what is added feature tha came with DDR DRAM and DDR2 DRAM compared to DRAM

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  [Prototype]

  To start, SDR DRAM is a single data rate asynchronous RAM. Having an asynchronous nature, it'll respond to commands as soon as it encounters one.
  
  The SDRAM on the other hand is synchronized with the clock of system bus, generally the positive edge, hence it'll respond only at the clock edge. The DDR or double data rate SDRAM are synchronous in nature, but are made to respond on either edge of the clock, thus, doubling the data rate as compared to SDRAM.
  
  Now, the difference between DDR I, II and III are, DDR I is capable of reading only 2 word while its 4 and 8 for DDR II and III respectively. To achieve this speed without manipulating the clock, the protocol at which the RAM works has been modified. This is the reason why the motherboards supporting DDR II ram will not be forward or backward compatible.