Well, by "relatively popular" I meant that AS/400 servers (aka AS400 or IBM iSeries or IBM System i: https://en.wikipedia.org/wiki/IBM_System_i) have been quite popular mostly only within certain industries and/or niches (e.g., banking, insurance, manufacturing, supply chain). They are still widely used there for legacy reasons, however demand for AS/400 developers (which implies proficiency in RPG language, since, as I understand, most legacy applications are written in RPG: https://en.wikipedia.org/wiki/IBM_RPG) is tiny; so, it's a good language to know to potentially expect a relatively high salary or consulting rate (hello, Delphi!), but a dead end for IT career growth. I would also venture to guess that some (many? most?) companies currently using AS/400 plan to migrate their legacy applications to the cloud.

I wouldn't want to work on the AS/400 systems, but its Technology-Independent Machine Interface (TIMI) was basically Android Runtime decades before flip phones were even a thing: ship all of your userspace binaries as bytecode, and compile that bytecode to optimized machine code for your CPU at program installation time.

That's quite fascinating. I appreciate your comment.

"Popular" was a touch sarcastic. AS/400s were most recently called the IBM system I, and are now a discontinued product. IBM still sells mainframes, and they have discontinued the AS/400.

They are most certainly not a discontinued product. We just purchased a brand new Power9 server at work. The model we purchased was just released. Everyone at work still calls it an AS400. They are bulletproof and run many large businesses today. They are also not mainframes but are a class of machines called minicomputers.

IBM Power Series is closer to the RS6000, no? Running real AIX and having a full JCL makes them a far sight better than the RPGII stuff on the AS/400. Night and day.

IBM AS/400 is not discontinued, but rather has been rebranded as IBM Power System: https://www.ibm.com/it-infrastructure/power.