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> The IBM midrange systems have a tremendous feature, backward-compatibility – anything you wrote 20 years ago can be compiled on current systems without any ch

> The IBM midrange systems have a tremendous feature, backward-compatibility – anything you wrote 20 years ago can be compiled on current systems without any change in source code. If this is saying OS/400 is backward compatible with old UNIX software, then it's kind of impressive. The team that developed OS/400 apparently had absolutely no background in UNIX. Reasons for AS/400 seem to mostly be big finance software and municipal software (apparently local governments trying to migrate away from AS/400 to Windows municipal software are still having serious trouble 5 years later). There are other reasons... very stable OS and the rare successful security by obscurity. Migrating AS/400 to Cloud is a thing now, but it is still less expensive to own and run the equipment locally.

Accujack 5y ago
He's talking about the binary compatibility of the various IBM system lines. The binaries of AS/400 systems include an intermediate code binary representation of the program that can be re-translated into a binary for a newer system when one is developed. If you wrote a POSIX based program in in the past on the AS/400, it would almost certainly run fine on the newer systems. Likewise, AS/400 programs written for AS/400 or iSeries chip based systems will run on the modern Power series OS/400 systems with a recompile. There are good and bad sides of choosing to build this level of compatibility, but IBM has made doing so a mainstay of their business for a long time.