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Comment by lispygem | Hacker News Reader

_The former is much faster and provides the same functionality. Why is not all RAM in a machine SRAM? The answer is, as one might expect, cost._

But since the density of transistors is pushing the cost of everything down how expensive is expensive? Does it make sense to product a SRAM-based laptop with 512MB of fast memory?

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Well, part of the faster-than-DRAM is because the cache is limited in size. Addressing 512 MB (and keeping it close enough to the CPU) would require a lot more address lines/multiplexing and CPU die real estate [1] that it would be prohibitively expensive and not as fast.

It'll still be faster than DRAM so you still have a point.

[1] In this die-shot (http://en.wikipedia.org/wiki/File:Barcelona_die.jpg) of AMD's Barcelona (4 core) all the regular grid like pattern is the cache (total is about 4MB + 512KB or so.) That takes about half the chip's real estate.

SRAM uses 6x times more transistors to store the same amount of information, so do your math for a hypothetical "512MB laptop". On the other hand maybe they do exist, did you look?

FTA: It takes more transistors and the packaging of DRAM is more regular. Even if you are willing to pay 6x as much for the needed transistors, you would also need to have your memory chips be much large physically, which, I assume, scales up the cost quite a bit.

In addition to being larger and harder to read data out of, as well as having far far lower yields, it uses easily several times more power to keep on than DRAM does.

Especially if you run it at speeds that would make it comparable to cache.

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