MVS/XA, or Multiple Virtual Storage/Extended Architecture, was a version of the IBM operating system that is now called z/OS. It was available only on the high-end 43xx, and 308x and later ranges of IBM computers, and was replaced by MVS/ESA, then OS/390 and later z/OS.

MVS/XA maintained the same multiprogramming and multiprocessing features supported by its predecessor MVS/370, combined with the capability of simultaneously servicing multiple programs and users. It not only supported virtual storage that was larger than the available real storage, but that separate processes (tasks) were run in separate address spaces. It made MVS/XA the perfect solution for business problems which resulted from the need to run more applications.

New features:

• 31-bit addressing, thus, a 2 gigabyte addressable memory area, ^[1]
• legacy addressing mode for older 24-bit applications (i.e. those that utilized bits above 24 bit for other purposes)

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