DDR Memory

• The first generation of memory to use the double data rate standard is referred to simply as DDR. This is the oldest and slowest generation of memory out of the three types of DDR memory. DDR memory is rated based on two numbers, the effective clock speed of the memory and the maximum amount of data it can transfer. Memory modules labeled DDR200 and PC-1600 are capable of transferring 1,600 megabytes of data per second at an effective clock speed of 200 megahertz. Memory modules in the DDR family are typically rated between DDR200 and DDR400, with speeds between 1,600 and 3,200 megabytes per second. However, some specialty memory modules are available at speeds as high as 433 MHz.
  
  

DDR2 Memory

• The next generation of DDR memory is DDR2. This type of memory is still found in many budget computer systems. DDR2 memory modules are rated at DDR2 400, 533, 667, 800 and 1066. These modules respectively have maximum data transfer rates of 3,200 MHz, 4,266 MHz, 5,333 MHz, 6,400 MHz and 8,533 MHz. DDR2 modules rated using the PC designation are listed as PC2-3200 through PC2-8500. Since the PC designation is designed to provide a basic approximation of data transfer rates, many numbers are rounded off. PC2-4200 memory for example, has an actual maximum transfer rate of 4,266 MHz.
  
  

DDR3 Memory

• The most current generation of memory is DDR3 memory. DDR3 supports significantly higher memory clock speeds than previous generations. DDR3 modules are commonly rated as DD3-800, 1066, 1333 and 1600. These modules are respectively rated at 6,400 MHz, 8,500 MHz, 10,666 MHz and 12,800 MHz. The PC designation for DDR3 modules respectively assigns these speed designations, including PC3-6400, PC3-8500, PC3-10600 and PC3-12800.
  
  

Memory Timing

• Another aspect of RAM that impacts its overall performance is its timing. The timing of a RAM module indicates the number of clock cycles required for that module to perform various types of actions. These numbers are listed in the format X-X-X-X, with lower numbers indicating faster performance. Two modules of the same type with different timing numbers will demonstrate markedly different levels of performance. Newer generations of RAM typically have higher timing numbers due to the large amounts of memory they support. However, this difference in latency is more than compensated by the higher clock speeds available with each successive generation.
  
  

CAS Latency

• CAS latency is an overall measure of memory timing. It measures the amount of time required for memory to deliver data to the CPU. Most low-end DDR3 memory has a CAS latency of eight or nine. High-end memory designed for gaming or server applications typically has a much lower CAS latency of four or five.