At least as it's normally used, stacked memory is not the same as on-chip memory. Stacked memory is when a separate memory chip is "stacked" on top of the CPU inside of the same package.
This lets you use a (cheap) bulk DRAM part, in conjunction with an (also usually pretty cheap) logic part.
There are two obvious alternatives. One is basically the same chips, but in separate packages. For large items (e.g., DVD players) this is perfectly fine, but for things like MP3 players and cell phones, stacking can save quite a bit of space (and reduce board design costs).
The other alternative is embedded DRAM (or just a large SRAM on the logic chip). The big disadvantage of both of these is that you generally pay quite a bit more per bit of storage. SRAM isn't nearly as dense, and embedded DRAM requires custom fabrication of your entire chip (I.e., you have to design an ASIC instead of using off the shelf CPU and memory parts). That tends to increase your design time and time to market, so unless you gain a pretty substantial benefit otherwise (e.g., from reduced DRAM access time) it's rarely worthwhile.
Can you comment on what the technologies are? this is mostly what I am looking for - tangible implementations of stacked design that is happening now and in the near future – Derek – 2010-08-24T19:17:29.967
It's not really the distance that counts. It's the capacitance - which increases with distance. The slowdown caused by distance is actually worse than merely physical signal propagation time. It's the signal rise time that is more damaging. – None – 2010-08-24T19:17:58.647
At these frequency capacitance of the wire does not count : you get a model which corresponds to a series inductor and parallel capacitances so you do not 'see' the complete capacitance from your driver, in the end this translates in a model with reduced light speed and a characteristic impedance. Which does matter are the gate capacitances which reduce the flank speeds and of course all other parasitic capacities. You also get nice reflections which corrupt the signal, there is ground bounce when a driver turns on. Fun stuff. You wonder how Intel and AMD can get these things out of the door. – Peter Tillemans – 2010-08-24T19:37:40.603
@Derek There is a cpu with small feature size on an analog sensor interface/power driver chip. Sensors with analog/logic conbined, etc... see e.g : http://melexis-sensor-interface.com/NewsDetail.aspx?nID=476
– Peter Tillemans – 2010-08-24T19:54:17.957