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The Memory Ladder

Computer Science How Computation Works Difficulty 1/5

Beginning of the lesson

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.

Learning goals: state the trade-off law of storage; name the steps of the ladder in order and say roughly how size and speed change per step; know which steps survive power-off; and see precisely what this refinement changes — and what it leaves untouched.

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.

The steps of the ladder

Registers Cache Operating memory (RAM) Disk fastest slowest a handful of cells oceans of cells Each step down: roughly tens-to-hundreds of times BIGGER and tens-to-hundreds of times SLOWER. Power-off erases the upper steps; the disk remembers. (The video card's own memory is a side-tower — its story comes later.)

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  • Worked examples
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  • Test questions: 2
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