IBM's flagship operating system for its System z (zSeries) mainframes. Introduced in 2000, z/OS is Unix compliant. It runs traditional IBM mainframe software, such as CICS, IMS and SNA, along with Unix/Linux mainstays such as Java and TCP/IP. Earlier versions of z/OS ran in System/390 mainframes with 31-bit addressing, but after z/OS Version 1.5 in 2007, z/OS only ran on 64-bit System z machines. See System z.