OS/390 is an IBM operating system for the System/390 IBM mainframe computers. OS/390 was introduced in late 1995 in an effort to simplify the packaging
Architecture), next to MVS/ESA (MVS/Enterprise Systems Architecture), and then extended to OS/390 for the System/390 systems, and finally to z/OS (when 64-bit support
inception z/OS has had tri-modal addressing (24-bit, 31-bit, and 64-bit). Up through Version 1.5, z/OS itself could start in either 31-bit ESA/390 or 64-bit
The IBM System/390 is a discontinued mainframe product family implementing ESA/390, the fifth generation of the System/360 instruction set architecture
between 16 MiB and 2 GiB (231 bytes). In z/OS, the storage layout is similar to that of MVS/XA, MVS/ESA and OS/390; storage above the 2 GiB bar is managed