How DDR Works

Computer memory gives your CPU working space. It feeds the data pipelines that keep the processor humming along; DDR memory gave it double the bandwidth, so it can feed the processor that much faster.

What's the Difference Between DDR and SDRAM?

The biggest difference between DDR SDRAM and its predecessor is bandwidth. Both types of RAM have the same 64-bit interface and run through a synchronous interface at the same 100 MHz to 200 MHz bus speeds. The big difference is that DDR has twice the memory bandwidth for the same basic interface speed. The primary technical difference comes down to the fact that DDR transmits data on both the rising and falling edges of the sine wave, so it effectively gets twice the mileage out of the same speed memory bus, which is why you refer to it by twice its base frequency. Another thing you need to be aware of is that the two types do come in different DIMM formats:

  • SDRAM: Standard package is a 168-pin DIMM running at 3.3 Volts. Memory bandwidth runs from 800 MB per second at 100 MHz to 1066 MB per second at 133 MHz.
  • DDR SDRAM: Standard package is a 184-pin DIMM running at 2.5 Volts. Memory bandwidth runs from 1600 MB per second at 200 MHz to 3200 MHz at 400 MHz. 

How is DDR Rated?

As with all memory, DDR rates in a standard fashion so you can easily tell its performance characteristics and wither it's compatible with your computer or motherboard. As a general rule, memory modules are described in terms of three main factors:

  • Capacity: Depending on size, this measures in either MB or GB. Common memory capacities include 128 MB, 256 MB, 512 MB, 1 GB, and 2 GB.
  • Frequency: This is how rapidly the memory communicates with the computer and is described by the effective rather than base frequency, thus Infineon DDR266 runs on a base clock of 133 MHz with an effective clock of twice that.
  • Bandwidth: Relating to frequency, bandwidth determines how much data your RAM can transfer in a second. For example, PC2100 DDR RAM can transfer 2.1 GB per second.

Using DDR Memory

Once you have it installed, using DDR memory is no different than using any other kind of RAM. You want matching pairs for dual-channel applications, such as two Infineon 256 MB DDR DIMMs rather than a single 512 MB DIMM so that you can transfer data in parallel for additional performance. The fact that each generation of memory uses a different physical interface ensures that it is impossible to accidentally install the wrong kind of RAM. It simply won't fit in the memory slot, so there's no way to damage a module with overvoltage.