z/Architecture and addressing
z/Architecture is the next step in the evolution from System/360 to System/370, System/370 Extended Architecture (370-XA), Enterprise Systems Architecture/370* (ESA/370),and Enterprise Systems Architecture/390 (ESA/390). In order to understand z/Architecture you have to be familiar also with the basics of ESA/390 and its predecessors.
An address space maps all of the available addresses, and includes system code and data as well as user code and data. Thus, not all of the mapped addresses are available for user code ...

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