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Two types of DIMMs: a 168-pin SDRAM module (top) and a 184-pin DDR SDRAM module (bottom). The SDRAM module has two notches (rectangular cuts or incisions) on the bottom edge, while the DDR1 SDRAM module has one. Also, each module has eight RAM chips, but the lower one has an unoccupied space for the ninth chip; this space is occupied in ECC DIMMs.
Three SDRAM DIMM slots on a ABIT BP6 computer motherboard.

A DIMM, or Dual In-Line Memory Module, is a popular type of memory module used in computers. It is a printed circuit board with one or both sides (front and back) holding DRAM chips and pins.[1] 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 133.35 mm (5.25 in) and laptop (SO-DIMM) which are about half the size at 67.60 mm (2.66 in).[2]


DIMMs (Dual In-line Memory Module) were a 1990s upgrade for SIMMs (Single In-line Memory Modules)[3][4] 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.[5] This allowed them to double the SIMMs 32-bit data path into a 64-bit data path.[6]

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.[6] Many enhancements have occurred to the modules in the intervening years, but the word "DIMM" has remained as one of the generic terms for a computer memory module.


There are numerous DIMM variants, employing different pin-counts:

  • 72-pin: FPM DRAM and EDO DRAM;[8] different pin configuration from 72-pin SIMM
  • 144-pin: SDR SDRAM,[8] sometimes used for DDR2 SDRAM
  • 200-pin: DDR SDRAM[8] and DDR2 SDRAM
  • 204-pin: DDR3 SDRAM
  • 260-pin: DDR4 SDRAM
  • 260-pin: UniDIMMs carrying either DDR3 or DDR4 SDRAM; differently notched than DDR4 SO-DIMMs
  • 262-pin: DDR5 SDRAM
  • 244-pin: DDR2 SDRAM
256 MB MicroDIMM PC133 SDRAM (double sided, 4 chips).
  • 144-pin: SDRAM[8]
  • 172-pin: DDR SDRAM[8]
  • 214-pin: DDR2 SDRAM


A 200-pin PC2-5300 DDR2 SO-DIMM.
A 204-pin PC3-10600 DDR3 SO-DIMM.
A SO-DIMM slot on a computer motherboard.

A SO-DIMM (pronounced "so-dimm" /ˈsdɪm/, also spelled "SODIMM") or small outline DIMM, is a smaller alternative to a DIMM, being roughly half the physical size of a regular DIMM. The first SODIMMs had 72 pins and were introduced by JEDEC in 1997.[8][9][10] Before its introduction, many laptops would use proprietary[11] RAM modules which were expensive and hard to find.[8][12]

SO-DIMMs are often used in computers 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.[13] 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[edit]

Notch positions on DDR (top) and DDR2 (bottom) DIMM modules.

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[edit]

16 GiB DDR4-2666 1.2 V Unbuffered DIMM (UDIMM).

DDR, DDR2, DDR3, DDR4 and DDR5 all have different pin counts and/or different notch positions, and none of them are forward compatible or backward compatible. DDR5 SDRAM is the most recent type of DDR memory and has been in use since 2020.


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).[14]

Error correction[edit]

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 an extra byte per 64-bit word. ECC modules usually carry a multiple of 9 instead of a multiple of 8 chips.


Sometimes memory modules are designed with two or more independent sets of DRAM chips connected to the same address and data buses; each such set is called a rank. Ranks that share the same slot, only one rank may be accessed at any given time; it is specified by activating the corresponding rank's chip select (CS) signal. The other ranks on the module are deactivated for the duration of the operation by having their corresponding CS signals deactivated. DIMMs are currently being commonly manufactured with up to four ranks per module. Consumer DIMM vendors have recently begun to distinguish between single and dual ranked DIMMs.

After a memory word is fetched, the memory is typically inaccessible for an extended period of time while the sense amplifiers are charged for access of the next cell. By interleaving the memory (e.g. cells 0, 4, 8, etc. are stored together in one rank), sequential memory accesses can be performed more rapidly because sense amplifiers have 3 cycles of idle time for recharging, between accesses.

DIMMs are often referred to as "single-sided" or "double-sided" to describe whether the DRAM chips are located on one or both sides of the module's printed circuit board (PCB). However, these terms may cause confusion, as the physical layout of the chips does not necessarily relate to how they are logically organized or accessed.

JEDEC decided that the terms "dual-sided", "double-sided", or "dual-banked" were not correct when applied to registered DIMMs (RDIMMs).


