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MS-DOS ( /ˌɛmɛsˈdɒs/ em-es-doss; short for Microsoft Disk Operating System) is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating systems offering a graphical user interface (GUI), in particular by various generations of the Microsoft Windows operating system.

Xenix is a version of the Unix operating system, licensed by Microsoft from AT&T in the late 1970s. The Santa Cruz Operation (SCO) later acquired exclusive rights to the software, and eventually superseded it with SCO UNIX (now known as SCO OpenServer).

Pilot was a single-user, multitasking operating system designed by Xerox PARC in early 1977. Pilot was written in the Mesa programming language, totalling about 24,000 lines of code. Pilot was designed as a single user system in a highly networked environment of other Pilot systems, with interfaces designed for inter-process communication (IPC) across the network via the Pilot stream interface. Pilot combined virtual memory and file storage into one subsystem, and used the manager/kernel architecture for managing the system and its resources. Its designers considered a non-preemptive multitasking model, but later chose a preemptive system based on monitors. Pilot was used as the operating system for the Xerox Star workstation.

86-DOS was an operating system developed and marketed by Seattle Computer Products for its Intel 8086-based computer kit. Initially known as QDOS (Quick and Dirty Operating System) the name was changed to 86-DOS once SCP started licensing the operating system. 86-DOS had a command structure and application programming interface that imitated that of Digital Research's (abbreviated DR or DRI) CP/M operating system, which made it easy to port programs from the latter. The system was purchased by Microsoft and developed further as PC-DOS and MS-DOS.

OS-9 is a family of real-time, process-based, multitasking, multi-user, Unix-like operating systems, developed in the 1980s, originally by Microware Systems Corporation for the Motorola 6809 microprocessor. It is currently owned by RadiSys Corporation. The OS-9 family was popular for general-purpose computing and remains in use in commercial embedded systems and amongst hobbyists. Today, OS-9 is a product name used by both a Motorola 68000-series machine language OS and a portable (PowerPC, x86, etc.) version written in C, originally known as OS-9000.

Seventh Edition Unix, also called Version 7 Unix, Version 7 or just V7, was an important early release of the Unix operating system. V7, released in 1979, was the last Bell Laboratories release to see widespread distribution before the commercialization of Unix by AT&T in the early 1980s. V7 was originally developed for Digital Equipment Corporation's PDP-11 minicomputers and was later ported to other platforms.

NLTSS, the Network Livermore Timesharing System, also sometimes known as the New Livermore Time Sharing System was an operating system that was actively developed at Lawrence Livermore Laboratory (now Lawrence Livermore National Laboratory) from 1979 until about 1988, though it continued to run production applications until 1995. NLTSS ran initially on a CDC 7600 computer, but only ran production from about 1985 until 1994 on Cray computers including the Cray-1, Cray X-MP, and Cray Y-MP models.

Unix (officially trademarked as UNIX, sometimes also written as Unix) is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna. The Unix operating system was first developed in assembly language, but by 1973 had been almost entirely recoded in C, greatly facilitating its further development and porting to other hardware. Today's Unix systems are split into various branches, developed over time by AT&T as well as various commercial vendors and non-profit organizations. The second edition of Unix was released on December 6th, 1972.

The TOPS-20 operating system by Digital Equipment Corporation (DEC) was the second proprietary OS for the PDP-10 mainframe computer. TOPS-20 began in 1969 as the TENEX operating system of Bolt, Beranek and Newman (BBN). TOPS-20 is almost entirely unrelated to the similarly named TOPS-10, but it was shipped with the PA1050 TOPS-10 Monitor Calls emulation facility which allowed most, but not all, TOPS-10 executables to run unchanged. As a matter of policy, DEC did not update PA1050 to support later TOPS-10 additions except where required by DEC software. TOPS-20 was preferred by most PDP-10 users over TOPS-10 (at least by those who were not ITS or WAITS partisans).

IBM Airline Control Program, or ACP, was an operating system developed by IBM beginning about 1965. In contrast to previous airline transaction processing systems, the most notable aspect of ACP is that it was designed to run on most models of the IBM System/360 mainframe computer family. This departed from the earlier model in which each airline would have a different, machine-specific transaction system.