Textbook › Part I The Physics of Semiconductors
CHAPTER 08
The Memory Hierarchy and the Memory Wall
This is the chapter that pays off most in real work.
However fast a processor is, it can do nothing if the data it needs is not at hand. And memory speed has never grown at anything like the pace of processor speed. The gap has widened year after year, and it has a name: the memory wall
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Kinds of memory and what each is like
| Kind | How it works | Speed / capacity | Where it is used |
|---|---|---|---|
| Register | A flip-flop, plain and simple | Fastest / a few hundred bytes | Right next to the arithmetic unit |
| SRAM | Holds one bit in six transistors | Fast / small and expensive | Caches, the internal buffers of an NPU |
| DRAM | Stores charge on a capacitor. Left alone the charge drains away, so it has to be rewritten periodically | Slow / large and cheap | Main memory, LPDDR |
| HBM | DRAM stacked vertically and joined by extremely wide wiring | Bandwidth in another league / expensive | GPUs and NPUs for the data center |
| Flash | Traps charge inside an insulating film | Slow / survives a power-off | Storage, holding models on disk |
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