DDR3 DIMM
– Image credit Wikipedia

Random access memory (RAM) is Dynamic RAM or DRAM and is stored on memory modules that are inserted in memory slots on the motherboard. DRAM requires constant refreshing and only holds is data and instructions temporarily before rapidly letting it go. The memory controller refreshes the temporary storage of data and instructions thousands of times a second while the CPU processes it. DRAM categories include:

  • DIMM or Dual Inline Memory Module – This is the type of RAM used by all new motherboards and uses a 64-bit data path.
  • SO-DIMM or Small Outline DIMM – This is a smaller version of DIMM, used by mobile devices such as laptops and other small form factor computers.
  • RIMM or Rambus Inline Memory Module – This is an older type of RAM designed by Rambus, Inc. This type of RAM is installed in pairs. Every slot must be filled by either a RIMM or a continuity RIMM (CRIMM). A CRIMM placeholder is the same size and shape as a RIMM but without the integrated RAM circuitry.
  • SIMM or Single Inline Memory Module – This type of RAM is used on outdated legacy motherboards and is no longer in use. Electrical contacts are only on one side of the module.
Desktop/large form factor RAM module type comparison
RAM Type Image Number of Pins Features
SDRAM DIMM

DDR-SDRAM DIMM
DDR-SDRAM DIMM – Image credit Wikipedia
168
  • First DIMM to run in sync with the system clock
  • 100-200 MHz bus clock speed
  • Electrical contacts are different on both sides
  • Has 2 notches
DDR DIMM

DDR DIMM
DDR DIMM – Image credit Wikipedia
184
  • Runs 2x faster than SDRAM
  • Electrical contacts are different on both sides
  • 100-200 MHz bus clock speed
  • Has 1 notch
DDR2 DIMM

2GB PC2 5300U 555
DDR2 DIMM – Image credit Wikipedia
240
  • 2x the bus clock multiplier of DDR at minimum
  • Electrical contacts are different on both sides
  • 200–533⅓ MHz bus clock speed
  • Uses less power than DDR
  • Has 1 notch
DDR3 DIMM

DDR3 DIMM
DDR3 DIMM – Image credit Wikipedia
240
  • 4x the bus clock multiplier of DDR at minimum
  • Electrical contacts are different on both sides
  • 400–1066⅔ MHz bus clock speed
  • Uses less power than DDR2
  • Has 1 notch
DDR4 DIMM

DDR4 DIMM
DDR4 DIMM – © apluscomputer.repair
288
  • 10x the bus clock multiplier of DDR at minimum
  • Electrical contacts are different on both sides
  • 1066⅔–2133⅓ MHz bus clock speed
  • Uses less power than DDR3
  • Has 1 notch
RIMM

RIMM
RIMM – Image credit Wikipedia
184
  • Has 2 notches located near center of connector edge
  • Proprietary memory from Rambus
  • Electrical contacts are different on both sides
  • Installed in pairs
  • Every slot must be filled. can use continuity RIMM’s (CRIMM)
72 pin SIMM

72 pin SIMM
72 pin SIMM – Image credit Wikipedia
72
  • Installed in groups of 2
  • Electrical contacts are the same on both sides
  • Has 1 notch
30 pin SIMM

30 pin SIMM
30 pin SIMM – Image credit Wikipedia
30
  • Installed in groups of 4
  • Electrical contacts are the same on both sides
  • Has 1 notch
Laptop/small form factor RAM module type comparison
RAM Type Image Number of Pins Size Features
SO-DIMM SDRAM

SO-DIMM SDRAM
SO-DIMM SDRAM – Image credit Wikipedia
144 2.66″
  • Has 1 notch slightly offset from center of connector edge.
SO-DIMM DDR SDRAM

DDR SO-DIMM
DDR SO-DIMM – Image credit Wikipedia
200 2.66″
  • Has 1 notch near one side of the connector edge.
SO-DIMM DDR2 SDRAM

DDR2 SO-DIMM
DDR2 SO-DIMM – © Matthieu Riegler, CC-BY, Wikimedia Commons
200 2.66″
  • Has 1 notch near one side of the connector edge.
SO-DIMM DDR3 SDRAM

DDR3 SO-DIMM
DDR3 SO-DIMM – Image credit Wikipedia
204 2.66″
  • Has 1 notch that is offset from the center of the connector edge.
SO-DIMM DDR4 SDRAM

