I think that it boils down to writing arbitrary data in memory (so that they can't follow the usual path of programs, as with compilers). For the anecdote, a fr

I think that it boils down to writing arbitrary data in memory (so that they can't follow the usual path of programs, as with compilers). For the anecdote, a friend of mine used to flash a "hello world" program into a microcontroller using push buttons only :)