Here's a taste:
https://qntm.org/zomg>System z is a living fossil; it is computing archaeology; computing palaeontology. System z is an example of the principle "If it ain't broke, don't fix it" carried to its logical extreme. System z and its predecessors haven't broken for forty-five years: the "z" stands for "zero downtime" and some of these machines have been running continuously for decades without a reboot. Nor, therefore, have any of them ever been "fixed".
>It uses EBCDIC, a character encoding wholly incompatible with ASCII featuring such delights as non-contiguous letter sequences. It was, from 1983 to 2000, a 31-bit operating system. (Yes, thirty-one.) It does things with virtualisation of system resources and operating systems themselves that were thirty years ahead of their time, but all the interaction with z machines is still performed through an archaic green screen system from the 1970s, running in an emulator, with a 24x80 character screen. The screen is 80 characters wide because that is how many characters you could fit on a punch card.
>The output from the Job Entry System, however, is 132 characters wide, because that was how many characters you could fit on the line printer where the output came out. Nowadays, of course, the output comes to the green screen. So you always have to scroll left and right to see all of it. The keys for scrolling left and right are F7 and F8. For scrolling up and down? F10 and F11. No, Page Up and Page Down will do nothing.
>To enter a command? You hit the right Ctrl key. "Return" will do nothing. "Enter" will work, of course, but, AS YOU KNOW, "Return" and "Enter" are separate keys on your keyboard. "Return" is just above the right Shift key. "Enter" is way over next to the numeric keypad.