What is a DIMM?

DIMM, which stands for dual in line memory module, is a type of computer memory used in Pentium or higher computers. A DIMM consists of a small printed circuit board that holds either four or nine (with parity) synchronous dynamic random access memory (SDRAM) or double data rate (DDR) SDRAM chips per side. The connecting edge of pins plugs into a socket on the motherboard and transmits data to the processor 64 bits at a time. DIMMs may come in 64-bit non-parity error-correcting code (ECC) modules or in 72-bit modules with parity. Several types of DIMMs exist; however, the three main types are SDRAM, DDR SDRAM and DDR2 SDRAM. DIMMs range in size from 64 Megabytes (MB) for older SDRAM to 128 Gigabytes (GB) or more per module of DDR2 RAM.

Of the different types of DIMMs, the 168-pin SDRAM is known as the original DIMM and had data and clock speeds up to 133 Megahertz (MHz). These modules had one notch on each side and two notches on the connecting edge. The 184-pin DDR and 240-pin DDR2 memory share a similar architecture, with two notches on each side and one notch along the connecting edge. DDR memory has clock speeds up to 200 MHz and double the data speeds of SDRAM. DDR2 memory has clock speeds up to 200 MHz and data speeds quadrupling SDRAM. Other types include DDR3 memory, a higher-speed and more expensive variety, and SO-DIMMs for laptops.

By around the year 2000, DIMMs had replaced SIMMs, or single in line memory modules, as the computer memory standard in Pentium computer systems. Unlike SIMMs, which contained 16-bit or 32-bit data paths and required addition in pairs to fully utilize the 64-bit data path of a Pentium PC, the data transfer rate of one individual DIMM fit the data bus width of the Pentium processor perfectly, thus eliminating the need for pairs.

Another difference between the two types of computer memory lies in the way the connecting edges work. The connecting edges of both sides of a DIMM point to different circuits that respond differently to electrical signals. This adds more power to a computing system since multiple lines of communication to the processor are possible. A SIMM, on the other hand, carries the same connectors on each side of the module and can manage one line of communication to the processor.

Although installing DIMMs is a fairly easy process, DIMM memory types are not interchangeable with one another. Always refer back to the motherboard or PC manuals to verify the type before replacing DIMMs or performing memory upgrades. First, turn off and unplug the computer. Remove the case and look for the memory slots on the motherboard. DIMM slots are usually black and located close to the processor. Dissipate extra static electricity by touching a metal object.

Next, set the ejector clips in the “down” position. Holding the memory module by the edges to avoid direct contact with the pins, line up the notches on the connecting edge against the keys in the socket so they match. Press the module into place until it clicks and both ejectors snap onto the module. Replace the cover of the computer, reconnect the cables, and turn on the computer. In most cases, the system should recognize the new memory.