The AS/400 (aka IBM i) appears to be of a different lineage, derived from the late-70s IBM System/38 minicomputers.

Nevertheless, it sports many IBM-isms in its design, including integrating database functionality directly into the operating system, making database tables and files almost coterminous with record-level file access being the primary mode of access; and the use of block-oriented, rather than character-oriented terminals (the IBM 5250).

Interestingly unique to IBM i among IBM operating systems is the pervasive use of object orientation throughout its system APIs; there are no pointers, everything is an object over which some operations are permitted and others forbidden. While the mainframe series retained darn-near-perfect backwards compatibility through system-level emulation of old architectures on top of new architectures, IBM i's solution to this problem is different: TIMI (Technology Independent Machine Interface), a kernel-level abstract machine to which all IBM i applications are targeted. No programmer access to the underlying machine code is provided. Rather, the first time the kernel loads a TIMI executable, it AOT-compiles it to whatever the underlying CPU architecture it is. This enables seamless CPU architecture changes with perfect backwards compatibility for application code; this already happened in the 90s as the AS/400 line migrated from a System/38-based architecture to PowerPC.