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- SIMM or Single Inline Memory Module – This type of RAM is used on outdated legacy motherboards and is no longer in use.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|
|72 pin SIMM||72||
|30 pin SIMM||30||
Laptop/small form factor RAM module type comparison
|RAM Type||Image||Number of Pins||Size||Features|
|SO-DIMM DDR SDRAM||200||2.66″||
|SO-DIMM DDR2 SDRAM||200||2.66″||
|SO-DIMM DDR3 SDRAM||204||2.66″||
|SO-DIMM DDR4 SDRAM||260||2.66″||
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.
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.
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).