The difference between PC100, PC133, and DDR memory

Question: Can I mix and match some of the newer and faster memory in my computer? –C.W.T.

Answer: I have had a few questions along these lines lately. Let’s go over some basics and then on to innovations in memory, where I will address some frequently asked questions about how the new RAM works.

RAM, as you may know, is Random Access Memory. It’s the part of a computer that loads programs when they are used. It is kind of a workbench where things get done in a computer.

SDRAM (or Synchronous Dynamic RAM) is a kind of memory chip that was introduced around the time that Pentium II computers appeared. If you have one of those computers today, chances are that you are in the market to add memory to it, in order to extend the life of the computer.

Adding RAM is the best and cheapest way to stave off obsolescence in any computer. My Pentium II 400 MHz computer, bought in early 1999, came with 128 MB of SDRAM. I have since added two more 128 MB SDRAM chips to the system (for about $20 US or $30 Canadian apiece!) to give the system better performance.

SDRAM was a huge improvement on previous types of RAM because it eliminated a lot of wait time. Traditionally, the system bus (which is all the circuitry on the motherboard that moves data around a computer) and the RAM had to wait for each other to do their part in processing information. SDRAM changed that — now the bus and the RAM work at the same time. There’s no tag team silliness going on.

When you go shopping for SDRAM, however, you’ll be asked whether you are interested in PC100 or PC133 SDRAM. Without going into too much painful detail, PC100 SDRAM is slightly slower than the PC133 version.

You can add both types to the same computer; but the machine will default to the speed of the slowest RAM installed. If you dump your PC100 RAM and replace it with PC133, you can take advantage of the faster technology.

You may also have heard of DDR SDRAM. This is an even faster kind of memory called double data rate memory. DDR memory moves data at 266MHz, but don’t call it PC266 SDRAM. More about that in a minute.

To use DDR memory, your system has to specifically support the DDR technology. Typically, PCs that run Pentium III or Pentium 4 processors that run at 1 GHz or faster will support DDR RAM, but don’t assume that all will. AMD Athlon processor-based machines also tend to support DDR SDRAM. Check with your computer maker to ensure that the motherboard (the big circuit board inside your PC) supports DDR RAM. Or look up your system on to find out.

When buying DDR SDRAM you’ll be faced with a choice of two types. (This is all so complicated, isn’t it?) — PC1600 or PC2100.

While your motherboard may support DDR memory, you will need to check to see that it supports PC2100 DDR SDRAM. Some early models of DDR-enabled motherboards only work with PC1600. You also need to ensure that the processor for your computer can support PC2100.

PC1600 DDR is designed for motherboards and processors with a 200 MHz bus, while PC2100 DDR is designed for a motherboards and processors with a 266 MHz bus. Remember that the bus is all those circuits on the motherboard that moves data around. They are kind of like data highways that run between the processor and RAM and all the other components in a computer.

Once again, a computer with a mix of PC2100 and PC1600 DDR SDRAM will default to the slower RAM speed. If you need more info, check out’s amazing RAM guide. Go to and click the Library tab for lots more useful RAM information.

Incidentally, I buy all my RAM from Crucial, because it’s easy to figure out what RAM your system needs. You just select your brand and make and model of the system and then Crucial shows you which RAM will work with your computer. With permission, I’ve duplicated their memory selector tool here.

Select your system and press go to find out what type of RAM you should use:

Memory upgrades from Crucial Technology – Computer memory, flash and video card upgrades

The Crucial Memory Advisor™

Select your system and press go!