RSX-11
| Company type | Proprietary software |
|---|---|
| Industry | Computer software |
| Predecessor | RSX-15 |
| Founded | 1973 |
| Founder | Digital Equipment Corporation |
| Fate | Rights transferred to Mentec Inc. in 1994; later acquired by Calyx and XX2247 LLC |
| Headquarters | United States |
Area served | Worldwide (historically) |
| Products | RSX-11A, RSX-11B, RSX-11C, RSX-11D, RSX-11M, RSX-11S, RSX-11M Plus, Micro/RSX, P/OS, IAS |
| Owner | Originally Digital Equipment Corporation; currently XX2247 LLC (for some versions) |
RSX-11 represents a now-obsolete series of multi-user real-time operating systems developed by Digital Equipment Corporation for the PDP-11 line of computers. Predominantly utilized during the late 1970s and early 1980s, RSX-11 played a pivotal role in shaping subsequent operating systems, including VMS and Windows NT.
Initially designated as Real-Time System Executive, RSX was engineered with a focus on real-time tasks, particularly in the realm of process control. Its reliable real-time performance also made it a popular choice for software development and general-purpose computing.
Name and origins
The RSX-11 system evolved as a port from the earlier RSX-15 OS designed for the PDP-15 minicomputer, which debuted in 1971. Dennis “Dan” Brevik was the principal designer behind RSX-15 (later known as XVM/RSX).
Concerning the acronym RSX, Brevik recounted that the original name was DEX-15 ("Digital's Executive - for the PDP-15"). However, trademark issues with a paper company necessitated a renaming. Brevik proposed several acronyms, favoring those containing an "X" due to its strong and dynamic connotation. After legal evaluation, a shortlist was created, and in collaboration with marketer Bob Decker, the name "RSX" was chosen, initially standing for "Real-Time System Executive." This was subsequently updated to "Resource Sharing Executive" to better align with its changing commercial objectives.
This naming process occurred on the third floor of building 5 at Digital Equipment Corporation’s original headquarters.
RSX-11D and IAS
The first porting stage produced real-time executives based on paper tape—RSX-11A and RSX-11C—with limited disk functionality introduced in RSX-11B. RSX-11B later matured into the fully disk-supported RSX-11D, released for the PDP-11/40 and PDP-11/45 models in early 1973. Up to version 4, project leadership was under Henry Krejci.
At the same time, Digital developed a more memory-efficient variant of RSX-11D named RSX-11M, which launched in 1973. From 1971 through 1976, operating system designer Dave Cutler led the RSX-11M project during his inaugural major role. Concepts from RSX-11M influenced future operating systems such as DEC's VMS, MICA, and Microsoft's Windows NT, underscoring Cutler’s enduring influence on OS design.
Under the guidance of Ron McLean, RSX-20F—a spin-off of RSX-11M—was created to operate on the PDP-11/40 front-end processor used with the KL10 PDP-10 CPU.
Meanwhile, from 1972 to 1976, Garth Wolfendale oversaw a redesign of RSX-11D aimed at commercial distribution that incorporated support for the 22-bit addressing PDP-11/70 system. Wolfendale also founded the UK team responsible for designing and prototyping the Interactive Application System (IAS), a time-sharing adaptation of RSX-11D. Further development of IAS was managed by Andy Wilson at Digital’s UK division.
Release dates
The table below provides approximate release dates for various RSX-11 versions and IAS based on documented printing dates; general availability usually followed soon after. When multiple manual dates exist, the most recent is cited. RSX-11S, a streamlined subset of RSX-11M, shares corresponding release timelines. The enhanced version known as RSX-11M Plus appeared later.
