ARC (compression format)

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ARC was for a time (1985-89) the leading file archiving and file compression format in the [http://en.wikipedia.org/wiki/Bulletin_board_system 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.
 
ARC was for a time (1985-89) the leading file archiving and file compression format in the [http://en.wikipedia.org/wiki/Bulletin_board_system 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.
+
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."
 
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."
Line 21: Line 21:
 
An ARC file consists of a sequence of zero or more archive members, followed by an end-of-archive marker: the bytes <code>0x1a 0x00</code>. It is common for ARC files to have padding or other data after the end-of-archive marker.
 
An ARC file consists of a sequence of zero or more archive members, followed by an end-of-archive marker: the bytes <code>0x1a 0x00</code>. 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.
+
Each member begins with a <code>0x1a</code> byte, then a byte indicating the compression method used for that member file. (For files beginning with <code>0x1b</code>, see [[ArcMac]].)
  
 
=== Compression methods ===
 
=== Compression methods ===
Line 33: Line 33:
 
|0 || End-of-archive marker ||
 
|0 || End-of-archive marker ||
 
|-
 
|-
−
|1 || Uncompressed || Uses old-style header.
+
|1 || Uncompressed || With old-style header.
 
|-
 
|-
−
|2 || Uncompressed ||
+
|2 || Uncompressed || With new-style header.
 
|-
 
|-
 
|3 || Packed || [[RLE90]]
 
|3 || Packed || [[RLE90]]
Line 41: Line 41:
 
|4 || Squeezed,<br>Packed+Squeezed || [[RLE90]] + [[Huffman coding|Huffman]]. See [[Squeeze#Compressed data section|Squeeze]] for more information.
 
|4 || Squeezed,<br>Packed+Squeezed || [[RLE90]] + [[Huffman coding|Huffman]]. See [[Squeeze#Compressed data section|Squeeze]] for more information.
 
|-
 
|-
−
|5 || crunched || Hashed [[LZW]] (old hash)
+
|5 || crunched || Hashed [[LZW]] (old hash). Derived from [[LZWCOM]]. Introduced in ARC v4.00.
 
|-
 
|-
−
|6 || crunched,<br>Packed+crunched || [[RLE90]] + hashed [[LZW]] (old hash)
+
|6 || crunched,<br>Packed+crunched || [[RLE90]] + method 5. Similar to [[Crunch|CP/M Crunch]] v1.x format. Introduced in ARC v4.10.
 
|-
 
|-
 
|7 || crunched,<br>Packed+crunched || [[RLE90]] + hashed [[LZW]] (new hash). Source code comment says "inadvertent release of a developmental copy forces us to leave [method 7] in".
 
|7 || crunched,<br>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,<br>Packed+Crunched || [[RLE90]] + [[LZW]]. Probably the most common compression method.
+
|8 || Crunched,<br>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,<br>Deviant || [[LZW]]. Used by PKARC/PKPAK.
+
|9 || Squashed,<br>Deviant || [[LZW]]. Used by [[PKARC/PKPAK]].
 
|-
 
|-
−
|10 || Trimmed || Supported by ARC 7.x.
+
|10 || Trimmed || [[RLE90]] +  {[[LZ77 with Huffman coding|LZH]] with [[adaptive Huffman coding]]}. Supported by ARC 7.x.
 
|-
 
|-
−
|10 || Crushed,<br>Packed+Crushed || Used by [[PAK (ARC extension)|PAK]]. (Possibly another RLE+LZW scheme?)
+
|10 || Crushed ||rowspan="2"| [[PAK (ARC extension)|PAK]] extensions: Refer to [[PAK (ARC extension)#Compression methods]].
 
|-
 
|-
−
|11 || Distilled || Used by [[PAK (ARC extension)|PAK]]. ([[LZ77]]+Huffman?)
+
|11 || Distilled
 
|-
 
|-
−
|20-29 || || Reserved for extensions / informational items (?)
+
|20-29 || || Used/reserved for informational items
 
|-
 
|-
−
|20 || Archive info (?) ||  
+
|20 || Archive info ||  
 
|-
 
|-
−
|21 || Extended file 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.
 
|30 || Subdir || Nested ARC-like format. Created by the "z" option introduced in ARC v6.
Line 77: Line 81:
 
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.
 
