What is the difference between DDR3, DDR2, DDR, and SDRAM?
Single data rate (SDR) SDRAM is the older type of memory, commonly used in computers prior to 2002.
Double data rate (DDR) SDRAM hit the mainstream computer market around 2002 and is a straightforward evolution from SDR SDRAM. The most significant difference between DDR and SDR is that DDR reads data on both the rising and falling edges of the clock signal, enabling a DDR memory module to transfer data twice as fast as an SDR memory module.
The generation after DDR, called DDR2, began to appear in mid-2004. DDR2 is faster than DDR, delivering bandwidth of up to 8.5 GB per second. Frequently, DDR2 based systems can use memory installed in pairs to run in "dual channel mode" to increase memory throughput even further.
The most recent generation of memory technology, DDR3, began to appear in systems in late 2007. DDR3 is the next evolutionary step beyond DDR2 and operates at lower voltages, which means it consumes less power, and it can transfer data at rates up to 12.8 GB per second.
Your Mac supports only the memory generation that it first shipped with. You cannot mix and match SDRAM, DDR, DDR2, or DDR3 memory in a Mac. The different memory types don’t function together, they will not even fit in the same sockets.
Because of this, you need to make sure that your upgrade is of the same memory generation as the memory that shipped with your Mac. The easiest way to find compatible memory is to look up your system in Crucial's Memory Advisor tool, that will list only compatible memory for your Mac.
If you don't know the exact model of your Mac, the Crucial Mac Scanner will automatically detect your system configuration and give you a report on which upgrade is right for you.