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:

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/360 MFT (Multitasking with a Fixed number of Tasks) provided multitasking: several memory partitions, each of a fixed size, were set up when the operating system was installed and when the operator redefined them. For example, there could be a small partition, two medium partitions, and a large partition. If there were two large programs ready to run, one would have to wait until the other finished and vacated the large partition.

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