Dual In-line Memory Module DIMM

A DIMM, or dual in-line memory module comprises a series of random access memory integrated circuits. These modules are mounted on a printed circuit board and designed for use in personal computers. DIMMs began to replace SIMMs (single in-line memory modules) as the predominant type of memory module as Intel's Pentium processors began to control the market.

The main difference between SIMMs and DIMMs is that SIMMs have a 32-bit data path, while DIMMs have a 64-bit data path. Since Intel's Pentium has (as do several other processors) a 64-bit bus width, it required SIMMs installed in matched pairs in order to use them. The processor would then access the two SIMMs simultaneously. DIMMs were introduced to eliminate this inefficiency. Another difference is that DIMMs have separate electrical contacts on each side of the module, while the contacts on SIMMs on both sides are redundant.
The most common types of DIMMs are:

72-pin-DIMMs, used for SO-DIMM
144-pin-DIMMs, used for SO-DIMM
200-pin-DIMMs, used for SO-DIMM
168-pin-DIMMs, used for FPM, EDO and SDRAM
184-pin-DIMMs, used for DDR SDRAM
240-pin-DIMMs, used for DDR2 SDRAM

There are 2 notches on the bottom edge of 168-pin-DIMMs, and the location of each notch determines a particular feature of the module.

The first notch is DRAM key position. It represents RFU (reserved future use), registered, and unbuffered.

The second notch is voltage key position. It represents 5.0V, 3.3V, and Reserved.

The upper DIMM in the photo is an unbuffered 3.3V 168-pin DIMM. A DIMM's capacity and timing parameters may be identified with SPD (Serial Presence Detect), an additional chip which contains information about the module type.

ECC DIMMs are those that have extra data-bits which can be used by the system memory controller to detect and correct errors. There are numerous ECC schemes, but perhaps the most common is Single Error Correct, Double Error Detect (SECDED) which uses a 9th extra bit per byte.