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 seems to understand archive info items, though it's unclear if it ever creates them. It does not support Trimmed decompression.
+
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 ===
 
=== PK-style comments ===
−
The PKARC/PKPAK software supports comments, apparently using a custom ARC format extension that appears after the end-of-archive marker.
+
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]].
−
 
+
−
''Information based on reverse engineering:'' An ARC file with comments ends with an 8-byte trailer that begins with the signature {{magic|'P' 'K' 0xaa 0x55}}. This is preceded by a sequence of 32-byte ''records'', each containing a comment, except for one that has a special purpose. The last 4 bytes of the file contain the offset of the special record. The special record somehow indicates whether an archive comment and/or file comments are present. An archive comment, if present, is in the record preceding the special record. File comment records come after the special record, in the same order as the members appear in the ARC file.
+
  
 
=== PAK extended records ===
 
=== PAK extended records ===
 
This is another kind of data that can appear after the end-of-archive marker. Refer to [[PAK (ARC extension)]].
 
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:
 +
* [[PKARC/PKPAK#Self-extracting archives]]
 +
* [[PAK (ARC extension)#Self-extracting archives]]
 +
 +
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 [[MS-DOS EXE|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 (<code>/R</code> option), OS/2-only (<code>/P</code>), and DOS-OS/2 hybrid (<code>/F</code>).
 +
 +
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 ==
 
== Identifiers ==
Line 93: Line 113:
  
 
== See also ==
 
== See also ==
 +
* [[PKARC/PKPAK]]
 
* [[PAK (ARC extension)]]
 
* [[PAK (ARC extension)]]
 
* [[Spark]]
 
* [[Spark]]
 +
* [[ArcMac]]
 +
* [[BTSPK]]
 
* [[RLE90]]
 
* [[RLE90]]
 
* [[AXE (executable compression)]] - Another SEA product
 
* [[AXE (executable compression)]] - Another SEA product
 +
 +
Other formats called ARC (or something similar) are listed at [[ARC]].
  
 
== Specifications ==
 
== Specifications ==
Line 103: Line 128:
 
* [http://apple2.org.za/gswv/a2zine/GS.WorldView/Resources/The.MacShrinkIt.Project/ARCHIVES.TXT Archive format info, including ARC]
 
* [http://apple2.org.za/gswv/a2zine/GS.WorldView/Resources/The.MacShrinkIt.Project/ARCHIVES.TXT Archive format info, including ARC]
 
* {{CdTextfiles|simtel/simtel20/MSDOS/ARCHIVRS/ARC_FILE.INF|ARC-FILE.INF}}
 
* {{CdTextfiles|simtel/simtel20/MSDOS/ARCHIVRS/ARC_FILE.INF|ARC-FILE.INF}}
 +
* {{CdTextfiles|currentsw199407/compress/tm0402.zip|SEA Technical Memorandum #0402}}: ARC 6.02 Extended Data
  
−
== Sample files ==
+
== Software ==
−
* {{CdTextfilesURL|powerprogramming/PROGTOOL/}} ...
+
−
* [https://www.dan.info/sampledata/CHRISTIE.ARC CHRISTIE.ARC]
+
−
* [https://www.dan.info/sampledata/POLIPREF.ARC POLIPREF.ARC]
+
−
* {{CdTextfilesURL|pcblue/}}
+
−
 
+
−
== Programs and Utilities ==
+
 
* [http://www.svgalib.org/rus/nomarch.html nomarch] by Russell Marks, c. 2001 (Unix/GPL2) -- extract only.
 
* [http://www.svgalib.org/rus/nomarch.html nomarch] by Russell Marks, c. 2001 (Unix/GPL2) -- extract only.
 
