In some early computers, the bootstrap was actually a matrix of diodes where you'd remove a diode to get a one and leave it in for a zero. I had a bunch of these boards sometime in the mid 1970's and found you could program a fully populated board with a 9V battery - basically connect it across a diode in a bit position where you wanted a '1', there would be a small but pretty flash from inside the glass case as a zero turned into a one.

When things like the 74S188 were available, we had so much fun squeezing bootstrap code for PDP11's into 2 of them; 32 words by 16 bits was more than enough (later I got code that would boot five different devices into 256 words).