What is DDR Memory?

Double data rate synchronous dynamic random access memory, or DDR SDRAM for short, is a type of computer memory used in Pentium III or higher computers. DDR RAM is the computer’s primary working memory, storing program instructions and data for use by the central processing unit (CPU) through the bus controller. DDR memory belongs to the dual in-line memory module (DIMM) architectural family and is a faster form of SDRAM.

Each module of DDR memory has one notch on the connecting side and two notches on the side that attach to latches on the PC motherboard to hold it in place. DDR RAM can come in either single-sided or double-sided chip configurations. Desktop DDR has 184 pins, as compared to 168 pins on SDRAM. Laptop versions, called SO-DIMMs, contain 200 pins.

                                          
There are three major types of DDR ram. DDR or DDR1, has clock speeds ranging from 100Mhz up to 200 Mhz. The 240-pin DDR2 supersedes DDR1 and quadruples the speed of SDRAM, peaking at 266MHz. DDR3, a 240-pin module, uses less power and transfers data eight times faster than SDRAM. DDR3 is also the most expensive and least common of the ddr memory types.

One of the primary differences between the SDRAM and DDR memory pertains to the way it uses the clock signal in data exchanges. SDRAM transfers 64 bits of data at a time to the CPU using the rising edge of a clock signal. In contrast, ddr ram passes data using both the rising and falling edges of the system clock without changes to the clock frequency. This means that up to 128 bits can be transferred at a given time, making it twice as fast as SDRAM. The faster the method of data exchange, the more improved the performance.

To determine maximum bandwidth of this type of computer memory, multiply the memory bus clock by the number of channels—1 for SDRAM, 2 for DDR, 4 for DDR2, and 8 for DDR3—and 64, the number of bits transferred. Then divide the product by 8, number of bits per byte.

Thus, for a motherboard with a minimum bus frequency of 100, the peak transfer rate would be 1600 Megabytes per second (MB/s). For computers using a 133MHz bus, the peak caps off at 2100 MB/s (DDR-266 or PC-2100). Finally, a DDR2 module with a clock speed of 266Mhz peaks at 8500 MB/s.
The standard names of the memory modules also incorporate the bandwidth measurements. DDR-200 or PC-1600 correlates to a DDR1 module with a clock speed of 100MHz and 1600 MB/s transfer rate. DDR2-400 or PC2-3200 likewise refers back to a DDR2 module with clock speed of 100MHz and transfer rate of 3200 MB/s.

In addition to speed, DDR memory also uses less power than SDRAM, operating at 2.5V (2.6V for DDR-400) compared to the 3.3V of SDRAM. A spinoff of DDR for mobile devices, called MDDR, uses considerably less voltage than desktop DDR memory.
Unfortunately, ddr memory is not backward compatible with older forms of memory. Thus, DDR2 modules cannot replace DDR RAM. In fact, the computer memory used is determined by the motherboard chipset, and not every motherboard will support the faster speeds. It is best to consult the motherboard or PC manual for specifics before performing a memory upgrade.