Types of Computer Memory (RAM) Explained

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

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.


Name Bus clock Data rate Transfer rate Module name
DDR-200 100 MHz 200 MT/s 1600 MB/s PC1600
DDR-266 133 MHz 266 MT/s 2100 MB/s PC2100
DDR-333 166 MHz 333 MT/s 2700 MB/s PC2700
DDR-400 200 MHz 400 MT/s 3200 MB/s PC3200

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: 


Name Bus clock Transfer rate DIMM pins SO-DIMM pins
DDR 100-200 MHz 200-400 MT/s 184 200
DDR2 200-533 MHz 400-1066 MT/s 240 200
DDR3 400-1066 MHz 800-2133 MT/s 240 204
DDR4 1066-2133 MHz 2133-4266 MT/s 288 256

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. 

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