| Date | RSX-11A, C | RSX-11D | IAS | RSX-11M, S | RSX-11M Plus | Micro/RSX | Comment | ||
|---|---|---|---|---|---|---|---|---|---|
| March 1973 | RSX-11A 1.0 | ||||||||
| May 1973 | RSX-11D 1.0 | ||||||||
| December 1973 | RSX-11C 7A | Final release of RSX-11C | |||||||
| November 1974 | RSX-11M 1.0 | ||||||||
| June 1975 | RSX-11D 6.2 | Final version of RSX-11D | |||||||
| September 1975 | RSX-11M 2.0 RSX-11S 2.0 |
RSX-11S 1.0 never existed | |||||||
| December 1975 | IAS 1.0 | ||||||||
| April 1977 | RSX-11M 3.0 RSX-11S 3.0 |
||||||||
| December 1977 | RSX-11M 3.1 RSX-11S 3.1 |
||||||||
| May 1979 | RSX-11M 3.2 RSX-11S 3.2 |
RSX-11M Plus 1.0 | |||||||
| before October 1979 | IAS 3.0 | Final major IAS release | |||||||
| November 1981 | RSX-11M 4.0 RSX-11S 4.0 |
RSX-11M Plus 2.0 | |||||||
| April 1983 | RSX-11M 4.1 RSX-11S 4.1 |
RSX-11M Plus 2.1 | |||||||
| July 1985 | RSX-11M 4.2 RSX-11S 4.2 |
RSX-11M Plus 3.0 | Micro/RSX 3.0 | ||||||
| September 1987 | RSX-11M 4.3 RSX-11S 4.3 |
RSX-11M Plus 4.0 | Micro/RSX 4.0 | Final Micro/RSX version | |||||
| May 1988 | RSX-11M 4.4 RSX-11S 4.4 |
RSX-11M Plus 4.1 | |||||||
| January 1989 | , | , | , | "RSX-11M" "4.5" "RSX-11S" "4.5" |
"RSX-11M Plus" "4.2" | , | , | , | |
| "January" "1990" | , | , | , | "RSX-11M" "4.6" "RSX-11S" "4.6" |
"RSX-11M Plus" "4.3" | , | , | , | |
| "May" "1990" | , | , | "IAS" "3.4" | , | , | , | "Final IAS Release" | ||
| "February" "1993" | , | , | , | "RSX-11M" "4.7" "RSX-11S" "4.7" |
"RSX-11M Plus" "4.4" | , | "Last release from Digital Equipment" | ||
| "March" "1995" | , | , | , | , | "RSX-11M Plus" "4.5" | , | , | ||
| "November" "1998" | , | , | , | "RSX-11M" "4.8" "RSX-11S" "4.8" |
, | , | "Released by Mentec" | ||
| "February" "1999" | , | , | , | , | "RSX-11M Plus" "4.6" | "Micro/RSX" "4.6" | "Released by Mentec" |
Legal ownership, development framework, and distribution
RSX-11 is proprietary software created internally by Digital. Each RSX release includes a copy of the kernel source code, utilized during the system generation phase. The only exception is Micro-RSX, which was distributed with a pre-built autoconfiguring binary kernel. Complete source code was made available separately to holders of binary licenses for reference purposes.
In March 1994, the rights to RSX-11S, RSX-11M, RSX-11M Plus, and Micro/RSX were transferred from Digital to Mentec Inc. under a comprehensive agreement. Mentec Inc., the American branch of the Irish company Mentec Limited—specializing in PDP-11 hardware and software support—declared bankruptcy in 2006. Later that year, Mentec Ltd. was acquired by the Irish firm Calyx. Subsequently, XX2247 LLC purchased the PDP-11 software previously owned by Mentec Inc., becoming its current proprietor. It remains uncertain if new commercial licenses are presently offered. Nevertheless, enthusiasts may operate RSX-11M (up to version 4.3) and RSX-11M Plus (up to version 3.0) on the SIMH emulator under a complimentary license granted by Mentec in May 1998.
The legal rights to RSX-11A, RSX-11B, RSX-11C, RSX-11D, and IAS stayed with Digital and were transferred to Compaq when it acquired Digital in 1998, then passed on to Hewlett-Packard in 2002. After Hewlett-Packard split into HP Inc. and Hewlett Packard Enterprise in 2015, DEC’s intellectual property became part of HPE. Since at least October 1979 (for RSX-11A/B/C) and 1990 (for IAS), no new commercial licenses have been issued for these systems, nor have they ever been authorized for hobbyist use.