** Packaged for Debian-based Linux distributions: <tt>apt-get install nomarch</tt>
 
** Packaged for Debian-based Linux distributions: <tt>apt-get install nomarch</tt>
−
* [https://sourceforge.net/projects/arc/ ARC] - Portable (Unix, etc.) software based on ARC source code
+
* [https://sourceforge.net/projects/arc/ ARC (for Unix)] - Portable software based on ARC source code
−
* ARC - DOS binaries
+
* ARC - DOS binaries, etc.
−
** {{CdTextfiles|rbbsv3n1/atnu/arc51.exe|v5.10}} (1986-01-31) (bare executable)
+
** v3.10 (1985-05-01): See [http://old-dos.ru/index.php?page=files&mode=files&do=show&id=699 old-dos.ru]
 +
** v4.11 (1985-07-02): [https://archive.org/details/science-libraryb Libris Britannia 4] → science library(b)/COMMUNIC/0132.ZIP → ARC.EXE
 +
** v4.20 (1985-07-16): {{CdTextfiles|pcsig12/CLA/DISK0420.ZIP|DISK0420.ZIP}} → ARC.EXE
 +
** v4.30 (1985-08-22): {{CdTextfiles|pcblueii/PCBLUE/VOL148.ZIP|VOL148.ZIP}} → ARC.EXE
 +
** v4.50 (1985-11-18): {{CdTextfiles|californiacollect/his033/sideway.zip|sideway.zip}} → SIDEWRIT.ARC → ARC.EXE
 +
** v4.52 (1985-12-04): See [http://old-dos.ru/index.php?page=files&mode=files&do=show&id=699 old-dos.ru]
 +
** v5.00 (1986-01-21): {{CdTextfiles|pcsig12/CLA/DISK1000.ZIP|DISK1000.ZIP}} → ARC.EXE
 +
** v5.10 (1986-01-31): {{CdTextfiles|cleanhpvac/TRAX.ZIP|TRAX.ZIP}} → ARC51.COM
 
** {{CdTextfiles|originalsw/25/arc512.com|v5.12}} (1986-02-05) (For self-extraction to work, must be renamed to "ARC51.COM".)
 
** {{CdTextfiles|originalsw/25/arc512.com|v5.12}} (1986-02-05) (For self-extraction to work, must be renamed to "ARC51.COM".)
 
