Back in Apple Silicon, Xeon Phi, and Amigas I asked how Apple would scale the memory up in a hypothetical Mac Pro based on the M1. We still don’t know because there still isn’t one, although now we sort of do know.
The M1 Ultra uses a dedicated interconnect allowing two (maybe more, but definitely two) M1 Max to act as a single SoC. So in an M1 Ultra-powered Mac Studio, there’ll be two M1 packages connected together, acting as if the memory is unified.
It remains to be seen whether the interconnect is fast enough that the memory appears unified, or whether we’ll start to need thread affinity APIs to say “this memory is on die 0, so please run this thread on one of the cores in die 0”. But, as predicted, they’ve gone for the simplest approach that could possibly work.
BTW here’s my unpopular opinion on the Mac Studio: it’s exactly the same as the 2013 Mac Pro (the cylinder one). Speeds, particularly for external peripherals on USB and Thunderbolt, are much faster, so people are ready to accept that their peripherals should all be outside the box. But really the power was all in using the word Studio instead of Pro, so that people don’t think this is the post-cheesegrater Mac.