Check out the picture of the rope memory used to store the ROM: http://authors.library.caltech.edu/5456/1/hrst.mit.edu/hrs/apollo/public/visual3.htm http://aut

Check out the picture of the rope memory used to store the ROM: http://authors.library.caltech.edu/5456/1/hrst.mit.edu/hrs/apollo/public/visual3.htm http://authors.library.caltech.edu/5456/1/hrst.mit.edu/hrs/a... Each ROM word is a wire. If the wire goes through the first magnetic core, its first bit is a one; if it skips the core, its first bit is zero, and so on. Each of the 36,000 words of ROM needed to be threaded through these cores by hand. Amazing. http://en.wikipedia.org/wiki/Rope_memory http://en.wikipedia.org/wiki/Rope_memory (I imagine it was chosen because it's much more resilient to cosmic rays flipping a bit. But it's pretty much impossible to alter the program without creating the whole thing from scratch. The ultimate spaghetti code!)