In my OS course, in 1969, I wrote an IPL (bootstrap) program that copied cards to the printer, running in supervisor mode. It ran on a 360 virtual machine under the MTS time-sharing system. Before that, I wrote programs in assembly language for an IBM 7044, and I even was allowed to operate the 7044 on some graveyard shifts. I also wrote programs that ran on a CDC 3600, a Univac 1108, and a Honeywell 200.
But I liked the PDP-8 best.
So there!