What operating systems were written for the PDP-11?

CAPS-11
Cassette Based Programme development System.
CTS-300
There's some confusion as to whether this is just a re-packaged version of RT-11, or a time-sharing system layered on top of RT-11.
CTS-500
A repackaged RSTS. As remembered by Paul Koning, a former RSTS developer:
CTS-500 is indeed repackaged RSTS. RSTS itself is a timesharing system, you don't need to layer anything on top of it. The packaging included, if I remember correctly, some stuff aimed at business applications such as an indexed file system. (This predates the support of RMS-11 in RSTS, and RMS-11 basically eliminated the reason for having CTS-500.)

There were some PDP-11s with special paint jobs (blue rather than maroon) and bezels, and also I think lower cabinets, as part of the CTS-500 effort.

DOS/BATCH
Duress
Written for the PDP-11/45 in 1972-1973 mainly by Russ Atkinson, Roger Haskin and Perry Emrath, all of University of Illinois Urbana, for a Chicago company named Datalogics (which was consumed by Frame, and subsequently by Adobe).

Duress was similar to Dec's DOS to the outside user. The motivation for writing the system was to take full advantage of the addressing modes of the 11/45, and may indeed be the only working system to use all three addressing modes. The system used K-space for the low- level kernel, device drivers and task management; S-space (Supervisor) for the disk file system; and U (user) for application code. Almost all other systems ignored S-space and put the entire OS in K-space. This implementation effectively doubled the amount of resident memory available for the OS.

The system was used for a phototypesetting system for the State of Wisconsin Legislature, and later was used for a variety of financial and newspaper typesetting and editing systems.

FUZZBALL University of Delaware, David L Mills.
Used for TCP/IP, NTP, Packet Radio, Hello Routing protocol research, complete with FAX facility, boots from an RT11 system, uses DEQNA UNIBUS, PROTEON Unibus ethernet cards. Some still hiding away.
GAMMA-11.
RT-11-derived operating system for Nuclear Medicine, supporting VT11 and VSV11 displays, with image analysis software and ultimately sold by DEC around 1985/6 to Philips Medical Systems.
HT-11
Heathkit's hacked version of RT-11, wouldn't run on a "real" PDP-11.
IAS
Interactive Application System. Multi-purpose multiprogramming system. This was a derivative of RSX11-D, with additional features to make it more of a time-sharing system. It also was specifically aimed at the 11/70, while RSX11-D was for the 18-bit machines like the 11/45. There is some suspicion that the intention was to make RSTS go away, but RSTS started supporting more languages than just BASIC and ended up outliving IAS.
MERTS (MERTSS?)
A virtual operating system that could run Unix as a process. Several machines at BTL ran MERTS.
Micropower Pascal
Not just a compiler, but a complete RT kernel product that could also be (and often was) programmed in assembly language as well. Folks who never used it were often confused by the unfortunate choice of name. Many large corporations used it, eventually migrating to VAXELN, which was similar in many respects.

By V2.5, MPP was highly reliable. The customized Pascal compiler had some issues, but I did most of my development in MACRO-11. We built ROM-based high-speed trunked radio system controllers with it at GE, and found it very satisfactory.

MTS The Multi-Tasking System written in RTL/2 by SPL. It was used for most projects in the 1970s until RSX-11M with RTL/2 became reliable and easier to use (Philip Hull).
MUMPS-11
Massachusetts General Hospital Multi-User Multi-Processing System. A language, an operating system and a DBMS all in one.
PC11 (Decus 11-501).
Process Control Operating System, developed originally by Pilkington UK for Glass applications, extended by various users and licencees. Used by ACI Glass in Australia, with DEC IP11 industrial I/O, on five or six sites. Have DECUS Disks, plus repaired stuff from John Gaunt ex ACI Australia. From Ken Kirkby.
RSTS, RSTS/E
Resource Sharing/Time Sharing. General purpose time-sharing system.
RT-11
Real Time. Foreground/Background or Single Job operating system.
RSX-11
Resource Sharing eXecutive. Multiprogramming system.
RSX-11B/C, Micro/RSX
RSX-11/D
Large real-time multiprogramming system.
RSX-11/M
Small to moderate-sized real-time multiprogramming system.
RSX-11/M+
Extended RSX-11/M.
RSX-11/S
Execute-only real-time multiprogramming system.
RSX-20F Modified version of RXS-11M that ran in the PDP-11/40 that was the console front-end for the PDP-10 Model KL. Primary console terminal for KL, KLINIK dial-up maintenance console, async terminals (hardwire and dial-up), line printer, card reader. Used part of the PDP-10's RP06 as its file system. The PDP-11 was used to start, stop, and load microcode into the PDP-10.
Softech Microsystems UCSD(TM) System with UCSD Pascal.
Sphere, from Infosphere - Portland Oregon 1981-87.
Developed on PDP11/23's by Evan Solley, and Rick Braithwaite, ran on KXT11A-AB' Falcons, complete with RLO2/RlO1, TU58, and RX02 drivers - used for embedded systems. Stoic-like syntax. Multi-tasking, built-in editor, ...
TRAX
Transaction Processing system.
TRIPOS
TRIPOS - A Portable Operating System for Mini-Computers, Software Practice and Experience, Volume 9, pp 513-526 (1979) by M. Richards, A. R. Aylward, P. Bond, R. D. Evans and B. J. Knight.

The web page for TRIPOS is at http://www.cl.cam.ac.uk/users/mr/Tripos.html, but it does not mention the PDP-11 support. TRIPOS is written mostly in BCPL.

TSX-11
Multi-user enhancements to RT-11 (third-party).
Ultrix
Digital's implementation/port of BSD UNIX.
Unix (tm)
Much/most of Unix was developed on PDP-11s. The source code for PDP-11-relevant versions of Unix is owned by the Santa Cruz Operation. They have been offering a low-cost "Ancient Unix License" for a while now, and as of April 18, 2000, dropped the licensing fee completely. Some relevant web links are: The SCO no-license-fee press release:
http://www.sco.com/press/releases/2000/6927.html
The PDP Unix Preservation Society's home page:
http://minnie.cs.adfa.edu.au/PUPS/
Marshall Kirk McKusick's CSRG Archive CD-ROM of all the BSD releases:
http://www.mckusick.com/csrg/index.html
Venix
A third-party implementation/port of UNIX.

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