z/Architecture, initially called ESA Modal Extensions (ESAME), refers to IBM's 64-bit computing architecture for a current generation of its mainframe computers. IBM introduced its first z/Architecture-based system, the zSeries Model 900, in late 2000. Later z/Architecture systems included the IBM z800, z990, z890, System z9 and the System z10. z/Architecture retains backward compatibility with previous 31-bit architecture ESA/390 and its predecessors all the way back to the 24-bit System/360.
The operating systems running on z/Architecture systems vary in how much they exploit the 64-bit design. Currently most operating systems, including z/OS, continue to restrict code execution to the first 2 GB (31 bits) of each virtual address space solely for reasons of efficiency and compatibility, allowing only data objects to reach into the higher 64-bit addressing ranges. Since z/OS virtual memory implementation supports multiple 2 GB address spaces, this restriction still allows to utilize much more than 2 GB of resident code concurrently. The 64-bit version of Linux on System z allows code to execute from 64-bit address range.
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.
Platform Solutions, Inc., marketed Itanium-based servers that could run System z applications and operating systems. PSI was bought by IBM in July 2008. The z/Architecture is also emulated by the Hercules emulator.