Primary editions
- RSX-11A, C – compact real-time executives designed for paper tape
- RSX-11B – a small real-time executive derived from RSX-11C that added disk I/O capabilities; booted through DOS-11 with RSX-11B applications employing DOS-11 macros for disk operations
- RSX-11D – a multiuser disk-based operating system that served as a forerunner to IAS
- IAS – a timesharing adaptation of RSX-11D for the PDP-11/70; the first RSX variant to incorporate DCL (Digital Command Language), initially named PDS (Program Development System)
- RSX-11M – widely used multiuser version across PDP-11 platforms
- RSX-11S – a memory-resident form of RSX-11M tailored for embedded real-time applications; its development occurred within the RSX-11M framework
- RSX-11M-Plus – an enhanced iteration of RSX-11M created for the unreleased multiprocessor PDP-11/74; it found extensive use on PDP-11/70 and other models (PDP-11/44, /84, /94, /73, /83, /93). This version supported larger programs by separating instruction and data spaces, minimizing overlays and boosting performance
Hardware-targeted editions
- RSX-20F – a specialized version of RSX-11M designed for the PDP-11/40 front-end processor in DEC KL10 systems
- Micro/RSX – a fully configured edition of RSX-11M Plus featuring hardware autoconfiguration aimed at Micro/PDP-11 systems; prioritized ease of setup without requiring system generation and was eventually superseded by RSX-11M Plus
- P/OS – an RSX-11M Plus variant developed for the DEC Professional series of PDP-11-based personal computers
Cloning Efforts in the USSR and Eastern Bloc Nations
In 1968, Soviet authorities determined that producing replicas of IBM mainframes and DEC minicomputers collaboratively with other COMECON member states was a more feasible approach than developing original systems. The replication of DEC architectures commenced in 1974 under the designation SM EVM ( or ). Much like the ES EVM mainframes modeled on the System/360 framework, these Soviet and allied adaptations frequently incorporated substantial alterations to Western designs, causing numerous SM EVM units to lack binary compatibility with their DEC counterparts.
- DOS/RV, Russian: ОСРВ-СМ, ОСРВM – These three titles refer to an unauthorized RSX-11M clone produced within the Eastern bloc. ОСРВ is an acronym for . This system imitated RSX-11M closely but exhibited differences in binary file headers. Variations emerged due to hardware discrepancies between SM and PDP machines as well as bug corrections implemented by Soviet developers. Nonetheless, the original RSX-11M maintained wider adoption because it offered superior programmer support, quicker driver updates, and a more extensive patch repository, resulting in greater system stability compared to ОСРВ.
An RSX-11M operating system clone was also deployed on Romanian-manufactured CORAL series computers (such as the CORAL 2030, itself a PDP-11 replica).
Functionality
RSX-11 was engineered primarily for real-time operations on PDP-11 hardware, featuring capabilities such as locking a task into memory during startup and assigning elevated priorities to guarantee prompt execution—critical for managing peripheral device interactions. While optimized for real-time environments, it also served as a general-purpose timesharing system, competing with RSTS/E.
To accommodate large applications within the PDP-11’s constrained 64 KB virtual address limit, RSX-11 utilized an advanced semi-automatic overlay mechanism generated by its taskbuilder (TKB). Employing complex overlay configurations could considerably prolong compilation durations.
The typical RSX command prompt appears as ">" or "MCR>", with commands distinguished by their initial three characters. The login command "HELLO" (abbreviated as "HEL") provides access to help features prior to user authentication.
Unique console light sequences convey system status on PDP-11 processors: when idle, RSX-11M exhibits two sets of lights radiating outward from the center, differing from other DEC operating systems like IAS and RSTS/E. These visual indicators reflect processor workload and task activity.