Synchronous DRAM memory works differently from other memory types. It exploits the fact that most PC memory accesses are sequential and is designed to fetch all the bits in a burst as fast as possible. With SDRAM an on-chip burst counter allows the column part of the address to be incremented very rapidly which helps speed up retrieval of information in sequential reads considerably. The memory controller provides the location and size of the block of memory required and the SDRAM chip supplies the bits as fast as the CPU can take them, using a clock to synchronise the timing of the memory chip to the CPU’s system clock.
Double Data Rate (DDR) SDRAM transfers data on both the rising edge and the falling edge of a clock cycle.
DDR SDRAM modules for desktops have 184 pins, DDR2 SDRAM has or 240 pins.
Read page 38 of the textbook “Exploring Computer Hardware” for more information.