[zasdffaa]

Not sure if relevant but dram refreshes continuously.

[wahern]

On-die CPU caches typically use SRAM. Modern DRAM chips are already die-stacked in several layers. But maybe DRAM is more heat sensitive than SRAM, so more difficult to stack atop a stronger heat source?

[rbanffy]

IIRC, the RPi SoCs have a DRAM chip stacked on top of the processor.