SQ (''squeeze'') is a computer program, devised by Richard (Dick) Greenlaw circa 1981, which was used in the early 1980s on both
DOS
DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems.
DOS may also refer to:
Computing
* Data over signalling (DoS), multiplexing data onto a signalling channel
* Denial-of-service attack (DoS), an attack on a communicatio ...
and
CP/M
CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/ 85-based microcomputers by Gary Kildall of Digital Research, Inc. Initial ...
computer systems to compress files so they use less space.
Files compressed by SQ are identified by changing the middle initial of the extension to "Q", so that text files ended with the extension , executable files ended with the extension or , documents with , etc. SQ used static
Huffman coding
In computer science and information theory, a Huffman code is a particular type of optimal prefix code that is commonly used for lossless data compression. The process of finding or using such a code proceeds by means of Huffman coding, an algori ...
as the compression algorithm.
Groups of files were often combined into an archive using the LU program, which created
.LBR files containing all the files needed for a particular group, such as all the files needed to install an application. Typically such files were either individually compressed (because LU did not compress files) or the LBR archive was itself compressed with SQ (similarly to the use of
tar
Tar is a dark brown or black viscous liquid of hydrocarbons and free carbon, obtained from a wide variety of organic materials through destructive distillation. Tar can be produced from coal, wood, petroleum, or peat. "a dark brown or black bit ...
and
gzip
gzip is a file format and a software application used for file compression and decompression. The program was created by Jean-loup Gailly and Mark Adler as a free software replacement for the compress program used in early Unix systems, and in ...
together).
With the development of the ARC program (which combined both compression and archiving into one program) and its
ARC
ARC may refer to:
Business
* Aircraft Radio Corporation, a major avionics manufacturer from the 1920s to the '50s
* Airlines Reporting Corporation, an airline-owned company that provides ticket distribution, reporting, and settlement services
* ...
archive file format, SQ essentially became obsolete on most systems, except CP/M, which lacked an ARC
port
A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Ham ...
for several years. On CP/M systems, the CRUNCH compression program was written to implement the
LZW algorithm (as ARC did) and plug the gap.
References
Archive formats
Data compression software
CP/M software
{{Compression software