DDR4 SO-DIMM
DDR4 SO-DIMM – © apluscomputer.repair
260 2.66″
  • Has 1 notch that is offset from the center of the connector edge.
SO-RIMM

SO-RIMM
SO-RIMM – Image credit imgarcade.com
160 2.66″
  • Has 2 notches near center of the connector edge.
  • Proprietary memory from Rambus
Parity vs. non-parity

On older SIMM’s, parity was used as an error checking technology. Parity RAM has an additional parity bit (8 bits for data and an addtional 1 bit for parity) and does not have the ability to correct errors. With this technology, RAM uses either even or odd parity. Even parity uses the parity bit to add either a 1 or 0 in order to make the total number of the 9 bits even. Odd parity uses the parity bit to add either a 1 or 0 in order to make the total number of the 9 bits odd. If the memory controller reads an incorrect state (even number when supposed to be odd or odd when supposed to be even) a parity error occurs.

ECC vs. non-ECC

ECC (error correcting code) is a modern error correcting technology that is often used in a server environment. This technology will detect errors in the RAM and correct those errors on the fly. A DIMM with ECC will have an odd number of chips laid out on the module, as opposed to a DIMM without ECC, which will have an even number of chips. ECC is supported on SDRAM, DDR, DDR2, DDR3 and DDR4 modules. For ECC to work, both the motherboard and the modules must support the technology.

RAM configurations

On a motherboard, Multi-channel DIMM slots are colored to differentiate between channels. Each RAM slot uses a channel. Originally, DIMM’s only used a single channel which meant that the memory controller could only access one DIMM at a time. Since its introduction, there have been a quite a few improvements to the DIMM; dual channels double the speed by allowing the memory controller to access two DIMM’s at the same time, triple channels tripled the speed by allowing the memory controller to access three DIMM’s at one time and quad channels allow access to four DIMM’s at the same time. DDR, DDR2 and DDR3 RAM can use dual channels. DDR3 and DDR4 DIMM’s can also use triple and quad channels. For dual, triple, or quad channel technology to work, the motherboard and the DIMM must support it. When setting up dual, triple or quad channeling, the DIMM’s in each channel must be an exact match in size, speed, and features and should also come from the same manufacturer.

Single sided vs. double sided

A DIMM is either single or double sided. Single sided RAM has memory chips on one side of the module while Double sided RAM has memory chips on both sides of the module. Some double sided DIMM’s are dual ranked. This means that the module has more than one bank; the chips on the DIMM are grouped so that the RAM controller only accesses one group at a time. This is a slower and lower performing type of DIMM in contrast to modules where all of the memory is accessible at one time.

RAM Compatibility and speed

DDR runs 2x as fast as SDRAM and has one notch and 184 pins. DDR2 uses less power than DDR, has one notch, and has 240 pins. DDR3 uses less power than DDR2, also has one notch, and has 240 pins. DDR4 uses less power than DDR3 , also has one notch, and has 288 pins. DDR2, DDR3, and DDR4 are not compatible with one another, the notch in these modules is positioned differently to ensure that they are not inserted into the wrong memory slot.

Double Data Rate SDRAM (DDR SDRAM) is an improvement on SDRAM. The dual inline memory module (DIMM) gets its name because it has an independent set of electrical contacts on both (dual) sides of the module. RAM modules are keyed with small notches so that the RAM will only fit into the memory slot one way. Synchronous DRAM (SDRAM) was the first DIMM to run in sync with the computer system clock. The SDRAM DIMM has 2 notches and 168 pins.

NOTE: SDRAM should not be confused with SRAM (Static RAM) which is very fast, very expensive and maintains data without constant refreshing. SRAM is often used in CPU memory caches.

The speed of a DIMM is measured in either MHz (i.e. 1600 MHz – transfers per second) or PC rating (i.e. PC3-12800 – bytes per second). A PC rating is a DIMM’s bandwidth in Megabytes per second. The term PC differs between DDR, DDR2, and DDR3 RAM. DDR uses the term “PC” while DDR2 uses the term “PC2” and DDR3 uses “PC3”. To calculate PC rating, multiply the modules MHz x 8. (i.e. 1600 MHz x 8 = PC3-12800).

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