PS: PDP-11 Unix didn't, I think, do much (anything?) to solve the null pointer problem. (This is for early C versions; I don't know about the later BSD ones.) Location 0 was a usable address for both read and write for everything except 'pure-text' (0410 magic word) processes; in those it was only legal for read. Address 0 mostly did not contain a 0, either; for C programs using the stock run-time, it contained a 'setd' instruction, except in split I+D processes, in which case data space location 0 probably (I'm too busy to spin up my V6 emulator to check) contained some of the program's initialized data (unless special arrangements had been made). Noel