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 type of computer memory module used in desktop, laptop, and server computers. It is a circuit board that contains memory chips and connects to the computer's motherboard. A DIMM is often called a "RAM stick" due to its shape and size. A DIMM comprises a series of dynamic random-access memory integrated circuits that are mounted to its circuit board. DIMMs 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 133.35 mm (5.25 in) and laptop (SO-DIMM) which are about half the size at 67.60 mm (2.66 in).[1]

History

DIMMs (Dual In-line Memory Module) were a 1990s upgrade for SIMMs (Single In-line Memory Modules)[2][3] 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.[4] This allowed them to double the SIMMs 32-bit data path into a 64-bit data path.[5]

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.[5] 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.

Variants

There are numerous DIMM variants.

Common types of DIMMs include the following:

DIMM
SO-DIMM
  • 72-pin (not the same as a 72-pin SIMM), used for FPM DRAM and EDO DRAM[7]
  • 144-pin, used for SDR SDRAM[7] (less frequently for DDR2 SDRAM)
  • 200-pin, used for DDR SDRAM[7] 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
  • 144-pin, used for SDRAM[7]
  • 172-pin, used for DDR SDRAM[7]
  • 214-pin, used for DDR2 SDRAM

SO-DIMM

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" /ˈsoʊdɪm/, 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.[7][8][9] Before its introduction, most laptops would use propietary RAM modules which were expensive and hard to find.[7][10]

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.[11] 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

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

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.

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).[12]

Error correction

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.

Ranking

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).

Organization

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.

Speeds

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.

SDR SDRAM DIMMs
Chip Module Effective Clock
(MHz)		 Transfer rate
(MT/s)		 Voltage
(V)
SDR-66 PC-6666663.3
SDR-100 PC-1001001003.3
SDR-133 PC-1331331333.3
DDR SDRAM (DDR1) DIMMs
Chip Module Memory Clock
(MHz)		 I/O Bus Clock
(MHz)		 Transfer rate
(MT/s)		 Voltage
(V)
DDR-200 PC-16001001002002.5
DDR-266 PC-21001331332662.5
DDR-333 PC-27001661663332.5
DDR-400 PC-32002002004002.6
DDR2 SDRAM DIMMs
Chip Module Memory Clock
(MHz)		 I/O Bus Clock
(MHz)		 Transfer rate
(MT/s)		 Voltage
(V)
DDR2-400 PC2-32002002004001.8
DDR2-533 PC2-42002662665331.8
DDR2-667 PC2-53003333336671.8
DDR2-800 PC2-64004004008001.8
DDR2-1066 PC2-850053353310661.8
DDR3 SDRAM DIMMs
Chip Module Memory Clock
(MHz)		 I/O Bus Clock
(MHz)		 Transfer rate
(MT/s)		 Voltage
(V)
DDR3-800 PC3-64004004008001.5
DDR3-1066 PC3-850053353310661.5
DDR3-1333 PC3-1060066766713331.5
DDR3-1600 PC3-1280080080016001.5
DDR3-1866 PC3-1490093393318661.5
DDR3-2133 PC3-170001066106621331.5
DDR3-2400 PC3-192001200120024001.5
DDR4 SDRAM DIMMs
Chip Module Memory Clock
(MHz)		 I/O Bus Clock
(MHz)		 Transfer rate
(MT/s)		 Voltage
(V)
DDR4-1600 PC4-1280080080016001.2
DDR4-1866 PC4-1490093393318661.2
DDR4-2133 PC4-170001066106621331.2
DDR4-2400 PC4-192001200120024001.2
DDR4-2666 PC4-213001333133326661.2
DDR4-3200 PC4-256001600160032001.2
DDR5 SDRAM DIMMs
Chip Module Memory Clock
(MHz)		 I/O Bus Clock
(MHz)		 Transfer rate
(MT/s)		 Voltage
(V)
DDR5-4000 PC5-320002000200040001.1
DDR5-4400 PC5-352002200220044001.1
DDR5-4800 PC5-384002400240048001.1
DDR5-5200 PC5-416002600260052001.1
DDR5-5600 PC5-448002800280056001.1
DDR5-6000 PC5-480003000300060001.1
DDR5-6200 PC5-496003100310062001.1
DDR5-6400 PC5-512003200320064001.1
DDR5-6800 PC5-544003400340068001.1
DDR5-7200 PC5-576003600360072001.1
DDR5-7600 PC5-608003800380076001.1
DDR5-8000 PC5-640004000400080001.1

Form factors

A comparison between 200-pin DDR and DDR2 SDRAM SO-DIMMs, and a 204-pin DDR3 SO-DIMM module[13]

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.[14]

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).[15]

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.[16]

Regular DIMMs are generally 133.35 mm in length, SO-DIMMs 67.6 mm.[1]

See also

References

  1. 1 2 "Common DIMM Memory Form Factor". 2009-10-06. Retrieved 2021-05-13.
  2. ↑ Lyla, Das B. (September 2010). The X86 Microprocessors: Architecture and Programming (8086 to Pentium). Pearson Education India. ISBN 9788131732465.
  3. ↑ Mueller, Scott (March 7, 2013). Upgrading and Repairing PCs: Upgrading and Repairing_c21. Que Publishing. ISBN 9780133105360 – via Google Books.
  4. ↑ Jacob, Bruce; Wang, David; Ng, Spencer (28 July 2010). Memory Systems: Cache, DRAM, Disk. Morgan Kaufmann. ISBN 9780080553849.
  5. 1 2 Mueller, Scott (2004). Upgrading and Repairing PCS. Que. ISBN 9780789729743.
  6. ↑ Smith, Ryan (2020-07-14). "DDR5 Memory Specification Released: Setting the Stage for DDR5-6400 And Beyond". AnandTech. Retrieved 2020-07-15.
  7. 1 2 3 4 5 6 7 Mueller, Scott (2004). Upgrading and Repairing Laptops. Que. ISBN 9780789728005.
  8. ↑ https://www.jedec.org/standards-documents/docs/module-444
  9. ↑ Fulton, Jennifer (November 9, 2000). "The complete idiot's guide to upgrading and repairing PCs". Indianapolis, IN : Alpha Books – via Internet Archive.
  10. ↑ Norton, Peter; Clark, Scott H. (2002). Peter Norton's New Inside the PC. Sams. ISBN 9780672322891.
  11. ↑ Synology Inc. "Synology RAM Module". synology.com.
  12. ↑ Temperature Sensor in DIMM memory modules
  13. ↑ "Are DDR, DDR2 and DDR3 SO-DIMM memory modules interchangeable?". acer.custhelp.com. Retrieved 2015-06-26.
  14. ↑ JEDEC MO-269J Whitepaper., accessed Aug. 20, 2014.
  15. ↑ JEDEC MO-309E Whitepaper., accessed Aug. 20, 2014.
  16. ↑ ASUS DIMM.2 is a M.2 Riser Card., accessed Jun. 4, 2020.
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