A DIMM or dual in-line memory module, comprises a series of dynamic random-access memory integrated circuits. These modules are mounted on a printed circuit board and designed for use in personal computers, workstations and servers. DIMMs began to replace SIMMs (single in-line memory modules) as the predominant type of memory module as Intel P5-based Pentium processors began to gain market share.
The main difference between SIMMs and DIMMs is that DIMMs have separate electrical contacts on each side of the module, while the contacts on SIMMs on both sides are redundant. Another difference is that standard SIMMs have a 32-bit data path, while standard DIMMs have a 64-bit data path. Since Intel's Pentium has (as do several other processors) a 64-bit bus width, it requires SIMMs installed in matched pairs in order to complete the data bus. The processor would then access the two SIMMs simultaneously. DIMMs were introduced to eliminate this practice.
The most common types of DIMMs are:
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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-counts, and different notch positions.
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.
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. Since all ranks share the same buses, only one rank may be accessed at any given time; it is specified by activating the corresponding rank's chip select (CS) signal. All other ranks 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.
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.
Most DIMMs are built using "×4" (by 4) memory chips or "×8" (by 8) memory chips with 9 chips per side. "×4" or "×8" refer to the data width of the DRAM chips in bits.
In the case of the "×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).
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.