4 ms·

If the difficulty of DRAM is in creating capacitors at that size, why haven't we seen a shift toward SRAM (6T, for instance), which is purely transistor-based?

• formerly_proven 6y ago
  On Intel's 10 nm process you can make SRAM with a density of about 20 megabit/mm². [1] Older processes are much worse (<5 megabit/mm²). [2] A current-gen DRAM package achieves about 170 megabit/mm² (but that's two dies, probably stacked). This article [3] cites 8 Gb on 77 mm² on a 21 nm process, giving 105 megabit/mm², and 148 megabit/mm² for the DDR5 version with a die size of 54 mm². The same article shows a Samsung part with around 200 megabit/mm² density. So even if you were to manufacture SRAM on Intel's ultra-expensive 10 nm logic process, you'd need a massive amount of silicon for the same capacity. [1] https://fuse.wikichip.org/wp-content/uploads/2017/12/isscc-2018-intel-10-sram-testchip-density.png https://fuse.wikichip.org/wp-content/uploads/2017/12/isscc-2... [2] https://d3i71xaburhd42.cloudfront.net/f20203949a744276e338d6a4c647e9db3facfdf6/4-Figure13-1.png https://d3i71xaburhd42.cloudfront.net/f20203949a744276e338d6... [3] https://www.anandtech.com/show/13999/sk-hynix-details-its-ddr56400-dram-chip https://www.anandtech.com/show/13999/sk-hynix-details-its-dd...
  • vitus 6y ago
    Totally makes sense that you wouldn't get the same capacity from the same silicon, or even close, given that SRAM uses far more transistors per cell. But if you have issues scaling DRAM, and different scaling limits on transistor count / SRAM, it makes sense (to me at least) to start considering SRAM as an option (e.g. for lower latency, faster speeds, higher bandwidth transfers, etc). Just because you can't achieve the same capacity today doesn't mean there's no merit to it -- HDDs vs SSDs from a decade ago feels like the obvious comparison. Supposedly [1] TSMC's 5nm process yields 256Mb on a 5.376mm² die, at roughly ~50Mb/mm², which would translate to a 3.5Gb die of the same size as the SK Hynix chip. Sure, that's no 16Gb die, but you could easily make 32GB sticks (assuming that you could just combine these chips in the same way as in DDR4). I guess there's also a barrier to entry in that you'd also either need new hardware to deal with "SRAM sticks", or some sort of compatibility layer (a controller that implements the DDRx signaling logic, perhaps). [1] https://www.anandtech.com/show/15219/early-tsmc-5nm-test-chip-yields-80-hvm-coming-in-h1-2020 https://www.anandtech.com/show/15219/early-tsmc-5nm-test-chi...
  • g8oz 6y ago
    Forgive my naivete but: 20 megabit/mm^2 for SRAM...a 1u rack is 600mm X 914mm = 548,400mm^2. Multiply that by 20 megabits and that is about 70 Gigabytes. Does that mean in theory we could build a rackmount server with an external L1 cache of 70 Gigabytes? The cost would be horrendous but I'm sure there is a scenario where it could make sense.
    • opwieurposiu 6y ago
      This would require an impractical amount of wires. For an 8 core, 64 bit cpu with differential signaling would need something like 8(64+64)2 = 2048 wires, and the length of the wires would mean the latency would be much worse then an on-die cache.
• deleted 6y ago
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• ComputerGuru 6y ago
  No, I believe SRAM actually has the edge in that department, and by a fair margin too. The d in DRAM is for dynamic as contrasted with S for static. DRAM need to be constantly read and refreshed (the timings) while SRAM doesn't and that comes with a pretty big hit to energy consumption. As soon as you add DRAM to an embedded project, the thermal/power envelop increases. Of course that all depends on the generation of tech and only applies in an apples to apples scenario.