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What is Synchronous DRAM?
Synchronous DRAM (SDRAM) is the first DRAM technology designed to synchronize itself to the timing of the CPU. SDRAMs are based on a standard DRAM core and work just like standard DRAMs, but they incorporate several distinguishing and innovative features:
  • Synchronous operation - unlike conventional, asynchronous DRAMs, SDRAMs have a clock input, so the system clock that controls the step-by-step operation of the microprocessor can also control SDRAM operations. That means the memory controller knows the exact clock cycle on which the requested data will be ready. In effect, this frees the processor from having to wait between memory accesses.

  • Cell banks - the memory cells inside the SDRAM chip are divided into two, independent "cellbanks." Since both banks can be activated simultaneously, a continuous flow of data can be produced by switching between banks. This method, called interleaving, cuts down the total memory cycle and results in faster transfer rates.

  • Burst mode - bursting is a rapid data-transfer technique that automatically generates a block of data (a series of consecutive addresses) every time the processor requests a single address. The assumption is that the next data-address the processor will request will be sequential to the previous one, which is usually true (this is the same assumption behind cache memory). Bursting can be applied both to read operations (from memory) and write operations (to memory).

In a word, synchronous DRAMs are faster. Even though SDRAM is based on standard DRAM architecture, its combination of these three features allows a faster, more efficient data transfer process. SDRAMs can presently transfer data at speeds up to 125MHz as much as five times the performance of standard DRAMs. That puts SDRAM on a par with the more expensive SRAMs (static RAM) used in external cache.

For more information on the types and configurations of memory modules available today, you might want to check out two of the biggest memory manufacturers on the web, Micron and Kingston.