System 360 is very well documented by the Principles Of Operation: https://dl.acm.org/doi/pdf/10.5555/1102026 https://dl.acm.org/doi/pdf/10.5555/1102026 So muc

System 360 is very well documented by the Principles Of Operation: https://dl.acm.org/doi/pdf/10.5555/1102026 https://dl.acm.org/doi/pdf/10.5555/1102026 So much better written than Intel’s Software Developer Manuals, where anything complicated is just “look at these three pages of c-like pseudo code we probably pulled from our tests, good luck!” IBM themselves have had to maintain backwards compatibility for > 50 years, and there is a long history of compatible machines: https://www.computerhistory.org/revolution/mainframe-computers/7/169 https://www.computerhistory.org/revolution/mainframe-compute... So all the strange behaviours are well documented.