used to describe assembled DIMMs. Bandwidth is calculated by taking transfers per second and multiplying by eight. This is because DDR3 memory modules transfer data on a bus that is 64 data bits wide, and since a byte comprises 8 bits, this equates to 8 bytes of data per transfer. With two transfers per cycle of a quadrupled clock signal, a 64-bit wide DDR3 module may achieve a transfer rate of up to 64 times the memory clock speed. 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 in a byte). Thus with a memory clock frequency of 100 MHz, DDR3 SDRAM gives a maximum transfer rate of 6400 MB/s. The data rate (in MT/s) is twice the I/O bus clock (in MHz) due to the double data rate of DDR memory. As explained above, the bandwidth in MB/s is the data rate multiplied by eight. CL – CAS Latency clock cycles, between sending a column address to the memory and the beginning of the data in response tRCD – Clock cycles between row activate and reads/writes tRP – Clock cycles between row precharge and activate Fractional frequencies are normally rounded down, but rounding up to 667 is common because of the exact number being 666 and rounding to the nearest whole number. Some manufacturers also round to a certain precision or round up instead. For example, PC3-10666 memory could be listed as PC3-10600 or PC3-10700. Note: All items listed above are specified by JEDEC as JESD79-3F.All RAM data rates in-between or above these listed specifications are not standardized by JEDEC—often they are simply manufacturer optimizations using higher-tolerance or overvolted chips. Of these non-standard specifications, the highest reported speed reached was equivalent to DDR3-2544, as of May 2010. Alternative naming: DDR3 modules are often incorrectly labeled with the prefix PC (instead of PC3), for marketing reasons, followed by the data-rate. Under this convention PC3-10600 is listed as PC1333. Serial presence detect DDR3 memory utilizes serial presence detect. Serial presence detect (SPD) is a standardized way to automatically access information about a computer memory module, using a serial interface. It is typically used during the power-on self-test for automatic configuration of memory modules. Release 4 Release