** {{CdTextfiles|originalsw/25/arc520.com|v5.20}} (1986-10-24)
 
** {{CdTextfiles|originalsw/25/arc520.com|v5.20}} (1986-10-24)
−
** {{CdTextfiles|pcmedic/utils/compress/arc601.exe|v6.01}} (Dated "January of 1989", but timestamps suggest 1989-02-23.)
+
** "v5.21" (1987-04-22): [https://archive.org/details/HAMCALL_CD-ROM_Buckmaster_April_1991 HAMCALL CD ROM April 1991] → HAMCALL CD-ROM (Buckmaster)(April 1991).{BIN,CUE} → ziparc/arc/arc.exe (possibly an unofficial build)
−
** {{CdTextfiles|hof91/arc/arc602.exe|v6.02}} (Dated "January of 1989", but timestamps suggest 1989-03-14.)
+
** "v5.21": {{CdTextfiles|hobbesos29503/16BIT/ARCHIVER/ARC521_2.ZIP|ARC521_2.ZIP}} - Binaries for DOS and OS/2 built by K. U. Rommel, from source code via H. Chu.
−
** {{CdTextfiles|rbbsv3n1/atnu/arc602.exe|v6.02}} - Same ARC.EXE as above, but has updated documentation (1989-04-21) and other differences.
+
** v6.00 (Dated "January of 1989", but timestamps suggest 1989-02-10): [https://archive.org/details/PCMCDS Discos PCManía] → PCMania CD15.nrg → sharewre/zip/arc600.exe
−
* {{CdTextfiles|pcmedic/utils/compress/arc520s.zip|ARC v5.20 source code}}
+
** {{CdTextfiles|pcmedic/utils/compress/arc601.exe|v6.01}} (Dated "January of 1989", but timestamps suggest 1989-02-23)
−
* XARC - A minimal ARC extractor from SEA (DOS binaries)
+
** {{CdTextfiles|hof91/arc/arc602.exe|v6.02 &#x5b;variant 1&#x5d;}} (Dated "January of 1989", but timestamps suggest 1989-03-14)
 +
** {{CdTextfiles|rbbsv3n1/atnu/arc602.exe|v6.02 &#x5b;variant 2&#x5d;}} - Same ARC.EXE as above, but has updated documentation (1989-04-21) and other differences.
 +
** ARC+Plus v7.10 (1990-10-04): [https://archive.org/details/floppycollection_NumberedFloppies] → 370.img → ARC71.ZIP
 +
** {{CdTextfiles|svplatinum/CDR31/ARC712.ZIP|ARC+Plus v7.12}} (~1991-01-24)
 +
** [http://old-dos.ru/index.php?page=files&mode=files&do=show&id=699 Various versions, at old-dos.ru]
 +
* ARC - source code
 +
** {{CdTextfiles|pcmedic/utils/compress/arc520s.zip|ARC v5.20 source code}}
 +
** [https://www.funet.fi/index/msdos/packing/archiver/arc521s.zip ARC v5.21 source code]
 +
*** {{CdTextfiles|hobbesos29503/16BIT/ARCHIVER/ARC521_2.ZIP|ARC521_2.ZIP}}
 +
* XARC - A minimal ARC extractor from SEA (DOS binaries, etc.)
 
** {{CdTextfiles|carousel/013C/XARC.ZIP|v4.31}} (1985-10-10)
 
** {{CdTextfiles|carousel/013C/XARC.ZIP|v4.31}} (1985-10-10)
 
** {{CdTextfiles|microhaus/mhblackbox3/ARCHIVER/XARC500.ZIP|v5.00}} (1986-01-23)
 
** {{CdTextfiles|microhaus/mhblackbox3/ARCHIVER/XARC500.ZIP|v5.00}} (1986-01-23)
−
** {{CdTextfiles|microhaus/mhblackbox3/ARCHIVER/XARC71.ZIP|v7.1}} (1990-10)
+
** ARCv7-era version (documentation strangely calls it "XARC V1.0"), DOS-only (1990-07-15): {{CdTextfiles|swextrav1993/disk3/archive/xarcplus.zip|xarcplus.zip}}
−
** {{CdTextfiles|pier/pier01/001a/xarc712.zip|v7.12}} (1990-10?)
+
** Unknown ARCv7-era version, OS/2-only: [https://archive.org/details/os2sharewarebbs OS/2 Shareware BBS Full Collection] → 09-Archive.zip → XARC_OS2.EXE
−
* PKARC/PKPAK (DOS binaries)
+
** Unknown ARCv7-era version, DOS & OS/2: {{CdTextfiles|wildcatgold/compress/qarc.zip|qarc.zip}} → XARC.EXE
−
** {{CdTextfiles|microhaus/mhblackbox3/ARCHIVER/PK36.EXE|PKARC 3.6}}
+
** v7.10 [DOS only] (1990-10-04): [https://archive.org/details/pegasus-win-os-2 Pegasus Win & OS/2 Edition] → SUPPORT/PACKER/XARCPLUS.ZIP
−
** {{CdTextfiles|microhaus/mhblackbox3/ARCHIVER/PK361.EXE|PKPAK 3.61}}
+
** {{CdTextfiles|microhaus/mhblackbox3/ARCHIVER/XARC71.ZIP|v7.10 &#x5b;DOS & OS/2&#x5d;}}
−
* PAK - See [[PAK (ARC extension)#Software]].
+
** v7.12 [DOS only, variant 1]: {{CdTextfiles|svplatinum/CDR31/ARC712.ZIP|ARC712.ZIP}} → ... → ARCDOS.ARC → XARC.EXE
−
* {{CdTextfiles|simtel/simtel9510/disk2/DISC2/ARCHIVER/SQUASH.ZIP|SQUASH.ZIP}} - Public domain decompression code for one of the compression methods
+
** {{CdTextfiles|pier/pier01/001a/xarc712.zip|v7.12 &#x5b;DOS only, variant 2&#x5d;}}
 +
** {{CdTextfiles|swsii/072/XARC.ZIP|v7.12 &#x5b;DOS & OS/2&#x5d;}}
 +
** Included with some versions of ARC.
 +
* ARCE (or ARC-E), by Wayne Chin and Vernon D. Buerg - An optimized ARC extractor for DOS
 +
** {{CdTextfiles|simtel/simtel9510/disk2/DISC2/ARCHIVER/ARCE41A.ZIP|ARCE v4.1a}} (1992-04-12)
 +
** Included with some versions of ARC.
 +
* PKARC/PKPAK (DOS binaries) - See [[PKARC/PKPAK#Software]].
 +
* PAK (NoGate Consulting) - See [[PAK (ARC extension)#Software]].
 +
* {{Deark}}
 +
 
