The Memory Ladder
Three lessons ago we promised that the idealized memory — one uniform row of cells — would later be refined, never cancelled. This lesson pays the first installment: the real machine's memory is not one row but a ladder of storages, and one iron law governs every step of it.
Start with the question
If memory is just cells, why does your machine have both "16 GB of RAM" and a "1 TB disk"? Why not one big fast memory and be done with it?
Because nobody knows how to build one — and it is not for lack of trying. Engineering has run into a stubborn law: a storage can be fast, large, or cheap — pick, at best, two. Fast cells are expensive and can't be packed in bulk; enormous cells are slow. Since we want all three qualities and physics refuses to sell them together, machines buy each quality separately: a little of the fastest, more of the slower, oceans of the slowest — and arrange them into a ladder.