Which time-sharing operating systems can be run via emulation today?

By "time-sharing" I mean an operating system which is

1. multi-user - more than one person can use the system at the same time.
2. multi-programming - more than one job can appear to run on the system at the same time.
3. interactive - a user can interact with the system without having to go through an intermediate step like submitting a batch job.
4. general purpose - the system can be used to develop and run programs in more than one language.

I also include virtualised systems like CP/CMS in the list.

We start with pioneers such as CTSS and Dartmouth DTSS in the early 1960s, but time-sharing really took off with mainframes in the late 1960s, with IBM's System/360 and Digital Equipment Corporation's PDP-10 having several commercial and academic operating systems. Also very influential was Multics, built by a consortium for a GE machine. IBM had many other competitors at this time, and outside the US several systems were developed. Minicomputers such as the PDP-8 and -11 started to appear towards the end of the decade, along with the birth of Unix on the latter machine. As the 70s continued, more minicomputers arrived, along with later large scale systems like PRIMOS and VAX VMS. Coming into the 1980s, focus shifted to microcomputers for personal use, with some outliers like MP/M and the workstation orientatted Domain/OS.

I have grouped operating systems into families where there is a clear line linking them together. This may span many years and different computer hardware. Probably the biggest example is Unix which is listed once in the table for the original implementation but went on to appear in many different versions (BSD, commercial Unixes like Solaris, Linux) across many different platforms.

As an operating system evolves, it can take on many names: for example the OS listed as CP/CMS includes its derivatives VM/370, VM/SE, VM/SP, VM/XA, VM/ESA, z/VM etc.

In some cases this is a judgement call - for example WAITS was originally based on TOPS-10, but I have included it as it is evolved to be a quite different system,

In scope are operating systems first introduced before 1983 - this is a fairly arbitrary cut off date chosen to focus on the period time-sharing systems were dominant before the personal computer started its ascent.

The date column indicates the first year an operating system was introduced: for commercial systems, the year of its first release to customers; for academic systems, the first year it ran production workloads.

Some operating systems had a lifespan of many years, gaining features and adding architectures. Some started as batch operating systems and had time-sharing added later. I have used the year when time-sharing features were added in that case. Some are still being maintained today, albeit in quite different forms from the original version, such as MVS, MCP and VMS.

This column was by far the hardest to verify and I would appreciate any correctiobs.

One complication then is, when we look at an operating system which version should we select? Often the choice is made for us, so for CTSS we have a single source from around 1969; in other cases like TOPS-10 we have a choice of releases spanning 1972 to 1988.

I will generally choose to look at the most recent version of a system, but this does make a chronological comparison of different systems more difficult.

I concentrate on running operating systems under emulation here, which anyone can do if they have a personal computer. The gold standard is an open-source, cross platform emulator, but I do included single platform ones (eg emulators running on Windows only) and closed-source but freely available emulators where this is the only choice. I do not include commercial, paid-for emulators as the focus for this site is hobbyists and researchers.

Some systems have more than one emulator. I usually choose simh or hercules if available, if not the most fully features single architecture emulator. In some cases where there is sufficiently different functionality or approach to emulation I list more than one.

simh is the most frequently represented emulator on this list as it emulates more than 30 different architectures. There are several different forks, but in most cases a recent version of open-simh is a good choice.

hercules also appears several times due to the large number of IBM System/360 operating systems. Again there are several forks, but SDL hercules hyperion is my current preference.

I have not (yet) run all the emulators listed in the table, so it's quite possible some do not work fully - please let me know if so.

Alternatives to emulation running on your own personal computer:

• hardware for more recent systems such as PDP-8s or VAXes is available on sites like ebay, but is getting increasingly expensive.
• replicas running emulators on hardware like Raspberry Pis, such as the PiDP-11.
• some museums has put original hardware online where you can get an account, such as The Interim Computer Museum.
• there also collections of online-accessible emulators such as nostalgiccomputing.org.

In the "SW" column I link to the project page or archive site where software kits can be obtained. Be aware that some kits have specific licenses attached and others have no obvious current owner so their licensing status is unknown.

The "Doc" column links to the main source of documentation for the operating system in PDF format: this is usually Bitsavers.

This is not a comprehensive list of all time-sharing operating systems ever produced - to my knowledge, no such list exists but I estimate it would be at least double this size.

In order for a system to be available on emulation, several things need to happen (or to have already happened)

• the original software and documentation needs to have been preserved, ideally in digital form
• sufficient details of the hardware it ran on need to have been preserved
• someone has made the big effort to write an emulator for the system

Some examples of systems that are not available, at least today, via emulation

• early systems like AN/FSQ-32, JOHNNIAC or the PDP-1 systems developed by BBN and MIT, along with British systems like the Titan Supervisor
• Mainframe operating systems like GE GCOS, RCA TSOS, Univac Exec 8 and ICL VME.
• Academic systems like CAL-TSS, OS-3, CAP or EMAS.
• Many minicomputer systems.
• More recent systems that are only available commercially, eg Tandem or Stratus VOS.

And in many cases, even for those available, not all versions of each operating system has been preserved.

Please let me know if you think I am missing any systems, or if you think my methodology is not right. You can email me at rupert@timereshared.com and if it makes sense I will add it here and update the main text.

August 2025: Lars Brinkhoff suggested adding SITS.