Operating System 400 (OS/400) is the operating system running IBM's roster of AS/400 servers. It was first launched in the market along with its hardware counterpart in 1988.

Subsystems like OLOPS and RPG are built in the OS/400. This allows system/3x application to run on the AS/400- natively or with some plug-ins specially designed for AS/400.

Old and archaic it may seem but OS/400 is one secure operating system. Of course, who would want to hack a 15-year old machine? Believe it or not, a large group of companies worldwide are still using the said system because of its versatility, stability, flexibility and efficiency.

The current version of OS/400 is the OS/400 V5R2. Features of this new release include incorporation of support of Linux, Enterprise Identity Mapping (EIM) and integration of Websphere Application Server for the implementation and deployment of e-business solutions. Old features like support for Java, SQL, XML, etc. are still retained in this new release of OS/400.

That's all.