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A narrow printed circuit board that holds memory chips. The common memory module is the DIMM (dual in-line memory module). DIMMs with 184 pins are used for DDR SDRAM, while 240-pin DIMMs are used for DDR2 and DDR3 SDRAM. DDR, DDR2 and DDR3 DIMM modules are each keyed differently (notches in different locations) so that they cannot be inserted into the wrong slots. Because of space limitations, laptops use small outline DIMMs (SODIMMs).

     PCs use either nine-bit memory (eight bits and parity) or eight-bit memory without parity. Macs use eight-bit memory without parity.

Singles or Pairs
Earlier SIMM and Rambus modules were installed in pairs, whereas a single DIMM can often be used. However, installing pairs of DIMMs in machines that support dual channel DDR SDRAM increases performance. See SDRAM for details.

Upgrading Memory - Read the Manual (RTFM!)
To upgrade memory, read your motherboard or system manual to learn which combinations of modules can be plugged into memory slots. There may be restrictions. See SDRAM and memory types.




Common Memory Modules
DIMMs are widely used in desktops and servers while the smaller SODIMMs are used in laptops. SIMMs were used in older PCs. For identification, look at the pin pattern and notches between the pins and on the sides. How the chips are laid out (horizontal or vertical) is up to the manufacturer.





Desktop Memory
To change memory on desktop computers, you have to open the cabinet and locate the slots. These three DIMM slots on this Mac motherboard are easy to find and reach.





Laptop Memory
To change memory on laptops, you have to unscrew a plate on the bottom of the machine to get to the SODIMM slots.





Two Sticks of Memory
Memory modules are often called "sticks," because the chips are housed on long, thin printed circuit boards.