Double Data Rate SD-RAM is the next generation in SD-RAM memory. As the name implies, it can double the data rate that normal SD-RAM makes, which theoretically makes it twice as fast. Contrary to what some people think, although it is twice as fast as SD-RAM, it would not make a computer run twice as fast as the computer with normal SD-RAM. This is because the computer does not depend solely on memory for performance; there are a variety of other factors such as CPU performance, Video Card performance, etc. The computers that do need fast RAM for 3D modeling programs and other professional applications, DDR-SDRAM is a very good choice, because of its high speed, and the not so large price tag.

High speed infineon DDR RAM Module
© 2001 by www.infineon.com

So how does DDR SD-RAM achieve twice the data transfer rate of standard SD-RAM? The concept is actually quite simple. We know that SD-RAM transfers data every clock pulse. To be more exact, it transfers data on every rising clock pulse. You see, the clock pulse rises, and then it falls, then a new one does the same thing over and over. DDR takes the advantage of the rising and falling clock pulses and treats them as though they were two clock pulses instead of one, allowing more data to be transferred. Here is a simple example:

Another important improvement that DDR has over its predecessor is that it consumes less power, and only needs 2.5 Volts from the voltage supply. This makes it a very attractive option for notebook computers, which need to consume as little power as possible in order to extend battery life.

DDR is also built similarly to standard SD-RAM, which means that it will not be as expensive as some of the other modifications to SD-RAM.

DDR RAM was first named the same way as SD-RAM. For example, 100MHz DDR would be called PC200 because of its double data rate. Since Rambus, a DDR competitor, decided to name its memory modules PC800 even though they are not running at 800MHz, DDR needed a new naming scheme. They used the amount of Megabytes that DDR can transfer every second. They took the width of the BUS and multiplied it by the number of clock cycles per second, and multiplied this by two (because of the DDR property) to figure out the transfer rate in MB/sec:

64-bit bus * 100MHz (1,048,576) * 2 times the transfer rate = 134217728 bits/sec.

This is now converted into bytes by dividing the number by 8:

134217728 / 8 = 1677721600 bytes/sec.

Now, to convert this to MB per second, the number is divided by the amount of bytes in 1 MB (1,048,576)

1677721600 / 1,048,576 = 1600 MB/sec.

Since they knew that 100MHz DDR RAM transfers at 1600 MB/sec., they decided to call it PC1600.

Here are some more of the popular speeds of DDR-RAM, and what they are named:

RAM has advanced quickly over the years, and DDR is one of the newer and more promising additions to the SD-RAM family.