OS/390 Storage in Mainframe system

Logicaly an OS/390 system consists of DASD, one or more CPUs and I/O devices.

Central or Main Storage - is directly addressable by CPU where both programs and data must be loaded before processing.

Expanded Storage - may be used to relieve the central storage when it is heavily utillized. This is done by moving data from the central to the expanded storage. The combination together is called processor storage. The storage is organized in 4k byte blocks. This is also the smallest addressable unit and is called a frame.

Virtual Storage - consists of 4KB blocks called pages. A page may be stored in central, expanded or auxiliary storage. When residing in central or expanded storage it is referred to as frame while in auxiliary storage it is referred to as slot.

Dynamic Address Translation - is the process of translating the virtual address to the corresponding central during storage reference.

Swapping - is the process of physically or logically removing a user from central storage. This means moving all of his currently active data and programs to expanded storage or writing them to swap or page data sets or marking them as swapped.

Address Space - The sequence of virtual addresses associated with a virtual storage is called and address space. An address space can contain 2GB of virtual Storage.

Multitple Virtual Storage - is the basic concept of the OS/390 operating system.

Data Spaces and Hyper Spaces - are similar in that, both are areas of virtual storage that can be created by the system upon user request. The size of those spaces can range from 4KB to 2GB depending on the users request. Unlike address space, a data space or hyper space contains only user data or user programs stored as data. Program code cannot be directly executed in a data space or hyper space.