Coherent (operating system)

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Coherent 4.2.10 installer screenshot.png
Coherent 4.2.10 installer
Developer Mark Williams Company
OS family Unix-like
Working state Historic
Source model Closed source; open sourced in 2015
Initial release 1980
Latest release 4.2.10 / 1994
Available in English
Platforms PDP-11, 8088, 286, 386, 486, Zilog Z8000
Kernel type Monolithic
License BSD

The Coherent operating system was a Version 7 Unix clone by the now-defunct Mark Williams Company, originally produced for the PDP-11 in 1980. A port was introduced in 1983 as the first Unix-like system for IBM PC compatible computers.

Coherent was able to run on most Intel-based PCs with Intel 8088, 286, 386, and 486 processors. Coherent version 3 for Intel-based PCs required at least a 286, Coherent version 4 for Intel-based PCs required at least a 386. Like a true Unix, Coherent was able to multitask and support multiple users. From version 4 on Coherent also had support for X11 and MGR windowing systems.

Later versions of Coherent (version 4 and higher) supported features common in modern Unix-like systems, including a version of MicroEMACS, access to DOS FAT16 File systems, an optimizing C compiler with linker, and a modified version of Taylor UUCP. The final releases of Coherent also fully supported the iBCS COFF binary standard, which allowed binary compatibily with SCO Unix applications, including WordPerfect, Lotus 1-2-3, and several Microsoft applications including QuickBASIC, Microsoft Word, and MultiPlan. There was no support for virtual memory or demand paging. Coherent predates both MINIX and Linux by many years.

A Zilog Z8000 port of Coherent was also used by the canceled Commodore 900 system.

Coherent was not Unix; the Mark Williams Company had no rights to either the Unix trademark or the AT&T/Bell Labs source code. In the early years of its existence, MWC received a visit from an AT&T delegation looking to determine whether MWC was infringing on AT&T Unix property. The delegation included Dennis Ritchie, who concluded that "it was very hard to believe that Coherent and its basic applications were not created without considerable study of the OS code and details of its applications" but "that looking at various corners [for peculiarities, bugs, etc. that I knew about in the Unix distributions of the time] I couldn't find anything that was copied. It might have been that some parts were written with [AT&T] source nearby, but at least the effort had been made to rewrite. If it came to it, I could never honestly testify [...] that what they generated was irreproducible from the manual."[1]

Much of the operating system was written by alumni from the University of Waterloo: Tom Duff, Dave Conroy, Randall Howard, Johann George, and Trevor John Thompson. Significant contributions were also made by people such as Nigel Bree (from Auckland, New Zealand). Nigel went on to write "Ghost" - later bought out and fully commercialized as Norton Ghost.

The Mark Williams Company closed in 1995.[2]

On January 3, 2015, Coherent sources were released under the 3-clause BSD license.[3]

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