DDR Memory Modules
The DDR, DDR2 and DDR3 memory modules have been based on the design of SDRAM or Synchronous Dynamic Random Access Memory. This means that they make use of the clock signal to synchronize things. DDR can be termed as Double Data Rate which implies that the memories transfer two data chunks per clock cycle. The DDR-800 memory modules can work at 400MHz. The DDR2-1066 and DDR-1066 memories at 533MHz and the DDR3-1333 memories at 666.6 MHz and so on.
These clock rates are the maximum that the memory can use in the official sense. While installing a DDR2-1066 memory modules on the computer which can access the memory subsystem at 400MHz or 800 MHz DDR the memories would be accessible at 400 MHz or 800 MHz DDR and not at 533 MHz or 1066 MHz DDR. This is possible as the clock signal has a memory controller that is a circuit situated outside the memory.
This is based on the assumption that the data will get transferred on every single clock pulse. In order to arrive at the maximum theoretical transfer rate in MB/s i.e megabytes per second we can multiply the DDR clock in MHz by eight. It is important that the memory controller and the memory exchange their commands of instructing the memory to deliver data stored at a stated position.
