The AS/400 is completely different hardware and software. I don't mean that it just a reimplementation of what you are used to, but rather alien to what you are used to. It is designed to be a business system, and an evolution of the System 32/36/38.

I recommend Frank Soltis' books about it, although they are very expensive. For example an earlier edition is http://www.amazon.com/Inside-As-400-Frank-Soltis/dp/18824191...

As an example there is extreme backwards compatibility, pervasive database, single address space (a different meaning of virtual memory than you are used to), sharing, tagged and protected pointers, clustering etc.