> AS/400 is another interesting one. It was introduced on a System/38 base but intended for future transition to POWER.
When the AS/400 was released in 1988, they weren't thinking about POWER at all. The point of AS/400 was to unify their existing S/36 and S/38 midrange lines; the AS/400 was essentially a new version of the S/38 with added S/36 compatibility features. Its "technology independence" had been a feature of the S/38 since its initial release in 1978; S/38 in turn had inherited the idea from IBM's 1971–1975 "Future Systems" project, a failed attempt to build a successor to the 360/370 mainframe line. RISC wasn't the original motivation for the idea since the term hadn't even been coined when it was first developed.
In 1990, they began a project to develop a successor to the CISC IMPI CPU. They briefly evaluated POWER, but rejected it as unsuitable for their needs – so they started designing their own custom CPU architecture, "Commercial RISC" or "C-RISC" for short. Despite its name, it wasn't a true RISC architecture – it actually kept the CISC IMPI instruction set for backward compatibility, but added new simpler RISC-style instructions alongside it. I think the idea was new compilers would generate the new RISC instructions which would be hardwired, whereas the legacy CISC instructions would be microcoded.
By mid-1991, IBM HQ came to realise they had two different divisions working on "RISC" projects (PowerPC in Austin and C-RISC in Minnesota), and did not approve of the duplication – they ordered C-RISC killed, and the AS/400 was instead to use POWER/PowerPC, with the addition of any necessary extensions. However, while that decision was arguably best for the long-term, it did delay the project by at least a year, since the jump from IMPI to POWER was much bigger than from IMPI to C-RISC. The MI bytecode was only used by applications and the highest levels of the OS; huge parts of the OS were compiled directly to IMPI, and IMPI, while lower-level than MI, still had some rather high-level features which POWER lacks, such as hardware multitasking, and knowledge of the basic structure of the 128-bit capability/single-level-store addresses.
AS/400 is another interesting one. It was introduced on a System/38 base but intended for future transition to POWER.