 +
== Sample files ==
 +
* {{CdTextfilesURL|powerprogramming/PROGTOOL/}} ...
 +
* [https://www.dan.info/sampledata/CHRISTIE.ARC CHRISTIE.ARC]
 +
* [https://www.dan.info/sampledata/POLIPREF.ARC POLIPREF.ARC]
 +
* {{CdTextfilesURL|pcblue/}}
 +
* {{CdTextfiles|jcs/jcsma2v19/mktsales/tfo.zip|tfo.zip}} → *.ARC (v7)
 +
* {{CdTextfiles|smsharew/WORD/JORJ41-1.ZIP|JORJ41-1.ZIP}} → *.ARC (v7)
 +
* {{DexvertSamples|archive/arc}}
  
 
== References ==
 
== References ==
 
* [[Wikipedia:ARC (file format)|ARC (file format): Wikipedia]]
 
* [[Wikipedia:ARC (file format)|ARC (file format): Wikipedia]]
 
* [http://www.esva.net/~thom/arclicense.html Original ARC license]
 
* [http://www.esva.net/~thom/arclicense.html Original ARC license]
−
* [http://textfiles.com/computers/sea.txt SEA vs. PKWARE suit commentary] (which worries that tech people will end up spending more time in court than working on programs in the future, and is quaintly old-fashioned in that it refers to a legal case that was settled for $62,500, not the billions that are involved in modern-day tech legal cases)
 
−
* [https://www.youtube.com/watch?v=_zvFeHtcxuA BBS Documentary Part 8: Compression]
 
 
* [http://www.textfiles.com/programming/FORMATS/arc-lbr.pro ARC vs LBR comparison (1985)]
 
* [http://www.textfiles.com/programming/FORMATS/arc-lbr.pro ARC vs LBR comparison (1985)]
 +
 +
== Further reading ==
 +
* [http://textfiles.com/computers/sea.txt SEA vs. PKWARE suit commentary] <!-- (which worries that tech people will end up spending more time in court than working on programs in the future, and is quaintly old-fashioned in that it refers to a legal case that was settled for $62,500, not the billions that are involved in modern-day tech legal cases) -->
 +
* [http://ascii.textfiles.com/archives/2398 ARC/ZIP Perspective by Dean W. Cooper]
 +
* [https://www.youtube.com/watch?v=_zvFeHtcxuA BBS Documentary Part 8: Compression]

Latest revision as of 16:12, 24 November 2024

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

[edit] 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."

[edit] 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.

[edit] 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.)

[edit] 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.

[edit] 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.

[edit] 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.

[edit] PAK extended records

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

[edit] 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.

[edit] 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

[edit] See also

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

[edit] Specifications

[edit] Software

[edit] Sample files

[edit] References

[edit] Further reading

Retrieved from "http://fileformats.archiveteam.org/index.php?title=ARC_(compression_format)&oldid=49698"
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