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> Is it faster in ARM?

Not in the general case, but specifically with how Apple has brought the ram chips so physically close to the cpu cores

The physical distance confers no latency advantage.

Is this because of signal speed anyway reaching almost speed of light?

Maybe 70% the speed of light. That's a handful of cycles at a few GHz for any SDRAM channel. The latency limitation is caked in to the scanning, strobed nature of SDRAM. It's dense and cheap, but it will never respond faster than ~20 ns (100+ cycles).

I wonder what stops AMD, Intel in trying this, except loss of modularity. It is not like the architecture will change by bringing it into the die. As far as I remember, CPU is the only thing that does RAM access even in x86

I would be surprised if they’re not evaluating this. The only difference I can see is that it complicates the number of SKUs they’d need to provide to their customers. It’s a little easier for Apple in this regard because they’re also building the final machines, whereas Intel/AMD are making chips that are going in a wider range of devices.

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