Z / OS is an operating system for IBM mainframe, which is marketed since 2001. Its roots date back to the 1950s. Only the third generation of the former IBM mainframe operating systems from the 1960s was both scientific as well as for business purposes are used and if was their versatility in association with the 360 degrees of the compass rose called OS/360.


S/360, S/370, ESA/390, etc. were actually more the hardware components on which multiple operating systems (MVT, MVS, MVS / ESA, OS/390, VM, ...) could walk. The most important development stations in the area of ​​software were certainly the introduction of MVS in the 1970s (24 bit) and MVS / XA in the 1980s (31 bit - the 32nd bit is used for backward compatibility ) and the successor to OS/390. Around the turn of the millennium was introduced with z / OS 64- bit architecture. As the latest generation of 40-year development history takes z / OS still today a dominant position among the mainframe operating systems, a.

In Parallel Sysplex composite, a close coupled cluster of IBM mainframes, z / OS surpasses the time set for the previous generation OS/390 availability rate of 99.999 % (so-called five- nines, which corresponds to a downtime of five minutes per year ) The z in the name stands for zero downtime. By definition of an IBM downtime is always a non- scheduled service interruption.


The operating system supports the scalability of a large computer at up to 640 64-bit IBM processors, and almost all of high-level languages ​​. Various subsystems enable concurrent operation in dialogue and in batch ( batch processing). Through Unix System Services and the ability to install Linux, the connection to the Unix world and the open systems world is guaranteed. The connection to other computers or to the Internet via both proprietary protocols and TCP / IP. Extensive cryptographic functionality with support not only for standards such as TLS, SET, VPN, and hardware isolation allow the creation of secure application systems.

The combination of z / OS with the System z hardware is optimized for very high I / O traffic (see inputs and outputs, transactions ) in very many users and with the greatest possible security (data integrity, access protection).

Components and standard software that is often used under z / OS

  • System for the input of program calls and for flow control JES (Job Entry Subsystem ) for executing JCL scripts JCL (Job Control Language ), a control language for batch processing
  • Databases and transaction monitors CICS ( Customer Information Control System), a transaction system
  • DB2, a relational database system
  • Contains IMS ( Information Management System), an information system, inter alia, a transaction monitor (IMS / DC)
  • Interactive Access with command-line or menu access TSO (Time Sharing Option), an interactive command-line interpreter ISPF ( Interactive System Productivity Facility ), a menu-driven user interface for use under TSO
  • Other components WLM ( Workload Manager ) and SRM (Systems Resource Manager ) for the management of resources
  • RMF ( Resource Measurement Facility) and SMF (System Management Facility ) for the gathering and analysis of operational data
  • SMP / E (System Modification Program / ​​Extended) for the installation and maintenance of the operating system and components
  • Special hardware Assist Processors, special purpose processors for the execution of specific tasks (for example, Java VM)