A DIMM () (Dual In-line Memory Module), commonly called a RAM stick, comprises a series of dynamic random-access memory integrated circuits. These memory modules are mounted on a printed circuit board and designed for use in personal computers, workstations, printers, and servers. They are the predominant method for adding memory into a computer system. The vast majority of DIMMs are standardized through JEDEC standards, although there are proprietary DIMMs. DIMMs come in a variety of speeds and sizes, but generally are one of two lengths - PC which are and laptop (SO-DIMM) which are about half the size at .
History
DIMMs (Dual In-line Memory Module) were a 1990s upgrade for SIMMs (Single In-line Memory Modules) as Intel P5-based Pentium processors began to gain market share. The Pentium had a 64-bit bus width, which would require SIMMs installed in matched pairs in order to populate the data bus. The processor would then access the two SIMMs in parallel.
DIMMs were introduced to eliminate this disadvantage. The contacts on SIMMs on both sides are redundant, while DIMMs have separate electrical contacts on each side of the module. This allowed them to double the SIMMs 32-bit data path into a 64-bit data path.
The name "DIMM" was chosen as an acronym for Dual In-line Memory Module symbolizing the split in the contacts of a SIMM into two independent rows. Many enhancements have occurred to the modules in the intervening years, but the word "DIMM" has remained as a generic term for computer memory modules.
Variants
Variants of DIMMs support DDR, DDR2, DDR3, DDR4 and DDR5 RAM.
Common types of DIMMs include the following:
DIMM
100-pin, used for printer SDRAM
168-pin, used for SDR SDRAM (less frequently for FPM/EDO DRAM in workstations/servers, may be 3.3 or 5 V) 184-pin, used for DDR SDRAM 200-pin, used for FPM/EDO DRAM in some Sun workstations and servers. 240-pin, used for DDR2 SDRAM, DDR3 SDRAM and FB-DIMM DRAM 278-pin, used for HP high density SDRAM. 288-pin, used for DDR4 SDRAM and DDR5 SDRAM SO-DIMM 72-pin (not the same as a 72-pin SIMM), used for FPM DRAM and EDO DRAM 144-pin, used for SDR SDRAM (less frequently for DDR2 SDRAM) 200-pin, used for DDR SDRAM and DDR2 SDRAM 204-pin, used for DDR3 SDRAM 260-pin, used for DDR4 SDRAM 260-pin, with different notch position than on DDR4 SO-DIMMs, used for UniDIMMs that can carry either DDR3 or DDR4 SDRAM 262-pin, used for DDR5 SDRAM MiniDIMM 244-pin, used for DDR2 SDRAM MicroDIMM 172-pin, used for DDR SDRAM 214-pin, used for DDR2 SDRAM
SO-DIMM
A SO-DIMM (pronounced "so-dimm" , also spelled "SODIMM") or small outline DIMM, is a smaller alternative to a DIMM, being roughly half the physical size of a regular DIMM.
SO-DIMMs are often used in systems that have limited space, which include laptops, notebooks, small-footprint personal computers such as those based on Nano-ITX motherboards, high-end upgradable office printers, and networking hardware such as routers and NAS devices. They are usually available with the same size data path and speed ratings of the regular DIMMs though normally with smaller capacities.
SDR 168-pin SDRAM
On the bottom edge of 168-pin DIMMs there are two notches, and the location of each notch determines a particular feature of the module. The first notch is the DRAM key position, which represents RFU (reserved future use), registered, and unbuffered DIMM types (left, middle and right position, respectively). The second notch is the voltage key position, which represents 5.0 V, 3.3 V, and RFU DIMM types (order is the same as above).
DDR DIMMs
DDR, DDR2, DDR3, DDR4 and DDR5 all have different pin counts and/or different notch positions. As of October 2022, DDR5 SDRAM is a modern emerging type of dynamic random access memory (DRAM) with a high-bandwidth ("double data rate") interface, and has been in use since 2020. It is the higher-speed successor to DDR, DDR2, DDR3, and DDR4. DDR5 SDRAM is neither forward nor backward compatible with any earlier type of random access memory (RAM) because of different signalling voltages, timings, as well as other differing factors between the technologies and their implementation.
SPD EEPROM
A DIMM's capacity and other operational parameters may be identified with serial presence detect (SPD), an additional chip which contains information about the module type and timing for the memory controller to be configured correctly. The SPD EEPROM connects to the System Management Bus and may also contain thermal sensors (TS-on-DIMM).