.ARJ

ARJ archive

Introduction

ARJ is an archiver for MS-DOS written by Robert K. Jung, first released in 1991. In the early and mid-1990s it was one of the most used archivers on DOS, valued for its compression and for splitting archives across floppy disks, and many shareware programs and games of the time were shipped as .arj files. Split archives continue in files named .a01, .a02 and so on. Jung published the header layout and the source code of the free UNARJ extractor.

Structure

Every header starts with the bytes 60 EA and a 2-byte size of the basic header that follows, then ends with a CRC-32 of that header and any extended headers. Numbers are little-endian.

  1. 0Main header 60 EA, versions, host OS, flags, dates, archive name and commentvaries
  2. …File header 60 EA, method, sizes, CRC-32, time, attributes, name and commentvaries
  3. …Compressed data for that filevaries
  4. …More files a header and data for eachrepeated
  5. …End 60 EA with a header size of 04 bytes
An ARJ archive: main header, file entries, end marker.

Method 0 stores a file; methods 1 to 3 use LZ77 with Huffman coding at different speeds, and method 4 is a faster, simpler variant.

Links

Software

  • 7-Zip, extract only (Windows)
  • The Unarchiver (macOS)
  • ARJ and ARJ32, the original archivers (MS-DOS, Windows)

Reference

  • TECHNOTE.DOC, the header description that came with ARJ