ARJ

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File Format
Name ARJ
Ontology
Extension(s) .arj, .a01, ...
PRONOM fmt/610
Wikidata ID Q2693033
Released 1990 (beta), 1991 (v1.00)

ARJ is a compressed archive format, and associated software. It was developed by Robert Jung.

Contents

Discussion

ARJ was one of the leading compression tools during the 1990s. While it was a bit slower than PKZIP, it sported many more options, some of which were unique during that time (archives over multiple disks/volumes, fine-tuning of the compression algorithms used based on the data that was being compressed, recovery records to recover from simple transmission errors, etc.).

ARJ software expiration

If you wish to use or research the official ARJ software, be aware that some of the free/evaluation and beta versions of ARJ and ARJ32 have a hard-coded expiration date. They do not work, or do not work as well, if they think they are being used after that date. Expiration can manifest in several ways.

The 0.xx versions disable some features, based on the system clock, and the timestamp of the file being archived. Other versions only seem to care about the system clock.

Some beta versions inexplicably print "CRC error!" if they're expired.

At least one version prints a message about a time delay, but then seems to hang forever.

Newer versions have a time delay whose duration depends on how long ago they expired. The delay can be several minutes.

The newest versions that don't expire are apparently ARJ 2.76 and ARJ32 3.09.

Suggest running expired DOS versions in DOSBox-X, which lets you set the clock.

Format details

Compression methods

ID Name Description
0 stored No compression
1 compressed most LZ77+Huffman. These are all essentially the same as LHA's "lh6" method, but with the history window artificially limited to 26K, instead of 32K.

It is possible for ARJ files to exist that violate the 26K limit, e.g. if they were created by ARJZ, or by Open-Source ARJ with the -hdd32750 debug option.

2 ...
3 ...
4 compressed fastest An ARJ-specific implementation of LZ77/LZSS.

Note: This compression method was slightly different in ARJ v0.13–0.14.

8 no data, no CRC
9 no data

Self-extracting archives

ARJ can create self-extracting archives. This is usually done with the -je or -je1 option. The -je1 option creates an "SFXJR" archive, which is smaller, but has limited features.

According to the documentation, the ASCII signature "aRJsfX" appears somewhere in the first 1000 bytes of all such files made by ARJ v2.20+.

Files made by ARJ for DOS use EXE format. The different versions are summarized below.

Versions Option Remarks
<0.15 No such feature.
0.15–0.15a -x The executable part is pre-compressed with LZEXE. LZEXE's usual "LZ91" signature appears at offset 28.
0.20 -je LZEXE w/"LZ91" signature.
1.00–2.00 -je or -je1 Apparently uses DIET compression.
2.10 -je or -je1 LZEXE w/"LZ91" signature.
2.20+ -je or -je1 LZEXE w/"RJSX" signature.

Identification

An ARJ archive starts with signature bytes 0x60 0xea. Byte 0x02 appears at offset 10.

The full identification algorithm used by the ARJ software is given in its technical documentation.

See also

Specifications

Software

See also the notes about software expiration, elsewhere on this page.

Sample files

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