Why not etch dies with either tiles of octagons and squares or just hexagons?
Traditionally, DRAM-style memory (tricky cells, regular layout) and CPU-style logic (simple cells, irregular layout) use different manufacturing techniques and lines, so it's relatively easy to get them in the same chip but relatively difficult to get them on the same die.
It is possible to build entire systems with SRAM, but it's not usually cheap. The old line about Crays is that after you bought the memory, they'd throw in some supercomputer CPUs for free.
But I get plenty of hits for "embedded DRAM", so maybe this is now a solved problem?
-Dave
