DDR Speeds: PCxxxx to DDRxxx Cheat Sheet

Okay, while DDR has been around for a number of years now, the PCxxxx and DDRxxx ratings continue to be a source of counfusion. To get a better understanding of how this works, you need to understand how DDR works.

DDR=Double Data Rate. Specifically, this is DDR SDRAM (Synchronous Dynamic Random Access Memory), but it's easier and shorter to just use DDR. 'SDRAM' is typically used to to refer to SDR (Single Data Rate) SDRAM.

DDR transmits data on the rise and fall of the clock. DDR at 100MHz is equal to SDR at 200MHz. This is why PC1600 is also known as DDR200. DDR (and DDR2) are 64-bits wide, or 8-bytes wide (8 bits to a byte). 8B x 200MHz = 1.6GB/s peak bandwith.

Keep in mind that the calculations are maximum speeds guaranteed for that RAM. They will function fine at slower speeds, but only produce the bandwith at the given speed rating.

Here's a table to give you the low-down:

1.6GB/s=PC1600=DDR200
2.1GB/s=PC2100=DDR266
2.7GB/s=PC2700=DDR333
3.2GB/s=PC3200=DDR400
3.5GB/s=PC3500=DDR433
3.7GB/s=PC3700=DDR466
4.0GB/s=PC4000=DDR500
4.4GB/s=PC4400=DDR550
4.8GB/s=PC4800=DDR600

DDR2 uses the same mathematics, but to differentiate between DDR and DDR2, PCxxxx and DDRxxx ratings use PC2-xxxx and DDR2-xxx.

3.2GB/s=PC2-3200=DDR2-400
4.2GB/s=PC2-4200=DDR2-533
5.3GB/s=PC2-5300=DDR2-667 (may also show up as PC2-5400)
5.6GB/s=PC2-5700=DDR2-700
6.4GB/s=PC2-6400=DDR2-800
7.2GB/s=PC2-7200=DDR2-900
8.0GB/s=PC2-8000=DDR2-1000
8.5GB/s=PC2-8500=DDR2-1066