The various types of memory have different key positions that allow for fool proof installation and disallow incompatible memory types to be installed.
On the bottom edge of 168-pin DIMMs there are 2 notches, and the location of each notch determines a particular feature of the module.
DDR, DDR2 and DDR3 all have a different pin-count, and different notch position.
A DIMM's capacity and timing 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.
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.
The number of ranks on any DIMM is the number of independent sets of DRAMs that can be accessed for the full data bit-width of the DIMM ie 64 bits. The ranks cannot be accessed simultaneously as they share the same data path. The physical layout of the DRAM chips on the DIMM itself does not necessarily relate to the number of ranks.
DIMMs are often referred to as "single-sided" or "double-sided" in reference to the location of the memory devices or "chips" being on only one or both sides of the DIMM printed circuit board (PCB). These terms may cause confusion as they do not necessarily relate to how the DIMMs are logically organized or accessed.
For example, on a single-rank DIMM that has 64 data bits of I/O pins, there is only one set of DRAMs that are turned on to drive a read or receive a write on all 64 bits. In most electronic systems, memory controllers are designed to access the full data bus width of the memory module at the same time.
On a 64-bit (non-ECC) DIMM made with two ranks, there would be two sets of DRAM that could be accessed at different times. Only one of the ranks can be accessed at a time, since the DRAM data bits are tied together for two loads on the DIMM (Wired OR). Ranks are accessed through chip selects (CS). Thus for a two rank module, the two DRAMs with data bits tied together may be accessed by a CS per DRAM (e.g. CS0 goes to one DRAM chip and CS1 goes to the other). 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. JEDEC decided that the terms "dual-sided," "double-sided," or "dual-banked" were not correct when applied to registered DIMMs.
Most DIMMs are built using "x4" (by 4) memory chips or "x8" (by 8) memory chips with 9 chips per side. "x4" or "x8" refer to the data width of the DRAM chips in bits.
In the case of the "x4"-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 "x8"-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).
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.
SDR SDRAM DIMMs - These first synchronous registered DRAM DIMMs had the same bus frequency for data, address and control lines.
DDR SDRAM (DDR1) DIMMs - 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.
DDR2 SDRAM DIMMs - DIMMs based on Double Data Rate 2 (DDR2) DRAM also have data and data strobe frequencies at double the rate of the clock. This is achieved by clocking on both the rising and falling edge of the data strobes. The power consumption and voltage of DDR2 is significantly lower than DDR(1) at the same speed.
DDR3 SDRAM DIMMs - DIMMs based on Double Data Rate 3(DDR3) DRAM have data and strobe frequencies at double the rate of the clock. This is achieved by clocking on both the rising and falling edge of the data strobes. The power consumption and voltage of DDR3 is lower than DDR2 of the same speed.
Several form factors are commonly used in DIMMs. Single Data Rate (SDR) SDRAM DIMMs commonly came in two main heights: 1.7-inch and 1.5-inch. 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" high box. To alleviate this issue, the next standards of DDR DIMMs were created with a "Low Profile" (LP) height of ~1.2". These fit into vertical DIMM sockets for a 1U platform. With the advent of blade servers, the LP form factor DIMMs have once again been often angled to fit in these space-constrained boxes. This led to the development of the Very Low Profile (VLP) form factor DIMM with a height of ~.72" (18.3 mm). The DDR3 JEDEC standard for VLP DIMM height is 18.75mm. These will fit vertically in ATCA systems. Other DIMM form factors include the SO-DIMM, the Mini-DIMM and the VLP Mini-DIMM.