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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 ...
For example, DDR3-2000 memory has a 1000 MHz clock frequency, which yields a 1 ns clock cycle. With this 1 ns clock, a CAS latency of 7 gives an absolute CAS latency of 7 ns. Faster DDR3-2666 memory (with a 1333 MHz clock, or 0.75 ns exactly; the 1333 is rounded) may have a larger CAS latency of 9, but at a clock frequency of 1333 MHz the ...
JEDEC has set standards for the data rates of DDR SDRAM, divided into two parts. The first specification is for memory chips, and the second is for memory modules. The first retail PC motherboard using DDR SDRAM was released in August 2000. [10]
At higher clock rates, the useful CAS latency in clock cycles naturally increases. 10–15 ns is 2–3 cycles (CL2–3) of the 200 MHz clock of DDR-400 SDRAM, CL4-6 for DDR2-800, and CL8-12 for DDR3-1600. Slower clock cycles will naturally allow lower numbers of CAS latency cycles.
The naming convention for DDR, DDR2 and DDR3 modules specifies either a maximum speed (e.g., DDR2-800) or a maximum bandwidth (e.g., PC2-6400). The speed rating (800) is not the maximum clock speed, but twice that (because of the doubled data rate). The specified bandwidth (6400) is the maximum megabytes transferred per second using a 64-bit width.
Memory clock I/O bus clock Transfer rate Theoretical bandwidth DDR-200, PC-1600 100 MHz 100 MHz 200 MT/s 1.6 GB/s DDR-400, PC-3200 200 MHz 200 MHz 400 MT/s 3.2 GB/s DDR2-800, PC2-6400 200 MHz 400 MHz 800 MT/s 6.4 GB/s DDR3-1600, PC3-12800 200 MHz 800 MHz 1600 MT/s 12.8 GB/s DDR4-2400, PC4-19200 300 MHz 1200 MHz 2400 MT/s 19.2 GB/s
To emphasize the advantages of the DDR technique, this type of RAM was marketed at speeds twice the actual clock rate, i.e. the 400 MHz Rambus standard was named PC-800. This was significantly faster than the previous standard, PC-133 SDRAM , which operated at 133 MHz and delivered 1066 MB/s of bandwidth over a 64-bit bus using a 168-pin DIMM ...
DDR and DDR2 memory is usually installed in single- or dual-channel configuration. DDR3 memory is installed in single-, dual-, tri-, and quad-channel configurations. Bit rates of multi-channel configurations are the product of the module bit-rate (given below) and the number of channels.