PatchworkOS: a 64bit non-POSIX OS where everything is a file
Patchworkis 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 aconstant-time scheduler, fully preemptive mutitasking, SMP, file-based IPC (includingpipes,shared memory,socketsand Plan9 inspired signals" callednotes), 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.