
    ror @
    sta getBit_buffer
    pla
getBit_return
    rts

Compression

There are several ways to get DEFLATE compressed data. Originally I wrote a command-line utility called deflater which used the standard zlib library. However, better compression can be obtained with 7-Zip. It supports the gzip format which is a thin layer on top of DEFLATE. My program gzip2deflate extracts the DEFLATE stream from a gzip file. It reads gzip on its standard input and writes DEFLATE to its standard output. I recommend using it this way:

7z a -tgzip -mx=9 -so dummy INPUT_FILE | gzip2deflate >OUTPUT_FILE.dfl

If you don't have 7-Zip, use:

gzip -c -9 INPUT_FILE | gzip2deflate >OUTPUT_FILE.dfl

If you are looking for maximum compression, KZIP is also worth a try. It creates ZIP files, so I wrote zip2deflate to extract DEFLATE data from a ZIP.

Windows binaries of gzip2deflate and zip2deflate are available for download. For other platforms, you will need to compile these programs yourself.

zlib?

This project is named zlib6502, but only supports DEFLATE decompression. Someday I'm going to include more functions, including compression.

Meanwhile, you may look at cc65 zlib.h. This is my old code, which includes an old version of inflate plus zlib-compatible uncompress, adler32 and crc32.

License

This code is licensed under the standard zlib license.

Copyright (C) 2000-2017 Piotr 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.