Page "MVS" Paragraph 2

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, were mainly written in Assembler.

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.