Most DIMMs are built using "×4" ("by four") or "×8" ("by eight") memory chips with up to nine chips per side; "×4" and "×8" refer to the data width of the DRAM chips in bits. High capacity DIMMs such as 256GB DIMMs can have up to 19 chips per side.

In the case of "×4" registered DIMMs, the data width per side is 36 bits; therefore, the memory controller (which requires 72 bits) needs to address both sides at the same time to read or write the data it needs. In this case, the two-sided module is single-ranked. For "×8" registered DIMMs, each side is 72 bits wide, so the memory controller only addresses one side at a time (the two-sided module is dual-ranked).

The above example applies to ECC memory that stores 72 bits instead of the more common 64. There would also be one extra chip per group of eight, which is not counted.


For various technologies, there are certain bus and device clock frequencies that are standardized; there is also a decided nomenclature for each of these speeds for each type.

DIMMs based on Single Data Rate (SDR) DRAM have the same bus frequency for data, address and control lines. DIMMs based on Double Data Rate (DDR) DRAM have data but not the strobe at double the rate of the clock; this is achieved by clocking on both the rising and falling edge of the data strobes. Power consumption and voltage gradually became lower with each generation of DDR-based DIMMs.

Another influence is Column Access Strobe (CAS) latency, or CL which affects memory access speed. This is the delay time between the READ command and the moment data is available. See main article CAS/CL.

Chip Module Effective Clock
Transfer rate
SDR-66 PC-66 66 66 3.3
SDR-100 PC-100 100 100 3.3
SDR-133 PC-133 133 133 3.3
Chip Module Memory Clock
I/O Bus Clock
Transfer rate
DDR-200 PC-1600 100 100 200 2.5
DDR-266 PC-2100 133 133 266 2.5
DDR-333 PC-2700 166 166 333 2.5
DDR-400 PC-3200 200 200 400 2.6
Chip Module Memory Clock
I/O Bus Clock
Transfer rate
DDR2-400 PC2-3200 200 200 400 1.8
DDR2-533 PC2-4200 266 266 533 1.8
DDR2-667 PC2-5300 333 333 667 1.8
DDR2-800 PC2-6400 400 400 800 1.8
DDR2-1066 PC2-8500 533 533 1066 1.8
Chip Module Memory Clock
I/O Bus Clock
Transfer rate
DDR3-800 PC3-6400 400 400 800 1.5
DDR3-1066 PC3-8500 533 533 1066 1.5
DDR3-1333 PC3-10600 667 667 1333 1.5
DDR3-1600 PC3-12800 800 800 1600 1.5
DDR3-1866 PC3-14900 933 933 1866 1.5
DDR3-2133 PC3-17000 1066 1066 2133 1.5
DDR3-2400 PC3-19200 1200 1200 2400 1.5
Chip Module Memory Clock
I/O Bus Clock
Transfer rate
DDR4-1600 PC4-12800 800 800 1600 1.2
DDR4-1866 PC4-14900 933 933 1866 1.2
DDR4-2133 PC4-17000 1066 1066 2133 1.2
DDR4-2400 PC4-19200 1200 1200 2400 1.2
DDR4-2666 PC4-21300 1333 1333 2666 1.2
DDR4-3200 PC4-25600 1600 1600 3200 1.2
Chip Module Memory Clock
I/O Bus Clock
Transfer rate
DDR5-4000 PC5-32000 2000 2000 4000 1.1
DDR5-4400 PC5-35200 2200 2200 4400 1.1
DDR5-4800 PC5-38400 2400 2400 4800 1.1
DDR5-5200 PC5-41600 2600 2600 5200 1.1
DDR5-5600 PC5-44800 2800 2800 5600 1.1
DDR5-6000 PC5-48000 3000 3000 6000 1.1
DDR5-6200 PC5-49600 3100 3100 6200 1.1
DDR5-6400 PC5-51200 3200 3200 6400 1.1
DDR5-6800 PC5-54400 3400 3400 6800 1.1
DDR5-7200 PC5-57600 3600 3600 7200 1.1
DDR5-7600 PC5-60800 3800 3800 7600 1.1
DDR5-8000 PC5-64000 4000 4000 8000 1.1

Form factors[edit]

A comparison between 200-pin DDR and DDR2 SDRAM SO-DIMMs, and a 204-pin DDR3 SO-DIMM module.[15]

Several form factors are commonly used in DIMMs. Single Data Rate Synchronous DRAM (SDR SDRAM) DIMMs were primarily manufactured in 1.5 inches (38 mm) and 1.7 inches (43 mm) heights. When 1U rackmount servers started becoming popular, these form factor registered DIMMs had to plug into angled DIMM sockets to fit in the 1.75 inches (44 mm) high box. To alleviate this issue, the next standards of DDR DIMMs were created with a "low profile" (LP) height of around 1.2 inches (30 mm). These fit into vertical DIMM sockets for a 1U platform.

