Bobo

Many many many people around here have been very confused lately about the speed at which their RAM runs at. Let me just start by saying that DDR400 does NOT mean 400MHz. Now that you know that, let's see why.

In the beginning, there was SDR SDRAM, Single Data Rate Synchronous Dynamic Random Access Memory, also known as plain SDRAM. This type of RAM has designations such as:
PC-66: 66MHz
PC-100: 100MHz
PC-133: 133MHz

This type transfers data once per clock cycle, hence the name single data rate. This was just too darn slow, so they came up with DDR.

Double Data Rate SDRAM, generally just referred to as DDR, transfers data twice per clock cycle, on both sides of the cycle (think in terms of a circle), hence its name. Because of this, the data output is doubled, while the clock speed stays the same. However, the designation numbers (DDR400) doubled to show that the performance was doubled. Let's look at the DDR speeds and designations:

PC-2100--DDR-266--133MHz
PC-2700--DDR-333--166MHz
PC-3200--DDR-400--200MHz

Eventually, even DDR became too slow. Into the ring entered DDR2. This type uses the same type of data transfer, just a changed architecture to allow for higher speeds (while sacrificing latency). Here are the common types of DDR2:

PC2-4200--DDR2-533--266MHz
PC2-5300--DDR2-667--333MHz
PC2-6400--DDR2-800--400MHz
PC2-8000--DDR2-1066--533MHz

There are some variations on the exact speeds, but those are the general types of DDR2.

Now where do the PC-xxxx numbers come from? This is a reference to the throughput, or amount of data per second, that the RAM transfers.

Let's use DDR400 for this example.

The interface between the memory and the controller is 64bits.

64bits x 200MHz x 2 / 8bits per byte = 3200MB/s. Therefore, we have PC3200. Pretty simple, huh?


Please, don't anybody ever again say that their RAM runs at 800MHz. That is my biggest computer pet peeve.

If I made any mistakes on this, please tell me, as sometimes my memory fails me.

If you want more information on RAM, please read Praetor's RAM 101 (but be advised that it is a little out-of-date)