1. DDR stands for “Double Data Rate”. Now before we jump to DDR, we should know few terms related to RAM in order to understand DDR. 2. The basic function of RAM is to store data for a short term and transfer it to the CPU to process that data. How fast it transfers the data to other components is called its Transfer Rate (unit MT/S). 1 MT/s = (Mega transfers per second) = 10^6 Transfers per second. 3. BUS WIDTH This transfer of data takes place through the memory bus connecting the RAM to the CPU. This bus has a certain Bus Width which is number of bits the bus can send to the CPU at the same time. 4. Now Double Data Rate(DDR) refers to the transfer of double amount of data through the memory bus per second. This is accomplished by transferring data on both the rising and falling edges of the clock signal instead of transferring just one time(SDR). 5. BUS CLOCK A bus cycle occurs every time data is sent from RAM to CPU. Bus Clock refers to the number of such cycles taking place per second(unit MHz). 6. For example, A DDR4-2400 SDRAM is called so because it’s transfer rate is 2400 MT/s. Hence It’s real Bus clock would be half of that ie., 1200 MHz.(Although they are marketed as 2400 MHz) 7. By using both edges of the clock, the data signals operate with the same frequency(bus clock), but doubling the data transfer rate. (Transfer rate = 2 * bus clock).
