Question

DDR3 SDRAM is neither forward nor backward compatible with any earlier type of random access memory (RAM). What does it mean?

Answer 1

You cannot use DDR3 SDRAM with sockets or systems designed for other types of ram, like SODIMM or DIMM RAM.

Answer 2

It could also mean that if a motherboard has both slot types, you would not be able to use them at the same time as they are not compatible with each other.

Answer 3

You cannot use DDR3 in earlier sockets (DDR2, etc..) sockets because of physical mismatch (slot configuration prevents insertion) and logical addressing issues inherent to the chip sets that drive the memory subsystem. This is likely true of any future memory systems as well. You can however, typically use a DDR3 1066Mhz module, in a DDR3 slot capable of only 800Mhz, since the module will (maybe) run at the reduced speed. Compatibility of RAM modules is dependent upon the implementation by the motherboard manufacturer.