Hercules is an emulator for the IBM 360 and 370. And, if you didn’t know, OS/360 is in the public domain. So go wild.
I spent about four months of evenings wayBackWhen mucking about at the IBM Federal Systems Division. There was a local computer literacy thing that some friends and I went to. Of the 20-30 kids who showed up, they realized that four or five of us (a) really knew computers, and (b) were trouble-makers, so they (our IBM hosts) decided that they’d let us use their mainframes.
Before they sat us down at terminals, they gave us a tour: Literally acres of machine-room floor, the “mass store” that held a terabyte or so of video tape and two robot arms that scurred back and forth, loading and unloading the tape into readers. Machine after machine after machine. The tempting orange panic buttons mounted on pillars everywhere that shut everything down (“You don’t press them, you yank on them. Hard. Then we spend days picking up the pieces. Don’t do that.”) The laser printer the size of a living room (“the paper goes ballistic inside.”)
There was a lecture on 370 architecture. I don’t think I really knew what a cache or a TLB was in those days (microprocessors didn’t have them yet). I forget a lot of the dog’s breakfast of software that they exposed us to; mostly we did assembly language, and my programs mostly just blew up. I was learning Emacs at the time [hell, I still am], and the “everything is 80 columns” editing environment was pretty awful. I was learning C and Unix, too, and when I asked how to open a file, expecting maybe to push a string and make an OS trap or something, I got a 20-minute talk that just scraped the surface of the assembler’s macro facility.
Once, one of my programs blew up; it had used its quota of CPU time (a second?) and had been killed. The phone next to my terminal rang. It was the system operator (they knew where you were logged in), and he said that I had 5,000 pages of output queued to be printed, and did I really want that?
[Possibly unrelated, but: Our school district ran on IBM mainframes. A few weeks later we had the district IT division beg us not to crash their mainframe any more. Sigh. I guess these days we’d go to jail or something].