We used Multics when I was at Bristol Uni in the UK c.1980. I can only remember two things about it:
1) The system was initially deemed slow, so they installed an extra 256 KB of RAM (for a system serving dozens/hundreds of students - Bristol was regional computing center), and that made a difference! This was a big deal - apparently quite expensive!
2) Notwithstanding 1), it was fast, and typical student FORTRAN assignments of a 100 or so lines of code would compile and link essentially instantly - hit enter and get prompt back. I wish compilers were this fast today on 2025's massively faster hardware!
CSP_Jobs · 2h ago
Always fun to see Multics pop up; the influence it had on computing is pretty impressive and its influence lives on in many projects. As just one personally relevant example, the SCOMP mentioned in the glossary [0] and described in more detail on the history page under 5.4.1 [1] became the STOP operating system which is still in active development and is what I still work on today. (Technically, the SCOMP was the whole machine, and STOP "SCOMP Trusted Operating Program" was its operating system). Up until pretty recently, we still had a Multician working on STOP, and have a guy from the Honewell days still plugging away on it.
This operating system sounds very interesting! How active is the development? I would imagine it's the type of thing that eventually gets "complete"
CSP_Jobs · 50m ago
I've been gainfully employed for well over a decade working on it and it's been around in one form or another for over 40 years. We're constantly improving performance and capabilities, adding support for more hardware, supporting the specific needs of our customers etc... Just like any modern operating system, it's never really "complete". STOP is a "security from the ground up" OS, where security isn't just a first-order priority, it's the entire point, typically used in/as multilevel security solutions.
rbanffy · 39m ago
Are there any documents we can use to learn more about it? What does it look like to the user? Is it intended to be embedded?
CSP_Jobs · 14m ago
There's a link in my profile to the company products page for my group, which includes a link to the STOP OS page. There used to be additional documents you could download from those pages, but it looks like they're not working any more.
The short version is that it implements three different MAC (mandatory access control) policies (RBAC, Bell-LaPadula, Biba) and the standard *nix DAC policies. It's designed for safely handling/moving data on/between multiple classification levels. (See the SCOMP section in [0] for history). From a user perspective, it's very similar to Linux, with a largely Linux-like ABI and similar user interfaces, including a full X/xfce GUI environment if you want, though most actual deployments tend to run headless with only required software loaded. It runs on both small embedded boards and large enterprise servers and a bunch in between.
AIM and MAC seem like a very interesting system for enforcing security guarantees, and they partially solved the malicious dependencies problem as well.
My Operating Systems class as an undergrad used a book written by the Multics guys.
I hated it. It would present a bunch of apparently incompatible techniques for e.g. job scheduling, and then say that Multics implemented all of them. I immediately understood why UNIX came about: the Multics designers appeared incapable of having opinions, which led to an OS that was bloated and hard to understand.
That class was a long time ago, and I was a young, arrogant, and uninformed programmer, and maybe that take was wrong. But it left a strong impression at the time, and it was one of the few books from my undergrad days that I sold back instead of keeping.
Thanks! This is a really useful source for computer science history, I was just thinking to myself that I keep reading UNIX history, but that I know very little about Multics. Looks like I can keep my evening busy for quite some time with this one.
1) The system was initially deemed slow, so they installed an extra 256 KB of RAM (for a system serving dozens/hundreds of students - Bristol was regional computing center), and that made a difference! This was a big deal - apparently quite expensive!
2) Notwithstanding 1), it was fast, and typical student FORTRAN assignments of a 100 or so lines of code would compile and link essentially instantly - hit enter and get prompt back. I wish compilers were this fast today on 2025's massively faster hardware!
[0]https://www.multicians.org/mgs.html#SCOMP [1]https://www.multicians.org/history.html
The short version is that it implements three different MAC (mandatory access control) policies (RBAC, Bell-LaPadula, Biba) and the standard *nix DAC policies. It's designed for safely handling/moving data on/between multiple classification levels. (See the SCOMP section in [0] for history). From a user perspective, it's very similar to Linux, with a largely Linux-like ABI and similar user interfaces, including a full X/xfce GUI environment if you want, though most actual deployments tend to run headless with only required software loaded. It runs on both small embedded boards and large enterprise servers and a bunch in between.
[0] https://multicians.org/b2.html
https://www.multicians.org/myths.html
And naturally, B2 Security Evaluation,
https://multicians.org/b2.html
AIM and MAC seem like a very interesting system for enforcing security guarantees, and they partially solved the malicious dependencies problem as well.
I hated it. It would present a bunch of apparently incompatible techniques for e.g. job scheduling, and then say that Multics implemented all of them. I immediately understood why UNIX came about: the Multics designers appeared incapable of having opinions, which led to an OS that was bloated and hard to understand.
That class was a long time ago, and I was a young, arrogant, and uninformed programmer, and maybe that take was wrong. But it left a strong impression at the time, and it was one of the few books from my undergrad days that I sold back instead of keeping.
Recent Changes 08/03 Multicians: Jim Bush died in July 2025.
07/14 Multicians: Norm Barnecut died Dec 09, 2023.
05/24 Multicians: Nate Adleman died Sept 19, 2022.
05/15 Simulator: Release 3.1.0 of the DPS8M simulator was released.
05/12 Multicians: Richard Gardner died in 2025.
05/05 Multicians: Art Bushkin died in Feb 2024.
At this moment in time, 125.
Unix and Multics (2019) - https://news.ycombinator.com/item?id=40315724 - May 2024 (76 comments)
Unix and Multics History - https://news.ycombinator.com/item?id=40065928 - April 2024 (3 comments)
Multics and AS400：DPS8M on IBM PASE for I (OS/400) - https://news.ycombinator.com/item?id=39308420 - Feb 2024 (4 comments)
Multics Simulator - https://news.ycombinator.com/item?id=37304367 - Aug 2023 (15 comments)
So! You want to use Multics? (1979) [pdf] - https://news.ycombinator.com/item?id=33366716 - Oct 2022 (5 comments)
Wordmul: Wordle for Multics - https://news.ycombinator.com/item?id=31636949 - June 2022 (3 comments)
Nobody learned the most important lesson from Multics vs. Unix - https://news.ycombinator.com/item?id=29783423 - Jan 2022 (1 comment)
Multics MR12.7 released - https://news.ycombinator.com/item?id=28006036 - July 2021 (25 comments)
Multics Public Access - https://news.ycombinator.com/item?id=27128765 - May 2021 (33 comments)
Ban.ai Public Access Multics - https://news.ycombinator.com/item?id=25611468 - Jan 2021 (6 comments)
Multics Simulator - https://news.ycombinator.com/item?id=22994027 - April 2020 (20 comments)
Multics: An Ancestor of Unix - https://news.ycombinator.com/item?id=21892923 - Dec 2019 (33 comments)
Multics Intro Course (1978) - https://news.ycombinator.com/item?id=18715014 - Dec 2018 (9 comments)
Thirty Years Later: Lessons from the Multics Security Evaluation (2002) [pdf] - https://news.ycombinator.com/item?id=16956386 - April 2018 (3 comments)
Public Access Multics - https://news.ycombinator.com/item?id=16875552 - April 2018 (3 comments)
48-Year-Old Multics operating system resurrected - https://news.ycombinator.com/item?id=14728441 - July 2017 (9 comments)
Multics Execution Environment (2014) - https://news.ycombinator.com/item?id=10364234 - Oct 2015 (1 comment)
Introduction to Multics - https://news.ycombinator.com/item?id=6985277 - Dec 2013 (1 comment)