what is quad stack and Quad die DDR SDRAM

Dear All:
   Would you please help explian the quad stack and quad die ddr sdram?
   Any tips will be great appreciated！

Some question, you means the design of the stack up and the signal termination of the DDR SDRAM? Can you describe it detailed?

As far as I know, bardshen should be mentioned "Quad-stacked/quad-die DDR2 SDRAM".
There is no quad-stacked DDR SDRAM according to "Double Data Rate (DDR) SDRAM Specification(JESD79D)".