I am not sure whether that principle they taught us was holding 100% of the time, but it for sure helped -- we were fearless ;-). I must say I am glad that I started my developer career by doing systems programming in IBM OSes. It felt like officiating in a temple at the time and age. Compared to channel programming and starting I/O asynchronously, twiddling bits in some register on a PC in a loop on main (and only) CPU felt just wrong to me ;-). Who knew...
