Below you can see a typical stick of RAM (Random Access Memory). This is a 184 pin module that can store 256MB of data. This is the most common type of RAM at this time (early 2005). As you can see by the sticker, this ram is DDR SDRAM (Double Data Rate Synchronous Dynamic Random access Memory). The DDR part of the name means that it can read/write data at twice the memory bus speed. For example, this memory is rated at 333MHz. This means that the bus speed is 166MHz. It can pass data at twice the bus clock speed because it processes data on both the rising AND falling portions of the clock pulse. Older memory could only process data on the rising OR falling side of the clock pulse. You will also see DDR-333 RAM labeled as PC2700 RAM. The 2700 in PC2700 means that it can process 2.7GB (2.7 billion bytes) of data per second.

The following memory module is DDR2 memory. It's a 240 pin module.

The following is DDR3 memory. It's also a 240 pin module. As you can see, the notch at the bottom of the module is in a different position, compared to the 240 pin DDR2. This prevents you from installing the wrong memory in a motherboard.

This next memory is old, virtually obsolete memory. It's a 128MB, 168 pin stick of PC133 SDRAM. This was used with the older Pentium 2 and 3 processors as well as other processors of that era. This type of RAM came in two significantly different variations. One was called a SIMM (Single Inline Memory Module). The other was a DIMM (Dual Inline Memory Module). They both looked pretty much the same except that the DIMM was actually 2 independent banks of memory. The DIMM's contacts were isolated from one side of the module to the other. The SIMM's contacts were connected through the circuit board (opposing contacts in the memory slot were actually connected together).

This is EDO ram and was used during the time of the first generation Pentium processors. As far as I know, it's no longer used in any of the motherboards being produced for the general public today.

Virtual Memory:
When an operating system is written, it has to be designed to work in all sorts of different configurations. Windows XP has a minimum requirement of 128MB of physical memory. This means it WILL run with that little memory (but it will be clunky). 128MB of memory is NOT enough to run XP plus other programs. If you build a budget system and only use 128MB of RAM, XP will need more memory to run. To allow the OS to run with only 128MB of physical memory without crashing, it uses 'virtual' memory. When it's low on physical memory, it will write some information that would have gone to physical memory to the hard drive. It uses the hard drive to compensate for the lack of physical memory. This works well but reading and writing to the hard drive takes much more time than reading and writing to electronic/physical memory. Having to use a lot of virtual memory will slow your computer down. In the computer I'm using for this tutorial (originally written in 2005), I used 256MB and thought it would be plenty but I was wrong. I replaced the 256MB stick with a 512MB stick and it was much faster at loading and switching between programs.

Now it's 2010 and things have changed a bit. The same rules apply to XP but the newer operating systems like Windows Vista or Windows 7 require much more memory. When Microsoft recommends a minimum of 1GB for Windows 7 (32 bit version), that's barely enough to make it run smoothly when you have only a few other programs running. When you have multiple programs open (graphics editors, document editors, web browsers...), the computer requires more memory to run smoothly. For older computers, having to write to virtual memory (due to a lack of physical memory) slowed things down a bit. With the newer, faster multi-core processors, having to use virtual memory makes the computer MUCH slower. As an example, a computer running a 4-core AMD Phenom processor with Windows 7 (64 bit version) and 1GB of memory wasn't much faster than a 5 year old computer running XP with 2GB of physical memory. After upgrading to 4GB of memory, the newer computer became lightning fast compared to the older computer (as it should have). The virtual memory (even when using fast hard drives) should be used as little as possible. if you're using Windows 7, I'd recommend a minimum of 2GB of memory. For the 64 bit version, 4GB would be a better choice.

Note:
Without a bit of tweaking, windows (any version) WILL use some amount of virtual memory no matter how much physical memory you have. Even if you have enough physical memory Windows still uses virtual memory for some things. To try to force it to use as much physical memory as possible, you can set the virtual memory to 0 for all drives. In XP, if you RIGHT-CLICK on MY COMPUTER and select properties, you will see the following dialog box. Select the ADVANCED tab. In the PERFORMANCE section, click on SETTINGS.

In the PERFORMANCE OPTIONS DIALOG BOX, select the ADVANCED tab again. In the VIRTUAL MEMORY section, select SETTINGS.

In the VIRTUAL MEMORY dialog box, you will see a list of the drives available to be used for virtual memory. In the computer below, the virtual memory has been removed from all drives. To change the virtual memory, you will need to shighlight a drive by clicking on it. You'd then choose one of the options below. To switch the virtual memory off, you'd select NO PAGING FILE and then click SET. After making the changes, exit all of the dialog boxes by clicking OK. The computer may have to be restarted after making the changes. DO NOT try this if you don't have significant physical memory (1GB for XP and 2GB for Windows 7 - as an absolute minimum).

In Windows 7, you right click on my computer and select properties as in XP but you then see a different dialog box. In this box, you select ADVANCED SYSTEM SETTINGS. This brings up the same dialog boxes as you saw above for XP.

Speed:
This is simply the speed at which the memory is rated to run. For your processor to run at it's full potential, you need memory that's rated to run as fast as or faster than the FSB (Front Side Bus) speed of your processor. Processors WILL run on memory rated slower than their FSB speed but they will not run as fast. If your memory is rated at DDR400(PC3200), it's rated to run on a 200MHz FSB. Since it processes data on both the rising and falling edges of the clock pulse, it acts as 400MHz RAM. The PC3200 indicates that the data throughput is 3.2GB per second.

Cas Latency:
The CAS latency tells how many clock pulses will occur between various operations in the memory. Lower numbers are better. Unless you're into over-clocking, these numbers are of little concern. Buy memory that's made to run at the correct speed for your system and you won't have any problems.

Support Voltage:
This is the voltage on which the memory operates.

NOTE:
Virtually all computer circuit boards and cards are static sensitive. In some areas, static electricity is a huge problem. The problem tends to be worse in cool, dry climates but everyone, no matter their location, has experienced static discharge. Even though you don't realize, there are discharges that are too insignificant to feel but they happen. These small discharges are still in the range of hundreds of volts. Since many of the electronics components on a circuit board can be damaged by as little as 20 volts, even a very small static discharge can be harmful to your computer components. If you have to work on your computer (for example, if you add memory to your computer), you will need to avoid dangerous static discharge. One way to do this it to make sure that you are in contact with your computer at all times. When working on your computer, try to keep one hand or arm in contact with the metal case (the computer will be unplugged, of course). When you pick up a computer component, you pick it up by its package ('static sensitive' bag or other conductive enclosure). While removing it from its package, don't lose contact with the computer case. Until the memory (or other component) is in it's slot and locked down, do not let go of the case. If you have to let go of the case and the component is not yet installed, put it back into its package or set it into the bottom of the computer case on top of the conductive package. If it's never been installed, it's OK to set it directly into the bottom of the computer case. Before touching the device, again make contact with the computer's case. I know that this sounds like a pain but you can easily damage an expensive card without even knowing.

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