IBM i (formerly OS/400) is a unique operating system developed by IBM since the 1980s, notable for integrating a relational database (Db2 for i) directly into its kernel, treating all system resources as objects rather than files. Its architecture includes a technology-independent machine interface (TIMI) ensuring software compatibility across hardware generations, and a Single Level Storage model that unifies memory and disk addressing, enabling efficient data management and high reliability. While deeply tied to specialized IBM Power hardware, the system provides a distinctive approach to application hosting, security, and workload management, exemplified by features like message-based error handling and a text-based 5250 terminal interface alongside modern web-based tools.
https://osadmins.com/en/ibm-i-os-400-the-database-operating-system/