ARC (compression format)

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File Format
Name ARC (compression format)
Ontology
Extension(s) .arc, .ark
UTI public.archive.arc
Wikidata ID Q296496
Released 1985

ARC is a compressed archive format, mostly used in MS/PC-DOS, though a CP/M version also existed. It supports a number of different compression schemes, the most common of which are based on LZW.

Contents

Discussion

ARC was for a time (1985-89) the leading file archiving and file compression format in the BBS world, replacing the formats used by earlier utilities which generally only did one of the two functions (either combining multiple files in one file for convenient download, or shortening the file length to take less download time and disk space). Combining the two functions in one utility simplified the process of preparing files for download and extracting them at the other end, leading to a rapid rise in popularity for the utility (also called ARC) and format both.

However, the ARC format suffered an equally rapid decline in its popularity after the company that published the ARC utility (called System Enhancement Associates or SEA, run by Thom Henderson who was very active in FidoNet) brought a successful trademark and copyright suit against rival Phil Katz, whose PKARC and PKXARC utilities were compatible with the ARC file format. The lawsuit was widely regarded by the BBS community as being a "David vs. Goliath" case of a faceless corporation bullying a "little guy", though in fact both companies were small, home-based operations. Nevertheless, the fallout from the suit led to rapid adoption of the competing ZIP format, introduced by Katz in 1989, and ARC files are no longer commonly encountered.

The fact that archives from an early period of BBSing are often in this format encourages bad puns referring to those who trawl such old archives as "Raiders of the lost ARC."

Disambiguation

There are, unfortunately, also several other incompatible file formats that have been used over the years with an "ARC" designation or file extension, so it's possible that a data set that is purportedly of type "ARC" is not actually of this format. Others include the FreeArc format and the Internet Archive ARC format, as well as a Commodore ARC that's similar in concept but not compatible to any of the other ARCs.

Format details

An ARC file consists of a sequence of zero or more archive members, followed by an end-of-archive marker: the bytes 0x1a 0x00. It is common for ARC files to have padding or other data after the end-of-archive marker.

Each member begins with a 0x1a byte, then a byte indicating the compression method used for that member file. (For files beginning with 0x1b, see ArcMac.)

Compression methods

The compression method byte identifies a member's compression method, and/or other information about the type of member. The usual compression methods are in the range 1 through 9.

Unfortunately, there are several different compression methods named "crunched" or "Crunched".

ID Name(s) Description and remarks
0 End-of-archive marker
1 Uncompressed With old-style header.
2 Uncompressed With new-style header.
3 Packed RLE90
4 Squeezed,
Packed+Squeezed 		RLE90 + Huffman. See Squeeze for more information.
5 crunched Hashed LZW (old hash). Derived from LZWCOM. Introduced in ARC v4.00.
6 crunched,
Packed+crunched 		RLE90 + method 5. Similar to CP/M Crunch v1.x format. Introduced in ARC v4.10.
7 crunched,
Packed+crunched 		RLE90 + hashed LZW (new hash). Source code comment says "inadvertent release of a developmental copy forces us to leave [method 7] in".
8 Crunched,
Packed+Crunched 		RLE90 + LZW. The LZW layer has a dynamic code size. There is a header byte giving the maximum LZW code size, but only 12 bits is generally supported. Introduced in ARC v5.00. This is probably the most common compression method.
9 Squashed,
Deviant 		LZW. Used by PKARC/PKPAK.
10 Trimmed RLE90 + {LZH with adaptive Huffman coding}. Supported by ARC 7.x.
10 Crushed PAK extensions: Refer to PAK (ARC extension)#Compression methods.
11 Distilled
20-29 Used/reserved for informational items
20 Archive info
21 Extended file info
22 OS-specific info
30-39 Used/reserved for "control" items
30 Subdir Nested ARC-like format. Created by the "z" option introduced in ARC v6.
31 End-of-subdir marker
72 No known use in ARC, but see Hyper archive.
83 No known use in ARC, but see Hyper archive.
≥128 Refer to Spark.

ARC Plus

The ARC v7 software is named "ARC Plus" or "ARC+Plus". By default, it uses Trimmed compression, and the files it creates begin with an archive info item ("compression method" 20). Some format identification tools identify files beginning this way as "ARC+" format.

ARC v6.02 understands archive info items, though it's unclear if it ever creates them. It does not support Trimmed decompression.

PK-style comments

The PKARC/PKPAK software supports comments, using a custom format extension that appears after the end-of-archive marker. Refer to PKARC/PKPAK#PK-style comments.

PAK extended records

This is another kind of data that can appear after the end-of-archive marker. Refer to PAK (ARC extension).

Self-extracting archives

A number of different ARC self-extracting archive formats exist. This section mainly covers those made by the official software. Others include:

Starting with v6.00, self-extracting archives can be made by the included MKSARC.EXE utility, which converts an ARC file to a self-extracting archive.

For the 6.x series, the executable format is always DOS EXE. There are three known self-extractor versions:

  • v6.00 and 6.01 (MKSARC v1.00) - Files made by these two versions are identical.
  • v6.02 [variant 1] (MKSARC v1.01)
  • v6.02 [variant 2] (MKSARC v1.01)

Technical note: The embedded ARC file was surely intended to start at the start of the overlay segment (refer to MS-DOS EXE#Special file positions), but for v6.02 (both variants), due to a bug, it actually starts 480 bytes before the overlay segment.

Versions 7.10 and 7.12 each offer three self-extractor formats, all in some EXE format: DOS-only (/R option), OS/2-only (/P), and DOS-OS/2 hybrid (/F).

A v7.x self-extracting archive is really just a copy of one of the XARC.EXE files, with a small patch that puts it into self-extracting mode (well, two small patches for hybrid files), and with the ARC file appended. A single "flag byte" in the middle of the patch seems to be the important one -- it is 0x00 in XARC.EXE, and 0x01 in self-extracting archives. For example, for v7.12 DOS-only (both known variants), the flag byte is at offset 8527.

Identifiers

  • File extension: .ARC (or conventionally .ARK on CP/M)
  • MIME type (Internet media type): Has no specific registered type; generic binary application/octet-stream is generally used, or perhaps unregistered custom types with an x- prefix
  • Uniform Type Identifier (Apple): public.archive.arc

See also

Other formats called ARC (or something similar) are listed at ARC.

Specifications

Software

Sample files

References

Further reading

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