SDRAM vs DDR

DDR DIMMs and SO-DIMMs have the same physical dimensions as their SDRAM counterparts, but have a different footprints. DDR DIMMs have 184 pins compared to 168-pins for SDRAM DIMMs. DDR SO-DIMMs have 200 pins compared to 144 pins for SDRAM SO-DIMMs.

DDR DIMMs and SO-DIMMs also have a different notch (called 'key') that will only fit motherboards and systems especially designed for DDR. DDR modules will not fit into PC100 or PC133 sockets and are not backward compatible.

The difference between DDR and SDRAM DIMM modules is shown below.

DDR - PC1600, PC2100, PC2700 or faster		184 Pin DIMM Single Notched (or Keyed) Two operations per clock cycle
VS
168 Pin DIMM Double Notched (or Keyed) One operation per clock cycle		SDRAM - PC100 or PC133