Coherent (operating system)

Coherent was a Unix-like real-time operating system (RTOS) developed and sold by Mark Williams Company (MWC) starting in 1980. It was designed to run on IBM PC compatibles and other Intel x86 architecture systems. Significantly, Coherent was a completely independent implementation, not derived from AT&T Unix source code. This allowed MWC to sell it at a much lower price than AT&T Unix licenses, making it an attractive option for hobbyists, educational institutions, and small businesses.

Coherent aimed to provide a Unix-like environment with features such as a hierarchical file system, command-line interface (shell), utilities like vi and cc, and a C compiler. Its design, while not fully compliant with the Single UNIX Specification, followed many of the principles and conventions of Unix. It was known for its relatively small size and efficient resource usage, allowing it to run on systems with limited memory and processing power.

Different versions of Coherent were released over the years, supporting various hardware configurations and adding features. It was initially available for 8088/8086 processors and later adapted to run on more advanced CPUs, including the 80286 and 80386. Version 4, released in 1992, was notable for its improved support for protected mode and virtual memory.

Although Coherent was popular for a time as a low-cost Unix alternative, its market share gradually declined with the rise of other operating systems, most notably Linux. MWC discontinued commercial sales of Coherent in 1995. In 2015, Caldera International (the successor to SCO) released the source code of Coherent Version 4.2 under a BSD-style license, making it open-source. This allows developers to study, modify, and redistribute the system.