A little wrong on that date. Even the PDP-11/20 (the first -11) had a boot ROM: https://gunkies.org/wiki/BM792_ROM which appreared in mid-1971 (about a year after the release of the /20). DEC sold them pre-programmed, but one could 'program' one onself, if one wanted - with a soldering iron! (Check out the image! I actually did that to one that I was given, that had been eviscerated by someone.) From then on (follow the category link), the rest used PROM chips. It's worth remembering that in the early period, that restriction to low addresses was built into the hardware (in an amusing way :-). Take the DL11: https://gunkies.org/wiki/DL11_asynchronous_serial_line_interface which was sort of mandatory as the 'console' serial interface on most early -11's (until the DL11-W appeared; more on its big improvement in a second). It set the interrupt vector with _jumpers_. (You want to change the interrupt vector? Dig out your soldering iron! :-) There were only 6 jumpers - one each for address bits 3 through 8. So the largest vector you could set was 0770. The DL11-W was a big step forward - it replaced the jumpers with a DIP switch! :-) Still only six bits, though. :-) Noel