Editing MVS (section)

==Overview==
First released in 1974, MVS was extended by program products with new names multiple times, retaining the term MVS in the nomenclature:
* first to MVS/SE (MVS/System Extensions),<ref group="NB">some print media used the singular, MVS/System Extension: Computerworld, 15 Dec 1980 -
Page 5; 26 June 1978 - Page 8</ref>
* next to MVS/SP (MVS/System Product) Version 1, 
* next to MVS/XA (MVS/eXtended Architecture), 
* next to MVS/ESA (MVS/Enterprise Systems Architecture),

and then extended

* to [[OS/390]] for the [[IBM System/390|System/390]] systems, and 
* finally to [[z/OS]] (when [[64-bit computing|64-bit]] support was added with the [[zSeries]] models). IBM added UNIX support (originally called [[OpenEdition MVS]]) in MVS/SP V4.3 and has obtained [[POSIX]] and [[Unix|UNIX™]] certifications at several different levels from [[IEEE Computer Society|IEEE]], [[X/Open]] and [[The Open Group]]. 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 is, in fact a major rewrite. OS/VS2 release 1 is an upgrade of [[Multiprogramming with a Variable number of Tasks|OS/360 MVT]] that retained most of the original code and, like MVT, is mainly written in [[assembly language]]. The MVS core is almost entirely written in [[IBM Basic assembly language and successors#Assembler XF|Assembler XF]], although a few modules were written in [[IBM PL/S|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 [[Time Sharing Option|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.