What is the oldest, still supported OS?

Possibly Burroughs MCP[0] from 1961, currently Unisys ClearPath MCP.

Not to be confused with Encom MCP[1], which was defeated by Flynn and Tron in 1982.

[0]https://en.wikipedia.org/wiki/Burroughs_MCP [1]https://www.imdb.com/title/tt0084827/

Burroughs MCP was released in 01961 and seems to still be supported. The latest release was 20.0 in May. That's probably the oldest.

z/OS was released in 01966. BOS/360 made it out the door earlier, in 01965, thanks to the disastrous delays in z/OS, but it's no longer supported; DOS/360 (z/VSE) also beat z/OS out, is still supported, and is arguably the continuation of BOS. https://en.wikipedia.org/wiki/DOS/360_and_successors

Unix wasn't released in 01969. I think it wasn't released until Fifth Edition in 01974, though Thompson and Ritchie described the Fourth Edition in CACM in 01973. Fourth Edition had "over 20" installations, but I think all within AT&T. https://en.wikipedia.org/wiki/Research_Unix

This is slightly off topic -- it's about an old supported microarchitecture -- but Linux still supports DEC Alpha, despite no chip with that architecture having been developed since 2004.

DEC Alpha has extremely weak memory ordering. [1] In fact, it's the weakest ordering of any arch supported by linux, which includes extra fence instructions to support it. The memory model is crazy weak, but it apparently allows for extra speculative execution parallelism.

[1] Awesome Raymond Chen post, totally worth a read: https://devblogs.microsoft.com/oldnewthing/20170817-00/?p=96...

The oldest one I know is PDP-11. It's still being used in nuclear power plants and many plants will continue using it until at least 2050 (maybe even longer if they remain working). [1] PDP-11 was released in 1970 so your TSOS find is even older. I'm sure there's something even older than these two that's being used by various gov orgs and industrial systems. There's plenty of small consulting firms that support ancient systems. These contracts provide them with steady and stable income.

[1] https://www.theregister.co.uk/2013/06/19/nuke_plants_to_keep...

UNIVAC 1108 EXEC-8, now OS-2200. Still in use, 55 years later, with mostly the same API and commands.

[1] https://en.wikipedia.org/wiki/OS_2200

Master Control Program of Burroughs Large Systems fame was first released in 1961 - latest release is from May 2021

OS/360 dates to 1966. I believe z/OS in theory still provides compatibility back to applications of that vintage, though AFAIK it is a separate codebase.

Along these lines I was curious what the oldest running program is. Found [0]. In use since 1959(ish).

[0] https://www.technologyreview.com/2015/08/06/166822/what-is-t...

What does "oldest" OS even mean? I haven't used TSOS/BS2000 but its Wikipedia article suggests that while the OS lineage dates back to 1968 that anyone using it currently is using some currently maintained developed version of it.

In my mind supporting an old OS means e.g. that someone is still running old terminals that still run actual Windows 95 (emulator or not), DOS etc.

It wouldn't be meaningful to say "this runs on an OS from the mid-80s!". Only to find that it's all C# code targeting last year's release of Microsoft Windows (which has its eventual origins in the mid-80s).

THEOS might be a contender. I don’t know when it was created, but I ran into a user 15 years ago. They were old then and still providing active support to customers.

http://www.theos-software.com/

Edit: Looks like it died in 2014 or 2016 or, at least, that’s the latest update I see.

Update: Yup, it’s now listed as suspended. Looks like it was started in 1983.

Possibly TPF, if you consider it's predecessors to be just TPF with different names.

https://web.archive.org/web/20041204030934/http://www.blackb...

The OS for IBM system/34 and system/36 mainframes is currently virtualized on their z/os POWER systems.