Z/Architecture

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z/Architecture (formerly and briefly known as ESAME) refers to IBM's 64-bit computing architecture for its top-of-the-line enterprise servers. IBM introduced its first z/Architecture-based system, the zSeries Model 900, in 2000. Other z/Architecture systems include the IBM z800, z990, z890, and System z9. z/Architecture supersedes but retains backward compatibility with ESA/390 and its predecessors all the way back to the 24-bit System/360.

The various operating systems running on z/Architecture systems vary in how much they exploit the 64-bit design. Most operating systems, including z/OS, continue to restrict code execution to the first 2 GB (31 bits) of each virtual address space, allowing only data objects to reach into the higher 64-bit addressing ranges, for reasons of efficiency and compatibility. (Since z/OS supports multiple 2 GB address spaces for code execution, this "restriction" really isn't.) Linux on zSeries is an exception, allowing code to execute from 64-bit addresses.

Unlike most other processor architectures, z/Architecture supports running multiple concurrent operating systems and applications even if they use different addressing modes. Thus developers can choose whichever addressing modes are most advantageous for their applications and data structures.

References

[Development and Attributes of z/Architecture], IBM Journal of Research and Development, 2002.
[z/Architecture Principles of Operation and Reference Summary]

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