RAM, Random access memory. Computer goes directly to the memory location specified in machine instructions.
DDR2 SDRAM is a double data rate synchronous dynamic random access memory interface. It supersedes the original DDR SDRAM specification and the two are not compatible. In addition to double pumping the data bus as in DDR SDRAM (transferring data on the rising and falling edges of the bus clock signal), DDR2 allows higher bus speed and requires lower power by running the internal clock at one quarter the speed of the data bus. The two factors combine to require a total of 4 data transfers per internal clock cycle.
With data being transferred 64 bits at a time, DDR2 SDRAM gives a transfer rate of (memory clock rate) × 2 (for bus clock multiplier) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus with a memory clock frequency of 100 MHz, DDR2 SDRAM gives a maximum transfer rate of 3200 MB/s.
Since the DDR2 internal clock runs at half the DDR internal clock rate, DDR2 memory operating at the same external data bus clock rate as DDR results in DDR2 being able to provide the same bandwidth but with higher latency. Consequently, DDR2 RAM possesses inferior performance. Alternatively, DDR2 memory operating at twice the external data bus clock rate as DDR may provide twice the bandwidth with the same latency. The best-rated DDR2 memory modules are at least twice as fast as the best-rated DDR memory modules.
In computing, DDR3 SDRAM or double-data-rate three synchronous dynamic random access memory is a random access memory interface technology used for high bandwidth storage of the working data of a computer or other digital electronic devices. DDR3 is part of the SDRAM family of technologies and is one of the many DRAM (dynamic random access memory) implementations.
DDR3 SDRAM is an improvement over its predecessor, DDR2 SDRAM, and the two are not compatible. The primary benefit of DDR3 is the ability to transfer at twice the data rate of DDR2 (I/O at 8× the data rate of the memory cells it contains), thus enabling higher bus rates and higher peak rates than earlier memory technologies. In addition, the DDR3 standard allows for chip capacities of 512 megabits to 8 gigabits, effectively enabling a maximum memory module size of 16 gigabytes.
With data being transferred 64 bits at a time per memory module, DDR3 SDRAM gives a transfer rate of (memory clock rate) × 4 (for bus clock multiplier) × 2 (for data rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus with a memory clock frequency of 100 MHz, DDR3 SDRAM gives a maximum transfer rate of 6400 MB/s.
DDR3 is a DRAM interface specification; the actual DRAM arrays that store the data are the same as in any other type of DRAM, and have similar performance.