PatchworkOS: a 64bit non-POSIX OS where everything is a file

Patchwork is a 64-bit monolithic NON-POSIX operating system for the x86_64 architecture that rigorously follows a “everything is a file” philosophy. Built from scratch in C it takes many ideas from Unix, Plan9, DOS and others while simplifying them and sprinkling in some new ideas of its own.
↫ PatchworkOS GitHub page

Patchwork is a surprisingly advanced operating system considering it’s a hobby project. It has multithreading with a constant-time scheduler, fully preemptive mutitasking, SMP, file-based IPC (including pipes, shared memory, sockets and Plan9 inspired “signals” called notes), and much more. It also uses a Linux-style VFS and has a custom C standard library. On top of that, there’s a modular window manager that supports themes, in which everything is a window, and so much more.

It supports x86_64, but only supports running in RAM. It’s licensed under the MIT license.