NuttX is a real-time operating system (RTOS) with an emphasis on standards compliance and small footprint. Scalable from 8-bit to 32-bit microcontroller environments, the primary governing standards in NuttX are Posix and ANSI standards. Additional standard APIs from Unix and other common RTOS's (such as VxWorks) are adopted for functionality not available under these standards, or for functionality that is not appropriate for deeply-embedded environments (such as fork()).

This list is an exploration of the world of UNIX®, including UNIX history, the relevance of UNIX today, and lists select awesome UNIX and UNIX-like projects. This list also contains resources for UNIX standards, programming, communities, and free software.

This is the announcement, more details can be found on https://github.com/EtchedPixels/FUZIX
Note that it is not limited to 8-bit processors: core code can be built for 6502, 6809, 68000, 8086, MSP430, pdp11 and Z80/Z180.

A security-oriented operating system (OS).

Explanation of the possible states of a Linux process.

An emulator for the hardware that ran the Multics operating system.

Most documentation is on the project wiki at
https://github.com/MichaelMondy/multics-emul/wiki

Node9 is a hosted 64-bit operating system based on Bell Lab's Inferno OS, but using the Lua scripting language instead of Limbo and the LuaJIT high performance virtual machine instead of the Dis virtual machine. It also uses the libuv I/O library for maximum portability, efficient event processing and thread management.

The Genode OS Framework is a tool kit for building highly secure special-purpose operating systems. It scales from embedded systems with as little as 4 MB of memory to highly dynamic general-purpose workloads.