What is it?
IBM's current flagship operating system is z/OS, released in 2000 at the same time as the 64-bit eServer zSeries 900 mainframe. Mainframe and operating system had both been redesigned primarily for e-business.
However, IBM mainframe users have plenty of choice, including z/OS's predecessors, OS/390 and its siblings VSE/ESA and VM/ESA.
VM/ESA has in turn given rise to z/VM, which uses IBM's virtualisation technology to create a number of virtual mainframes on a single mainframe, each of which can run a different operating system. For budget mainframe users there is z/OS.e, a kind of z/OS-lite. And of course there is Linux, which can run alongside z/OS under z/VM.
Where did it originate?
2004 is the 40th anniversary of IBM's mainframe range, which began with the System/360. The System/390 was released in 1990. Linux was first implemented on IBM mainframes in 1999.
What's it for?
Mainframe users who do not move to z/OS will soon find themselves up a blind alley. IBM is gradually ending support for OS/390, and z/OS 1.4 is the last release that OS/390 users will be able to migrate to in a single step.
New versions of key products such as DB2, Lotus Domino, Cics and the Java SDK (software developer's kit) will all require z/OS 1.3 or higher.
What makes it special?
IBM's former 31-bit architecture was insufficient to handle the size and complexity of today's applications. Previous IBM mainframe operating systems (and AS/400 and RS6000) had offered some 64-bit capabilities, but z/OS represents a complete move to 64-bit general registers, 64-bit operations and 64-bit virtual and real addressing.
As a 64-bit address space is eight billion times larger than a 31-bit space, this is bigger than any foreseeable computing requirement. But the need to retain compatibility with 31-bit has held IBM back from making full use of the z/Architecture, which is why the writing is on the wall for OS/390.
How difficult is it to master?
There is a two-day "Introduction to the z/OS and OS/390 environment" for IT professionals without previous mainframe experience. But you will need to take a number of courses to become proficient in any mainframe operating system area. There are 25 courses under the heading "z/OS and OS/390 Basics" alone.
Where is it used?
z/OS is intended for Java-based e-business. z/VM provides a test environment and one in which multiple operating systems can run side by side. Linux provides a way of consolidating multiple servers on a single mainframe.
What systems does it run on?
IBM mainframes - advisory firm IDC said IBM sold 2,700 last year, up from 2,300 in 2002.
Not many people know that...
IBM has moved from a six-month to a 12-month release cycle for z/OS, finally recognising that most customers (who are actually on 12-18 month cycles) want to minimise cost and disruption.
What is coming up?
Additional autonomic computing capabilities - the self-healing, self-managing features such as dynamic memory sparing and dynamic disc balancing by which zSeries systems fine-tune themselves.
Training is available from IBM and third-party trainers. A great deal of material can be downloaded from IBM's various operating system home pages, but you will need the structure provided by formal training to slot it all together.
Rates of pay
Mainframe operators and technical support staff can look for £28,000 to £35,000.