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6502 inflate routine
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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. In 2017 I fixed bugs causing invalid expansion of some streams. These were edge cases, unlikely to encounter unless intentionally triggerred.


Use xasm.

The routine uses three memory areas:

  • inflate - code and constants (508 bytes)
  • inflate_data - uninitialized data (765 bytes)
  • inflate_zp - variables on zero page (10 bytes)

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).


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:

    lsr getBit_buffer
    bne getBit_return
    stx getBit_buffer
    jsr readByte
    ldx getBit_buffer
    ldy #0
    ror @
    sta getBit_buffer


There are several ways to get DEFLATE compressed data.

If you are looking for maximum compression, use Zopfli. For example:

zopfli --deflate --i1000 INPUT_FILE

will compress to INPUT_FILE.deflate. I have compiled a Windows exe for you. Increasing the number passed to the --i option can improve the compression by a few bytes at the cost of increased compression time.

Historically, I have used:

  • my programs using the Deflater Java class
  • my deflater program written with the zlib library.
  • my gzip2deflate program that extracted the DEFLATE stream from a GZ file created with gzip or 7-Zip
  • my zip2deflate program that extracted the DEFLATE stream from a ZIP created with KZIP


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. In addition to inflate, it supports zlib-compatible uncompress, adler32 and crc32.


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.