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Comment by brucethemoose2 | Hacker News Reader

More DDR5 is tricky, as the SoC very much depends on the packaged RAM. The M chips physically don't support it.

For $3000 I would expect a different SoC that has a second integrated memory controller with DDR5 support, even if it leads to an asymmetric memory configuration.

This is easy to say, but tape out costs for a reticle size slab of silicon on an interposer are ~10 figures, at least.

It makes zero economic sense for a low volume product like the Mac Pro, even with Apple margins.

EDIT: Also, they would have to seriously compromise performance. A number of GPU cores would have to be removed to make room for the bigger memory controller. The bus would have to be even wider with regular DDR5 DIMMs, and it would suck much more power.

The Studio should have upgradable RAM too IMHO, so they could use such a chip for that too, which would amortize the costs.

There is no space for ~16 DIMMs.

Make it taller by one Mac Mini height, no one would care. Also CXL memory modules don’t necessarily have to be the same dimensions of a DIMM.

The M2 series does not support CXL, as far as I know.

They would presumably use UDIMMs anyway, so maybe there is theoretically space?

Still, that would be one heck of a socket/motherboard. I can't imagine how many motherboard layers and socket pins they would need for 16+ channel DDR5 in such a tiny space... If its even possible to manufacture, it would be hilariously expensive.

Is that true even with a chiplet or north bridge approach? I feel like we’ve seen plenty of examples of CPUs laid out this way. Yeah with a performance penalty but that 192GB of L4 with direct GPU access would go a long way to making that better.

Apple could indeed fab a small memory controller chip, but I am not sure the M2 Ultra die has the interconnect to support such a thing? AMD makes this looks "easy," but it is not easy, and they built for Infinity Fabric from the start.

As said below, the answer is CXL. Apple would be stupid not to support CXL devices on the M3 or M4, though I would not be surprised if they skip it

I actually wasn't familiar with CXL, I'll check it out. Thanks!

It still wouldn't hurt to have as an option for, say, a swap disk in RAM instead of the SSD, or expose it as storage to macOS for use as scratch disks, or even just use it as a buffer/cache for the flash storage. There are all kinds of use cases where it would be beneficial to have a wicked-fast storage medium working between system storage and memory.

You are saying Apple needs CXL support.

Yes, this is 100% true. CXL devices are exactly what the Mac Pro needs.

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