MVS
Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers. It was developed by IBM, but is unrelated to IBM's other mainframe operating systems, e.g., VSE, VM, TPF.
First released in 1974, MVS was extended by program products with new names multiple times, first to MVS/SE (MVS/System Extension), next to MVS/SP (MVS/System Product) Version 1, next to MVS/XA (MVS/eXtended Architecture), next to MVS/ESA (MVS/Enterprise Systems Architecture), next to OS/390 and finally to z/OS (when 64-bit support was added with the zSeries models). IBM added Unix support (originally called OPEN EDITION) in MVS/SP V4.3 and has obtained POSIX and Unix certifications at several different levels. The MVS core remains fundamentally the same operating system. By design, programs written for MVS run on z/OS without modification.
At first IBM described MVS as simply a new release of OS/VS2, but it was, in fact a major rewrite. OS/VS2 release 1 was an upgrade of OS/360 MVT that retained most of the original code and, like MVT, was mainly written in assembly language. The MVS core was almost entirely written in Assembler XF, although a few modules were written in PL/S, but not the performance-sensitive ones, in particular not the Input/Output Supervisor (IOS). IBM's use of "OS/VS2" emphasized upwards compatibility: application programs that ran under MVT did not even need recompiling to run under MVS. The same Job Control Language files could be used unchanged; utilities and other non-core facilities like TSO ran unchanged. IBM and users almost unanimously called the new system MVS from the start, and IBM continued to use the term MVS in the naming of later major versions such as MVS/XA.
OS/VS2 (SVS)
Single Virtual Storage (SVS) refers to Release 1 of Operating System/Virtual Storage 2 (OS/VS2); it is the successor system to the MVT option of Operating System/360. OS/VS2 (SVS) was a stopgap measure pending the availability of MVS, although IBM provided support and enhancements to SVS long after shipping MVS.
SVS provides a single 16MiB address space which is shared by all tasks in the system, regardless of the size of physical memory.
Differences from MVT
OS/360 used the Interval Timer feature for providing time of day and for triggering time-dependent events. The support for S/370 made limited use of new timing facilities, but retained a dependency on the Interval Timer. SVS uses the TOD Clock, Clock Comparator and CPU Timer exclusively.
In the wake of the Applied Data Research lawsuit IBM decided to develop chargeable versions of several applications, mostly language processors, although it's not clear whether the lawsuit was actually the deciding factor. As a result, SVS does not include a sort/merge program or any language processor other than the new Assembler (XF), which is required for the system generation process.
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