Your computer uses random access memory, or RAM, to both start and run the programs you're currently using, including the operating system. Double Data Rate-2 and -3, or DDR2 and DDR3, are two specific types of RAM often used in computers.
DDR2 was released in 2004 and approximately doubled the speed of regular DDR memory. Released in 2007, DDR3 approximately doubled the speed of DDR2 memory.
DDR2 memory has a transfer speed between 400 and 800 megabytes per second, depending on the specific memory. DDR3's transfer speed is between 800 and 1600 Mbps.
Voltage, Internal Banks and Prefetch
DDR3 memory, while faster than DDR2, requires less voltage. DDR2 requires 1.8 volts, while DDR3 only requires 1.5 volts. DDR3 also doubles the memory's internal banks from four to eight, raising the prefetch data from 4 bits to 8 bits. Internal banks can only process 1 bit of information at a time, so doubling the banks means DDR3 can theoretically process twice as much data in the same amount of time.
DDR2 and DDR3 memory are not compatible with one another. If your computer uses DDR2 memory, you will not be able to install DDR3 memory without first buying a new motherboard compatible with DDR3 memory. To avoid accidentally using DDR2 in a DDR3 system, the key notches for installing the RAM are in different positions, making it so DDR3 won't fit in a DDR2 memory bay.
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