> The reason you don’t is because it is basically impossible to get access to a mainframe you can play with and learn on.

Once upon a time, we made a product that was selling to Fortune 500s, and IBM loaned one of the smaller zSeries to us to make sure the product was running well in a Linux VM on it. The thing only fit in the elevator after we took it out of the wood crate...

A lot of the porting effort happened on Hercules, an emulator: http://www.hercules-390.org/