not-os is a minimal, embedded-focused Linux distribution built entirely as a Nixpkgs overlay, demonstrating how to construct a bootable OS from scratch (kernel, initrd, squashfs root) using NixOS modules while replacing systemd with runit. The repository provides configurations for x86_64 QEMU, Raspberry Pi (ARM), and Xilinx Zynq targets, along with an iPXE boot chain featuring cryptographic signature verification for secure boot. It illustrates advanced Nix techniques for building custom initrds, integrating firmware, and creating reproducible embedded system images, though some components (like the dummy stage-2 init script placeholder) indicate a focus on flexibility over polish. The codebase is well-structured with clear module boundaries and includes Hydra CI integration for automatic testing.