Coherent (operating system)

Coherent

Coherent 4.2.10 installer
Developer	Mark Williams Company
OS family	Unix-like
Working state	Discontinued
Source model	Closed source; open sourced in 2015
Initial release	1980
Latest release	4.2.10 / 1994
Available in	English
Platforms	PDP-11, x86 (8088, 286, 386, 486), Zilog Z8000
Kernel type	Monolithic
License	BSD

Coherent is a clone of the Unix operating system for IBM PC compatibles and other microcomputers, developed and sold by the now-defunct Mark Williams Company (MWC). Historically, the operating system was a proprietary product, but it became open source in 2015, released under a 3-clause BSD License.

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." However, he also stated 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.

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), the later author of Ghost.

Coherent was originally written for the PDP-11 range of minicomputers in 1980, then ported to various early 1980s microcomputer systems including IBM PC compatibles and machines based on the Zilog Z8000 and Motorola 68000. Initially sold to OEMs, starting 1983 it was available on the consumer market from MWC directly. At this point, Coherent 2.3 offered roughly the functionality of Version 7 Unix on PC hardware, including the nroff formatter but not the BSD extensions offered by competing Unix/clone vendors; compared to its competitors, it was a small system distributed on only seven double-sided floppy disks, costing only US$500 for a license.