The full-sized DIMM form factor was the original form factor that was created for DRAM modules. It is in wide use with desktops, servers, and systems used in embedded and telecommunications applications. This module is 133.5 mm wide and is 30 mm or more tall. Standard full-sized DIMM form factors have been defined with pin counts of 168 (SDRAM), 184 (DDR), and 240 (DDR2 and DDR3).

The Very Low Profile (VLP) DIMM was created to save height for space constrained systems, including blade servers and ATCA (Advanced Telecommunications Architecture) applications. The DIMMS are also 133.5 mm wide and have the same pinout as the DDR, DDR2, and DDR3 full-sized DIMMs. However, these modules will typically have a height between 18-19 mm. This shortened height allows the designer to use a vertical socket and still fit the DIMM into his constrained chassis space.

The So-DIMM is a shortened DIMM form factor that was created originally for use in notebook computers. This module has a width of 67.6 mm and these will typically use a horizontal socket when designed into the system. The original So-DIMM was defined without ECC for use in notebooks. But additional versions that include ECC have been created for embedded, telecomm, and other applications that require additional reliability. The So-DIMM can come in versions with pin-counts of 144 pins (SDRAM), 200 pins (DDR and DDR2), and 204 pins (DDR3).

The mini-DIMM is a relatively new form factor that was invented to provide the benefits of a shortened DIMM length but still provide additional pin-count to support new features in the DDR2 technology. Mini-DIMMs are currently available for only DDR2 in a 244-pin module and will always include ECC. These modules are 82 mm long. It is likely that future extensions to the DDR3 standard will also provide this form factor for DDR3 technologies.

The VLP mini-DIMM is a new form factor that combines the shorter length of the mini-DIMM with the shorter height of the VLP DIMM. This form factor is a good solution for highly space constrained systems that only need a small to moderate amount of memory. The typical size of a VLP mini-DIMM is 82 mm long with a height between 18-19 mm.

The most common widths of DRAM modules are either 64 or 72 bits wide. The ECC option refers to a module that is 72 bits wide. The 72 bits can be split into 64 data bits and 8 check bits. These modules work with chipsets that support Error Correction Codes which will provide additional reliability in the memory system. These codes can detect and correct one bad bit within the 64 bit data word and they can detect two or sometimes more than two bad bits. Modules with ECC are most commonly used in server or telecomm applications where reliability and uptime are extremely important.

Non-ECC modules are normally 64 bits wide. These modules do not provide any error detection or correction capabilities. They are typically used in single user desktop or notebook systems. Because these systems only service one use and do not require 24/7 operation, the standard reliability built into DRAM modules is adequate for these systems.

In large systems that use a lot of memory, there may be as many as 4 or 8 DIMM modules attached to the chipset over a single memory channel. Because of the heavy electrical load it may be difficult for the chipset to send out the Address and Command signals with enough strength to directly drive all the DRAM chips on all the modules and meet the speed requirements. For this reason, these types of systems will normally use Registered DIMM modules. These modules put a special chip on the DIMM, called a Register chip. This chip receives the Address/Command signals from the chipset and receives it into a latch. On the next clock cycle the Address/Command signals are then sent out to the DRAM chips from the Register instead of directly from the chipset. The advantage of this arrangement is that the Register presents just a single input load to the chipset instead of the 9 or 18 or 36 loads that the chipset would see when driving the DRAM chips directly. This can allow the interface logic to run at higher speeds. Registered DIMMs are often used in servers and other applications that use a lot of memory. Registered DIMMs will always be configured with ECC.

A DIMM module that does not have a Register is called an Unbuffered DIMM. These modules are satisfactory in small systems that have only one or two DIMM modules per channel. The chipset Address/Command signals have enough drive to directly drive all the DRAM chips without compromising on the bus speed. Unbuffered DIMMs are most often used in desktop and notebook systems which will only have two or four DIMM slots in the system. Unbuffered DIMMs can be configured either with ECC or without ECC.

The FB-DIMM is a new memory architecture that was recently introduced for use with Intel server chipsets. This architecture was designed for very large memory systems that may have as many as eight or sixteen DIMM sockets per channel and four or more memory channels. The FB-DIMM uses an advanced chip on the DIMM called the Advanced Memory Buffer (AMB). This chip not only buffers the Address/Command signals, like a Register chip, but it also buffers the outputs of the DRAM chips. The AMB chip also provides for a parallel to serial conversion and transfers the data to the chipset or other FB-DIMMs using a very high speed serial protocol. The FB-DIMM architecture is also characterized by a daisy-chain architecture and transfers the data from the chipset to the first FB-DIMM and then from the first to the second, second to the third, etc. This point-to-point connection has low loading which is needed to support the transfer rates on the very high speed serial bus. All FB-DIMMs are configured with ECC.