Principia Softwarica

A fork of Plan 9 from Bell Labs, curated for education.

See this video for a presentation of the project.

About Plan 9

Plan 9 is an operating system considered by many to be the successor to Unix (see Wikipedia). The code of the original Plan 9 is available at https://9p.io/plan9/.

Other forks of Plan 9:

• 9front
• 9legacy
• 9legacy-bootable
• Nix
• 9atom (DEAD in 2025)
• Harvey (DEAD in 2025)
• Jehanne (DEAD in 2025)

Close descendants of Plan 9:

• Inferno

About this fork

The specificity of my fork is to see Plan 9 as a great educational platform to learn about programming and computer science. The code of Plan 9 is so elegant and small that you can realistically understand the whole operating system. In fact, I explained the whole code in my Principia Softwarica book series (see https://principia-softwarica.org).

This fork, because it is used to support my Principia Softwarica book series, contains only the essential programs used by a programmer (e.g., compiler, linker, assembler, kernel, windowing system). It does not contain all the programs of the original Plan 9; I've selected only what I consider to be the essence of an operating system. Moreover, this fork supports only the ARM and x86 architectures. Even though the original Plan 9 supported more architectures, I think ARM and x86 are enough for the educational purpose of Principia Softwarica.

AI disclaimer

99.9% of the code in this repository was derived from Plan 9 fourth edition and was written by humans (Ken Thompson, Rob Pike, and many other Plan 9 contributors), and for the most part written more than 30 years ago. What I did was essentially to reorganize the code for my Principia Softwarica book series. Many of the explanations in those books were written using AI assistance but the code itself (and its organization in chapters, sections, appendix in those books) was not.

The 0.1% of code that was written using AI is clearly marked with a special claude: comment at the front and part of a commit authored by Claude code.

Building

As opposed to the original Plan 9, this fork can also be cross-compiled from Linux, macOS, or Windows, which makes it easy to experiment with. To cross-compile, you will need to install Goken.

See https://principia-softwarica.org/getting-started.html for the up-to-date build and installation instructions.

Enjoy.