EmuTOS is a complete, GPL-licensed replacement operating system for Atari ST/TT/Falcon, ColdFire, and Amiga computers. It reimplements the entire TOS stack: BIOS (hardware drivers, boot), GEMDOS (file system, process management), VDI (graphics primitives), AES (windowing and event management), a desktop environment (EmuDesk), and a command-line interpreter (EmuCON2). The codebase is highly modular with clear layer separation, extensive conditional compilation for over a dozen hardware variants, and a build system that generates ROM images for each target. It is not a traditional application but a firmware ROM; however, from a software engineering perspective, it exhibits application-like structure with entry points (startup.S, biosmain, gem_main), a GUI, and a CLI. The system is actively maintained, with a rich history of reverse-engineering Atari TOS behavior and fixing emulator quirks. It relies on a 68k/ColdFire cross-compiler toolchain and GNU Make.