OS-9 is a real-time operating system originally developed by Microware in the late 1970's to take advantage of Motorola's powerful new 6809E microprocessor. OS-9 had a concept of modules: all programs and OS components were modules, and could be loaded dynamically. Everything was re-entrant, using a link-count like that of the UNIX filesystem. The I/O system was broken into several layers using an approach that was object-oriented. OS-9 was the first program that I ever considered beautiful.