DRAM Demystified


Could you please explain the many ways DDR3 speed ratings are stated?

—Tom Ellison
DDR3 is commonly referred to in two different ways: by the “module name,” which denotes the module’s theoretical bandwidth, and by the effective clock speed at which it runs. For example, a common performance DIMM is a PC3/12800 module, which gives it a theoretical bandwidth of about 12,800MB/s. The same module can also be referred to as a DDR3/1600 DIMM which means it has an effective clock rate of 1,600MHz.

Why the two naming conventions? Politics and marketing. The module name actually goes back to the days when DDR was competing with Direct RDRAM. A PC-800 RIMM had an effective clock speed of 800MHz and a theoretical transfer rate of 1,600MB/s per module, with dual-channel offering 3,200MB/s transfer rates. Since the initial single-channel-only DDR/266 modules didn’t sound as impressive as a PC-800 module, the competing SDRAM industry decided to name it PC-2100 for its theoretical bandwidth of 2,100MB/s—a much bigger number than 800.

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