Their operating systems are unbelievably efficient as compared to the bloatware we see floating around today.
My first experience:
To give you an example, at the insurance company where I started back in ’79, we had an S/370 with (if I remember correctly) 8MB (yes, eight megabytes) of internal storage (storage is IBM lingo for ‘memory’). And apart from all kinds of batch jobs that also ran during day-time we serviced the entire company with over 200 people who could work simultaneously with online software to administer insurance policies etc…
Operating Systems in the public domain
There’s a couple of operating systems out there in the public domain. IBM was forced by the US government to do this, because the funding for the development of the systems was done by them. That gave hobbyists the opportunity to adapt these OS’s. They are: OS/VS1, VM/370 and MVS3.8j.
To be able to run these operating systems we used to need access to the very, very expensive mainframe hardware. Not anymore! A group of enthousiasts created an emulator for IBM mainframes that runs on all kinds of hardware, under Windows, Linux and MacOS. It’s called Hercules.
On the subpages you will find more information regarding this emulator and about The IBM 370 Operating Systems.