DIMM

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Two types of DIMMs: a 168-pin SDRAM module (top) and a 184-pin DDR SDRAM module (bottom).

A DIMM, or dual in-line memory module, comprises a series of random access memory integrated circuits. These modules are mounted on a printed circuit board and designed for use in personal computers. DIMMs began to replace SIMMs (single in-line memory modules) as the predominant type of memory module as Intel's Pentium processors began to control the market.

The main difference between SIMMs and DIMMs is that SIMMs have a 32-bit data path, while DIMMs have a 64-bit data path. Since Intel's Pentium has (as do several other processors) a 64-bit bus width, it required SIMMs installed in matched pairs in order to use them. The processor would then access the two SIMMs simultaneously. DIMMs were introduced to eliminate this inefficiency. Another difference is that DIMMs have separate electrical contacts on each side of the module, while the contacts on SIMMs on both sides are redundant.

The most common types of DIMMs are:

72-pin-DIMMs, used for SO-DIMM
144-pin-DIMMs, used for SO-DIMM
200-pin-DIMMs, used for SO-DIMM
168-pin-DIMMs, used for FPM, EDO and SDRAM
184-pin-DIMMs, used for DDR SDRAM
240-pin-DIMMs, used for DDR2 SDRAM

There are 2 notches on the bottom edge of 168-pin-DIMMs, and the location of each notch determines a particular feature of the module.

The first notch is DRAM key position. It represents RFU (reserved future use), registered, and unbuffered.
The second notch is voltage key position. It represents 5.0V, 3.3V, and Reserved.
The upper DIMM in the photo is an unbuffered 3.3V 168-pin DIMM.

A DIMM's capacity and timing parameters may be identified with SPD (Serial Presence Detect), an additional chip which contains information about the module type.

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 a 9th extra bit per byte.

1 Ranking
2 Organization
3 Speeds
4 Form factors
5 See also
6 External links

[edit] Ranking

The number of ranks on any DIMM is the number of independent sets of DRAMs that can be accessed simultaneously for the full data bit-width of the DIMM to be driven on the bus. The physical layout of the DRAM chips on the DIMM itself does not necessarily relate to the number of ranks. Sometimes the layout of all DRAM on one side of the DIMM PCB versus both sides is referred to as "single-sided" versus "double-sided". 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.

[edit] Organization

Most DIMMs are built using "x4" (by 4) memory chips or "x8" (by 8) memory chips. "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).

[edit] 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.

SDRAM DIMMs - These first synchronous registered DRAM DIMMs had the same bus frequency for data, address and control lines.

PC66 = 66 MHz
PC100 = 100 MHz
PC133 = 133 MHz

DDR SDRAM (DDR1) SDRAM 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.

PC1600 = 200 MHz data & strobe / 100 MHz clock for address and control
PC2100 = 266 MHz data & strobe / 133 MHz clock for address and control
PC2700 = 333 MHz data & strobe / 166 MHz clock for address and control
PC3200 = 400 MHz data & strobe / 200 MHz clock for address and control

DDR2 SDRAM 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 of DDR2 is significantly lower than DDR(1) at the same speed.

PC2-3200 = 400 MHz data & strobe / 200 MHz clock for address and control
PC2-4200 = 533 MHz data & strobe / 266 MHz clock for address and control
PC2-5300 = 667 MHz data & strobe / 333 MHz clock for address and control
PC2-6400 = 800 MHz data & strobe / 400 MHz clock for address and control

[edit] Form factors

Several form factors are commonly used DIMM's. Single Data Rate(SDR) SDRAM DIMM's commonly came in two main sizes: 1.7" and 1.5". When 1U rackmount servers started becoming popular, these form factor Registered DIMM's had to plug into angled DIMM sockets to fit in the 1.75" high box. To alleviate this issue, the next standards of DDR DIMM's 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 DIMM's 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). Other DIMM form factors include the SO-DIMM, the Mini-DIMM and the VLP Mini-DIMM.