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Comment by threecheese | Hacker News Reader

Thanks for that, just went down an interesting rabbit hole. Many of us were hoping this re-tooling would eventually trickle some fast RAM down to DRAM-exhausted PCs, but given it would require a rearchitecture of the motherboard it's unlikely.

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HBM4 has over 2048 signals to the processor’s PHY with tight signal integrity requirements that require the HBM stack to be < 0.5 mm from the processor die. That’s why HBM integration is done with interposers (soldered on the package). So, it’d be the CPU package that integrates it. Motherboard is too far away.

Kind of seems like we should be making chips with both. Big HBM stack on top as a sort of huge L5 cache like thing. And then a bunch of DRAM type sockets (like LPCAMM) around the exterior.

For chips with integrated CPU+GPU+NPU, it could be worth it tech-wise. The GPU and NPU can eat HBM bandwidth. For general purpose CPU code, the HBM would likely not be worth it. It's high bandwidth, but you trade latency, and general CPU code is branchy. Economics-wise, the HBM stacks alone will cost more than a consumer CPU (or APU).

[edit] The packaging needed to support HBM is also much more expensive too. If demand for current HBM applications tanks and the manufacturing lines need filled, then maybe. Currently, the price point would make it very very niche.

HBM also trades bandwidth for latency, and your regular computing is much more sensitive to latency than bandwidth.

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