BCS 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 (System Extension), next to MVS/SP (System
Product) Version 1, next to MVS/XA (eXtended Architecture), next to
MVS/ESA (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.