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 as the predominant
type of memory module as Intel P5-based Pentium processors began to gain market share.
While the contacts on SIMMs on both sides are redundant, DIMMs have separate electrical
contacts on each side of the module. 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, many processors
have a 64-bit bus width, requiring 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 practice. The most common types of DIMMs are:
72-pin SO-DIMM, used for FPM DRAM and EDO DRAM
100-pin DIMM, used for printer SDRAM 144-pin SO-DIMM, used for SDR SDRAM
168-pin DIMM, used for SDR SDRAM 172-pin MicroDIMM, used for DDR SDRAM
184-pin DIMM, used for DDR SDRAM 200-pin SO-DIMM, used for DDR SDRAM and DDR2
SDRAM 204-pin SO-DIMM, used for DDR3 SDRAM
214-pin MicroDIMM, used for DDR2 SDRAM 240-pin DIMM, used for DDR2 SDRAM, DDR3 SDRAM
and FB-DIMM DRAM 244-pin MiniDIMM, used for DDR2 SDRAM
260-pin SO-DIMM, used for DDR4 SDRAM 288-pin DIMM, used for DDR4 SDRAM
168-pin SDRAM
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. The first notch is DRAM key position. It represents
RFU, registered, and unbuffered. The second notch is voltage key position.
It represents 5.0 V, 3.3 V, and Reserved. The upper DIMM in the photo is an unbuffered
3.3 V 168-pin DIMM. Variants of DIMM slots support DDR, DDR2 and DDR3 RAM.
DDR DIMMs DDR, DDR2, DDR3 and DDR4 all have different
pin counts, and different notch positions. As of August, 2014, DDR4 SDRAM is a modern
emerging type of dynamic random access memory with a high-bandwidth interface, and has been
in use since 2013. It is the higher-speed successor to DDR2 and DDR3. DDR4 SDRAM is
neither forward nor backward compatible with any earlier type of random access memory because
of different signalling voltages, timings, as well as other differing factors between
the technologies and their implementation. SPD EEPROM
A DIMM's capacity and other operational parameters may be identified with serial presence detect,
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. 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 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. 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 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. 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.
Organization Most DIMMs are built using "×4" memory chips
or "×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 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.
Note: The above example applies to ECC memory which 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 DRAM have
the same bus frequency for data, address and control lines. DIMMs based on Double Data
Rate 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.
Form factors Several form factors are commonly used in
DIMMs. Single Data Rate Synchronous DRAM DIMMs were primarily manufactured in 1.5 inches
and 1.7 inches 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 high
box. To alleviate this issue, the next standards of DDR DIMMs were created with a "low profile"
height of around 1.2 inches. 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 form factor DIMM with a height of around 0.72 inches. The DDR3 JEDEC
standard for VLP DIMM height is around 0.740 inches. 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 by standards set by JEDEC. These form factors
include 240-pin DIMM, SODIMM, Mini-DIMM and Micro-DIMM.
Full-height 288-pin DDR4 DIMMs are slightly taller than their DDR3 counterparts at 1.23
inches. Similarly, VLP DDR4 DIMMs are also marginally taller than their DDR3 equivalent
at nearly 0.74 inches. See also
References
External links How to Install PC Memory guides
Very Low Profile DDR2 Whitepaper