I think on DOS in real mode address 0 was not special. Some(?) compilers would add code to programs to check that location on program exit to see if its value had changed and emit an error.<p>(I think I read about this somewhere, but it was a while ago, and I never really programmed for DOS (well, one little program to set the serial port to some specific settings, but that was about ten lines including whitespace and a little comment explaining what the code did).)