Questions about IBM AS/400 midrange computing platform, and its operating system OS/400.
The IBM AS/400, released in 1988, was successor to 1978's IBM System/38. Its operating system, OS/400 was a descendant of the System/38 CPF operating system. OS/400 was later renamed IBM i5/OS and more recently to IBM i; the contemporary descendant of the AS/400 hardware is the IBM Power Systems line (which also runs AIX).
Although descended from System/38 CPF, OS/400 introduced some incompatible changes, such as renaming some commands. It did however come with a backwards compatibility environment for running System/38 applications. It also came with an emulation environment for running software from IBM's (technically very different) System/36 midrange platform (and System/36's SSP operating system). Later, OS/400 added an AIX compatibility subsystem, PASE, to ease porting applications from Unix.