I wrote various device drivers for RT-11 back in the day, including for one of the first winchesters (Rodime 5Mb, which was a big step-up from 8" floppies - so

I wrote various device drivers for RT-11 back in the day, including for one of the first winchesters (Rodime 5Mb, which was a big step-up from 8" floppies - so much so I partitioned the driver into 8 separate areas). The boot ROM was 256 words, and in there was boot code for the winchester, 8" and 5 1/4" floppies and a CLI to select between them (including the unit number). One of the tricks I recall used the need to load locations 100 and 102 with the vector and processor status word for the BEV (bus event/timer) interupt. At the end, I needed 3 words to load the read sector command for the 8" floppy but only had 2 left. Noticing that the read sector command was 204, I got that by shifting the leftover address for the BEV vector processor status word up one bit (102 => 204) and finished the job with a word to spare.