The
Operating System for
s/390 mainframes. Works as a general control program,
partitioning the hardware and offering low level service where other
applications - but you would call them true operating systems in
PC world - like
MVS,
CICS and
TSO live in. Yes,
Linux too on recent
IBM hardware.
As most things related to the
mainframe world, it's usually considered quite boring by most hackers. On the other side, such machinery - and OS/390 itself - offers excellent uptime, data protection and security, complete scalability, error correction, the best database performance and storage access in a way that's simply impossible for lower end hardware.