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How to: Recognise Laptop Memory

It’s well known that the cheapest and easiest way to give your old laptop a new lease of life is to bung some extra RAM in it. In most laptops it’s as easy as turning it upside down, unscrewing a cover and pushing it into the right slot.

fitting laptop memory

Recognising what memory you need to buy can be a little more tricky.

Websites like crucial.co.uk know this and have a really good system where you can choose your exact laptop and they will show you what memory you need, but if you want to save a bit of money and buy used memory it’s a good idea to know what you’re talking about.

Any laptop outside a museum or science lab that you have lying around today will have one of 4 types of memory, all called SO-DIMM. It’s smaller than the memory that goes into most PCs, about 67mm wide, to save space.

144pin SO-DIMM Memory

144pin SO-DIMM

Oldish laptops will use 144pin SO-DIMMs, this is usually labelled PC100 or PC133 which refers to how fast the memory can transfer data. There are 72 pins (gold contact strips along the edge) on either side grouped into 30 and 42pins. There is a gap between the groups that a corresponding ridge in the memory slot lines up with to stop you fitting the wrong type of memory in your laptop. The centre of the gap is approximately 29mm along.

128mb, 256mb and 512mb sticks of this type of memory are still readily available both new and used, 1GB sticks are very rare and expensive and, to be honest, overkill for this age of laptop.

Newer memory is refered to as DDR. DDR1 (mostly labelled just DDR) and the even newer DDR2 have 200pins, 100 on each side grouped into 20 and 80 pins and look very similar.

200pin DDR SO-DIMM

200pin DDR SO-DIMM

DDR1 has the centre of the gap at about 15mm along and is usually found labelled PC2100 (267mhz), PC2700 (333mhz) or PC3200 (400mhz). There is plenty of this type of memory about both new and used and it is relatively inexpensive.

200pin DDR2 SO-DIMM

200pin DDR2 SO-DIMM

DDR2 has its gap at about 16mm along and can be bought as PC2-3200 (400mhz), PC2-4200 (533mhz), PC2-5300 (667mhz), PC2-6400 (800mhz) and, very rarely, as PC2-8500 (1066mhz). There is lots of this memory around and at the moment it is about two thirds of the price of DDR1.

The positioning of the gap means that you cannot fit DDR2 into a DDR1 slot and vice versa. However, they do look very similar and the pins line up exactly. From experience I have found that the easiest way to tell the difference (without cheating and looking at the label) is that if you hold it with the small group of pins to your left, in DDR1 the notch butts up against the smaller group of pins with a small space to its right before the longer group starts, while DDR2’s notch butts up against the larger group with a space to its left.

204pin DDR3 SO-DIMM

204pin DDR3 SO-DIMM

The latest and greatest memory is DDR3. This has 204pins, 102 each side arranged into groups of 36 and 66 pins. The gap in this memory is about 25mm along. At the moment it can be bought as PC3-8500 (1066mhz) and PC3-10600 (1333mhz) though I’m sure it will be readily available in faster speeds fairly soon.

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