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.