z/OS
The main operating system that runs on IBM mainframes, in the same way Windows or Linux runs on a laptop or server.
Also written zOS, MVS, OS/390
z/OS is the operating system most mainframe work happens on. It does the same job as any other operating system, managing memory, storage, programs and security, but the vocabulary and the tools it presents are its own.
A few things stand out to newcomers. Files are called datasets, and they are structured records rather than a stream of bytes. Work is organised into address spaces, each one an isolated pocket of memory running a program. Long-running background work is submitted as jobs rather than launched as commands. And security is not bolted on: every request to read a file or run a transaction is checked against a central security database.
z/OS is also relentlessly backwards compatible. Programs compiled decades ago still run, which is the whole reason organisations trust it with records they cannot afford to lose.
Other operating systems do run on the same hardware. Linux on Z is common, and z/VM, z/VSE and z/TPF exist, but z/OS is the one most learners and most jobs are concerned with.
Related terms
- MainframeA large, highly reliable computer built to run enormous volumes of transactions and batch work for an entire organisation at once.
- IBM ZIBM's current family of mainframe hardware, and the brand name you will see on most modern mainframe job adverts.
- Address spaceAn isolated pocket of memory in which a program runs, so that one program cannot interfere with another.
- DatasetWhat a mainframe calls a file: a named collection of records with a defined structure, rather than a loose stream of bytes.
- Unix System ServicesA full Unix environment built into z/OS, with a shell, directories and files alongside the traditional mainframe world.