IBM i (originally AS/400) is a unique operating system with an integrated Db2 database at the kernel level, treating database and OS as one inseparable unit.
It uses an object-based architecture instead of file-centric, with Single Level Storage (SLS) treating all memory and disk as one flat address space.
The TIMI abstraction layer ensures decades-long backward compatibility, allowing programs from 1990 to run on modern hardware without recompilation.