The z/Architecture was introduced in 2000 with the introduction of the zSeries 900. The ISA was designed as an extension to the ESA/390 and features 64-bit registers, 64-bit operations, and a 64-bit virtual and real address space. Development for z/Architecture started in 1996 when IBM determined the ESA/390 had to be extended to 64-bit to overcome its limitations. The new architecture increased the 31-bit address to 64 bit, enough to address 16 exbibytes.
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