Coherent (operating system)

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**Coherent**
Company/ developer:	Mark Williams Company
OS family:	Unix-like
Latest stable release:	4.2.10 / 1994
Kernel type:	monolithic
Working state:	Historic

The Coherent operating system was a UNIX Version 7 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 systems for IBM PC-compatible computers. Coherent was capable of running on most Intel-based PCs with Intel 8088, 286, 386, and 486 processors and, like a true Unix, was capable of multitasking and of having multiple users. Coherent also had support for X11 and MGR windowing systems. Later versions of Coherent (4 and higher) supported features common in modern Unix-like systems, including virtual memory with demand paging, a version of MicroEMACS, access to MS-DOS FAT16 filesystems, 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. Coherent predates both Minix and Linux by many years.

Coherent is sometimes mistakenly referred to as "Coherent Unix", which is incorrect. Coherent was based on the specifications of Unix Version 7, without reference to any of the Unix source code, either from Bell Labs or BSD.

Much of the operating system was written by ex-students from the University of Waterloo: Tom Duff, Dave Conroy, Randall Howard, and Johann George.

The arrival of Linux on the scene sounded the death-knell for Coherent. Coherent was unable to compete with the burgeoning GNU/Linux movement, which, owing to its vast body of contributors, quickly exceeded Coherent in feature set and quality. For example, Coherent never got a kernel-mode TCP/IP stack. As a result, The Mark Williams Company closed in 1995.

Some websites offer Coherent for download, although the copyright status of it nowadays is unclear. While Coherent does not have the features of a modern Unix-like OS such as Linux, it is still a viable solution for those looking to run Unix on a very old computer such as a 286-based machine.