Double Data Rate Double In Line Memory
DDR-DIMM is a 184-pin memory module that uses DDR-SDRAM memory chips.
DDR-DIMM memory modules are classified accordingly to their maximum theoretical transfer rate, in MB/s: PC1600, PC2100, PC2700, PC3200, PC3500, PC3700 and PC4000 (i.e. memory modules using DDR200, DDR266, DDR333, DDR400, DDR433, DDR466 and DDR500 memory chips, respectively). For example, PC3200 modules have a maximum theoretical transfer rate of 3,200 MB/s.
For a better understanding, read DDR-SDRAM.
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Figure 1: Comparison between a DIMM module and a DDR-DIMM one.