The Program Lies in Memory as Data
Every previous lesson was preparation for this one. Today the course states the central idea of all computing — the fact that makes languages possible, machines universal, and (much later) attacks conceivable. Like every deep truth here, it is derived, not announced: three old statements meet, and the conclusion is forced.
Start with the question — and the forced answer
The processor performs actions in sequence. Something must tell it which actions, in which order — the program. Where is the program, and what is it made of?
Assemble what the course already knows. The action set is small and fixed — so each action can be given a permanent number: say 1 = read, 2 = write, 3 = add, 4 = compare, 5 = move to another place in the program. A sequence of actions then becomes a sequence of numbers. And numbers live in memory cells — the same cells as everything else, because there are no other cells. Conclusion, with no exit:
The program is numbers, stored in ordinary memory cells, side by side with the data it works on. Not "loaded into a special instruction compartment" — there is no such compartment. One memory, one substance, two roles.
This design has a name worth knowing once — the von Neumann architecture, after the mathematician who articulated it — and virtually every machine you have ever touched is built this way.