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     • Windows Shortcuts
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Types of RAM

There are different types of RAM available which can be found on memory modules of the same size. There is the old type SDRAM (Synchronous Dynamic Random Access Memory), DDR SDRAM (Double Data Rate SDRAM), DDR2 SDRAM, DDR3 SDRAM, and DDR4 SDRAM is now available.

RAM - Comparing Desktop Computer memory modules

RAM - Comparing Desktop Computer memory modules.

You will notice from the picture above that SDRAM memory modules have two notches in the contacts while DDR, DDR2, and DDR3 have just one notch which is located in a different place on each memory module. This is because all these types of RAM are incompatible with each other and it makes it impossible to put the wrong type memory module into a motherboard's memory slots because of these notches.

Laptop RAM has the same types of RAM but are incorporated onto a physically smaller sized memory module called a SO-DIMM (Small Outline Dual in-line Memory Module). They also have notches in the memory module so it is impossible to mix them up.

SDRAM is so-called because the RAM is 'dynamic' meaning it has to be continuously refreshed while it is in use and when power is switched off then all the data is lost. It is 'Synchronous' because it is synchronised with the system bus and waits for a clock signal before sending or receiving data. DDR (Double Data Rate) SDRAM is quicker than ordinary SDRAM as it transfers data twice during each clock cycle. DDR2 runs twice as fast as DDR, and DDR3 runs twice as fast as DDR2.

Like DDR, DDR2 transfers data twice per clock cycle but DDR2 runs the internal clock at half the speed of the data bus which gives a total of four data transfers per internal clock cycle making it twice as fast as DDR. DDR3 has a total of eight data transfers per internal clock cycle which makes it twice as fast as DDR2.

Memory modules are available with different capacities which are measured in megabytes (MB) or more recently in gigabytes (GB). They are also available in different speeds, so for example DDR memory modules are available in DDR-200, DDR-266, DDR-333, and DDR-400. DDR-200 (also called PC1600) runs at 200 MT/s (million transfers per second), or 1600 MB/s (megabytes per second). The speed of memory that you require depends on the front-side-bus (FSB) speed and DDR-200 is designed for use in computers with a 100MHz FSB. This will be explained further here.

NameBus clockData rateTransfer rateModule name
DDR-200100 MHz200 MT/s1600 MB/sPC1600
DDR-266133 MHz266 MT/s2100 MB/sPC2100
DDR-333166 MHz333 MT/s2700 MB/sPC2700
DDR-400200 MHz400 MT/s3200 MB/sPC3200

To find the correct memory modules for your computer you can either look in your motherboard manual (The name of your motherboard is usually printed on the edge of the actual motherboard), or visit the Crucial website at: and run their system scanner, or take one of your existing memory modules out of your computer and read the label.

The following table shows the different types of DDR memory, the range of speeds for each type, and pin-count for DIMM, and SODIMM memory modules:

NameBus clockTransfer rateDIMM pinsSO-DIMM pins
DDR100-200 MHz200-400 MT/s184200
DDR2200-533 MHz400-1066 MT/s240200
DDR3400-1066 MHz800-2133 MT/s240204
DDR41066-2133 MHz2133-4266 MT/s288256

You may notice that some memory modules are labelled EEC or non-EEC. EEC stands for Error Correction Code and can detect errors and even correct them. EEC memory modules are more expensive than non-EEC but are also a little slower.