From Granizada

Build a Mainframe!

An IBM mainframe is a physically large computer, although vastly smaller than they used to be. There are plenty of other equally impressive bits of hardware (mainframes aren't nearly as reliable as Tandem Nonstop for example) but there are a lot of them and it is pretty impressive that most of the world's banks should be running computers that don't need to be rebooted more than every twenty years or so. Even though they have been extensively upgraded over that time. You can shift the work to one end of the computer while you replace the CPUs down the other end.

A group of hackers ported Linux to the mainframe, and eventually IBM agreed it would be useful. Now, Linux on mainframes is big business for IBM. Their slogan was "Never trust a computer you can lift".

An Englishman living in Paris decided he could encapsulate everything in the mainframe manuals lining his bookshelves in a program, so he did. What he did was write a software mainframe. It is so good that it boots operating systems intended only for use on mainframes, both Linux and the weird ones IBM uses. His work is called an emulator. It emulates mainframe disks (which are descended from physically massive drum storage) and tapes and even buttons on the front panel. All on a program you can run on your desktop. IBM supported Hercules at first and then realised it was becoming better than the lower-end hardware they were still supporting for huge sums of money, so they hastily removed all reference to Hercules in their manuals.

Here is some info about Hercules on Wikipedia: http://en.wikipedia.org/wiki/Hercules_emulator

Here is the official site: http://www.hercules-390.org/

Here is a step-by-step for installing Debian for 390 on Hercules: http://www.josefsipek.net/docs/s390-linux/ . You should be able to get full networking, in fact I've even played games on it. There are a lot of interesting reasons why it can be useful to use Hercules, happy to explain them another day.

As an uber-challenge, there is also a guide for installing OS/360 on Hercules. OS/360 was released in 1965 for the S/360 machine. Hercules is pretty thorough, it emulates the entire mainframe family from the most modern z-series back through all its ancestors, the S/370 and the S/360. IBM have released OS/360 to the general public because, well, 1965 is a long time ago. If you can do this then you're doing extremely well.