OS/400
The original operating system for the AS400 platform, released in 1988. Renamed to i5/OS in 2004 and IBM i in 2008. The OS lineage that all IBM i releases trace back to.
OS/400 was the name of the operating system that IBM released with the original AS/400 hardware in 1988. OS/400 was tightly integrated with the AS400 hardware, featuring a single-level object storage model, an integrated relational database (DB2/400), and a comprehensive security architecture. OS/400 received annual releases through the 1990s and early 2000s under the V1Rx, V2Rx, V3Rx, V4Rx, and V5Rx naming convention. IBM renamed OS/400 to i5/OS in 2004 with the release of V5R3, as part of the platform's iSeries rebranding. IBM then renamed i5/OS to IBM i starting with IBM i 6.1 in 2008. OS/400 is still used colloquially to refer to the IBM i OS by administrators who trained on the original platform.