4os was an operating system written in Forth. It was discussed on Hacker News in 2016: https://news.ycombinator.com/item?id=12709802 https://news.ycombinator.c

4os was an operating system written in Forth. It was discussed on Hacker News in 2016: https://news.ycombinator.com/item?id=12709802 https://news.ycombinator.com/item?id=12709802 In many ways many Forth systems were simultaneously the programming language and the operating system, especially on small systems. Having a single integrated system meant you didn't need a lot of space, which was important when you were trying to develop software on a 1MHz system with 8KiB RAM. There have been many operating systems written tightly integrated to a language. Oberon is one, there were several that were based on Lisp, and many Forth systems count. The early Smalltalk systems probably count as well. I agree with you that tightly language-integrated operating systems are dead. You're absolutely right that tight integration makes it hard to change things. The tight integration could potentially save space (memory and storage), which is why I think it was more popular years ago, but that's much less relevant today. If you have tight memory constraints, you'll probably develop on a beefier system and then transmit the result to the tiny system instead, and that approach doesn't require a tightly language-integrated operating system.