Crunch

Revision as of 20:41, 15 June 2022

This article is about the CP/M compressed file format. See the disambiguation section for other "Crunch" formats.

Crunch was a method of compressing single files popular on CP/M, devised by Steve Greenberg circa 1986. It superseded Squeeze and was succeeded by CrLZH, and crunched files were common in LBR archives. The underlying compression uses the LZW algorithm, combined with run-length encoding.

Similar to Squeeze, crunched files were signified in CP/M's 8.3 filename format by replacing the middle letter of the extension with Z (.?Z?), with the extension .ZZZ used for corner cases such as a blank extension.

There are two main versions of the compressed data format, and not all decompressors support both. The new (v2.x) format is apparently more common.

Disambiguation

Not to be confused with:

• The "crunched" compression methods used in ARC format, though they are related.
• Crunch-Mania - An Amiga file compression utility
• Cruncher - An executable compression utility for DOS, by Ori Berger [1]
• CRUNCH - A compression optimization utility for DOS, by Bruce Gavin. It does not appear to have any native file formats. [2]
• Crunch - An old ARC compression utility by Richard P. Byrne [3]
• CRUNCH - A PKARC automation utility by Chuck Zulker [4]

See also

• LZWCOM - predecessor
• Squeeze - predecessor
• CrLZH - successor
• LBR - container
• ZSQ (LZW compression)‎ - Similar format
• Zoo Z format - Same file naming convention

Format details

The file header follows a similar/compatible structure to CrLZH. It was derived from Squeeze, but bears only a little resemblance to it.

Note that, as explained in the format documentation, the "filename" field contains not only the filename, but also extension data. If extension data exists, the filename extension is padded with spaces until it is exactly three characters long.

In archives originating on CP/M systems, the high bit of each byte in the filename field may contain encoded CP/M file attributes. To extract the original filename, each byte should be masked with 0x7F.

Compression

V1.x compression is based on RLE90 and LZWCOM, very similar to ARC's method #6, except that Crunch reserves code 0 to mean "stop".

V2.x compression is considerably more complex. CRUNCH20.DOC shipped in CRUNCH20.LBR says: It embodies all of the concepts employed in the UNIX COMPRESS / ARC512 algorithm, but is additionally enhanced by a "metastatic code reassignment" facility. This is one of several concepts I am developing as part of an effort to advance data compression techniques beyond current performance limits. I believe this is the first time this principle has been proposed or implemented.

Identification

Files begin with bytes 76 fe.

Specifications

• The file header is described in the text file LZDEF20.DOC shipped with CRUNCH20.LBR.
  • An extracted copy is provided here.
• crunch.abs - "Technical Abstract" by Steven Greenberg, 16 November 1986
• crunch.izf → crunch.inf - Collected information about the format

Software

• CFX (DOS/Unix)
• lbrate by Russell Marks, c. 2001 (Unix, GPL2)
• The Unarchiver
• On CP/M (or emulators):
  • The canonical tools were CRUNCH and UNCR. Possibly Greenberg's last version (Feb 1988) is v2.4:
    • CRUNCH24.LBR
    • CRNCH24S.LBR (source code)
  • The later LT31 deals with extracting from all of Squeeze, Crunch, CrLZH and LBR formats. Widely available in CP/M archives, e.g. LT31.LBR
  • crunch12.lbr - Crunch 1.2 - Possible sources: [5], [6]
  • crunch20.lbr - Crunch 2.0
  • fcrnch11.lbr - FCRUNCH v1.1 - An improved version of Crunch 2.x, by C.B. Falconer
• UNCR version "UNCR231" - Crunch v2 decompression source code by Frank Prindle. Package includes a DOS binary.
  • UNCR233 - Based on UNCR231, with modifications by Skip Hansen (source code + DOS binary)

Sample files

• OAK CP/M archive → .../*.?z?
• Found in many LBR files. Note that you may have to tell your LBR utility not to decompress them (e.g. lbrate -n).
• https://telparia.com/fileFormatSamples/archive/crunch/