z/OS is a Operating system 64 - bits developed for the large computers of the type “Mainframes” ZSeries by IBM in 2001. It is the successor of the system OS/390. z/OS is a combination of MVS and UNIX System Services.

z/OS made its appearance in 1960 and with changed several times of name (according to the versions) during the years:

• MFT (Multiprogramming with has Fixed number off Task),

• MVT (Multiprogramming with has Variable number off Task),
• SVS (Individual Virtual Storage),
• MVS (Multiple Virtual Storage),
• MVS/370,
• MVS/XA (Extended Architecture) (1981)
• MVS/ESA (Enterprise Systems Architecture).
• OS/390 (1996)
• z/OS

All these versions are indistinctly called MVS in the current language.

Based on an extremely reliable architecture having proven reliable during the decades, it is voted by plebiscite by all the companies having very large needs for availability and performances, one finds it like central location (host computer = Mainframe) at the majority of the banks, insurances, mutual insurance companies, auto industries, distributers because it is only OS making it possible to answer their waitings in term of reliability, performances and accesses competitor.

The z/OS is an OS completely different from OS currents type Windows or Unix. The most obvious example is the management of the files by catalog and not in a hierarchical way. Thus, under z/OS, one does not find a repertory. The files are catalogued but not treated on a hierarchical basis in repertories. To indicate a file, one speaks about DSN, i.e. to Set Name Went back. DSN is composed of several alias connected by a point, of which only the first alias returns to a catalog.