With the advent of blade servers, angled slots have once again become common in order to accommodate LP form factor DIMMs in these space-constrained boxes. This led to the development of the Very Low Profile (VLP) form factor DIMM with a height of around 0.72 inches (18 mm). The DDR3 JEDEC standard for VLP DIMM height is around 0.740 inches (18.8 mm). These will fit vertically in ATCA systems.

Full-height 240-pin DDR2 and DDR3 DIMMs are all specified at a height of around 1.18 inches (30 mm) by standards set by JEDEC. These form factors include 240-pin DIMM, SO-DIMM, Mini-DIMM and Micro-DIMM.[16]

Full-height 288-pin DDR4 DIMMs are slightly taller than their DDR3 counterparts at 1.23 inches (31 mm). Similarly, VLP DDR4 DIMMs are also marginally taller than their DDR3 equivalent at nearly 0.74 inches (19 mm).[17]

As of Q2 2017, Asus has had a PCI-E based "DIMM.2", which has a similar socket to DDR3 DIMMs and is used to put in a module to connect up to two M.2 NVMe solid-state drives. However, it cannot use common DDR type ram and does not have much support other than Asus.[18]

Regular DIMMs are generally 133.35 mm in length, while SO-DIMMs are generally 67.6 mm in length.[2]

See also[edit]


  1. ^ "What is DIMM(Dual Inline Memory Module)?". GeeksforGeeks. 2020-04-15. Retrieved 2024-04-07. In the case of SIMM, the connectors are only present on the single side of the module...DIMM has a row of connectors on both sides(front and back) of the module
  2. ^ a b "Common DIMM Memory Form Factor". 2009-10-06. Retrieved 2021-05-13.
  3. ^ Lyla, Das B. (September 2010). The X86 Microprocessors: Architecture and Programming (8086 to Pentium). Pearson Education India. ISBN 9788131732465.
  4. ^ Mueller, Scott (March 7, 2013). Upgrading and Repairing PCs: Upgrading and Repairing_c21. Que Publishing. ISBN 9780133105360 – via Google Books.
  5. ^ Jacob, Bruce; Wang, David; Ng, Spencer (28 July 2010). Memory Systems: Cache, DRAM, Disk. Morgan Kaufmann. ISBN 9780080553849.
  6. ^ a b Mueller, Scott (2004). Upgrading and Repairing PCS. Que. ISBN 9780789729743.
  7. ^ Smith, Ryan (2020-07-14). "DDR5 Memory Specification Released: Setting the Stage for DDR5-6400 And Beyond". AnandTech. Retrieved 2020-07-15.
  8. ^ a b c d e f g Mueller, Scott (2004). Upgrading and Repairing Laptops. Que. ISBN 9780789728005.
  9. ^ "72 Pin DRAM SO-DIMM | JEDEC".
  10. ^ Fulton, Jennifer (November 9, 2000). "The complete idiot's guide to upgrading and repairing PCs". Indianapolis, IN : Alpha Books – via Internet Archive.
  11. ^ Inc, Ziff Davis (December 25, 1990). "PC Mag". Ziff Davis, Inc. – via Google Books. {{cite web}}: |last= has generic name (help)
  12. ^ Norton, Peter; Clark, Scott H. (2002). Peter Norton's New Inside the PC. Sams. ISBN 9780672322891.
  13. ^ Synology Inc. "Synology RAM Module". synology.com.
  14. ^ Temperature Sensor in DIMM memory modules
  15. ^ "Are DDR, DDR2 and DDR3 SO-DIMM memory modules interchangeable?". acer.custhelp.com. Retrieved 2015-06-26.
  16. ^ JEDEC MO-269J Whitepaper., accessed Aug. 20, 2014.
  17. ^ JEDEC MO-309E Whitepaper., accessed Aug. 20, 2014.
  18. ^ ASUS DIMM.2 is a M.2 Riser Card., accessed Jun. 4, 2020.

External links[edit]