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- A dual in-line memory module (DIMM) is defined as a 64-bit memory unit containing multiple RAM chips on a circuit board with pins that connect to the computer’s motherboard.
- DIMM is used in desktop machines, laptops, printers, servers, and high-end workstations.
- This article explains the fundamentals of DIMM, its characteristics, and types.
Table of Contents
What Is a Dual In-line Memory Module (DIMM)?
A dual in-line memory module (DIMM) is a 64-bit memory unit that contains multiple RAM chips on a circuit board with pins that connect to the computer’s motherboard. Under the DIMM architecture, every data bit is stored in separate memory cells. DIMM follows a 64-bit data path to enable faster data transfer. DIMM is used in desktop machines, laptops, printers, servers, and high-end workstations.
Going back to the DIMM roots, you stumble upon computer RAMs. Let’s understand the makeup of a simple RAM, how it operates, and its evolution into DIMM.
A DIMM is an electronic chip that stores temporary working data of a system that can be read and written. It is mounted on the system’s motherboard and requires an electric supply to function normally. This implies that when the electric supply is cut off, all the RAM data is wiped off.
RAM chips have less storage capacity and are not individually mounted on motherboards. Instead, several RAMs are soldered together to form a single integrated circuit (IC) board called a ‘module.’ Further, multiple such modules are then mounted on the motherboard with the help of pins or connectors. The below image reveals this illustration.
Illustration of a RAM Module
Source: GeeksforGeeksOpens a new window
The discussed modules are further divided into two types:
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- Single in-line memory module (SIMM): Under the SIMM configuration, pins are present on one side of the module and are shorted together. Its maximum data storage capacity is 32-bit/cycle, and its voltage capacity is 5V.
- Dual in-line memory module (DIMM): With time, SIMM became obsolete and was replaced by DIMM. DIMM has pins on both sides of the modules, which are independent of each other. Such configuration doubles the capacity of DIMM for the same RAM specifics. Thus, DIMM supports 64-bit processors. Moreover, the voltage capacity of DIMM is 3.3V. It is worth noting here that DIMM is not backward compatible. This implies that motherboards with SIMM slots cannot use DIMM.
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Thus, it is safe to say that DIMM outperforms SIMM in speed and power consumption parameters. DIMMs available in the market generally contain 168, 184, 214, or 244 pins.
With the emergence of dynamic random access memory (DRAM), the DIMM circuit board underwent further evolution. Modern-day DIMMs based on double data rate fourth generation (DDR4) SDRAM chips (Synchronous DRAM) use 288 pins to connect to the machine’s motherboard. This causes a substantial increase in the data throughput. Moreover, with the RAM chips gaining clock speeds, the 64-bit path can now handle increased data.
In addition, DIMMs are further evolving as cooling fins or structures are being attached to DIMM modules directly. This leads to an increase in chip density in 8 GB or 16 GB DIMMs. Also, there is a substantial increase in the clock speeds, which can lead to an increase in overall heat produced. The situation can further worsen if we consider 64 GB DIMMs that are based on DDR4 RAM chips. Hence, to compensate for this, the cooling structures are installed on DIMMs to help vent the excess into system enclosures far from the motherboard and CPU.
Today, if we look at the global device development landscape, the need for memory capacity has grown significantly in mobile and portable devices. As a result, the DIMM market has seen a spike in recent times. According to an October 2021 report by Exactitude Consultancy on dual in-line memory modules, the global DIMM market stood at $3 billion in 2021 and is estimated to reach $10 million by 2026. The market is dominated by global players like Intel, Ramaxel, Tigo, Micron, Samsung, and so on. Moreover, as per the report, North America, which includes the U.S., Mexico, and Canada, is witnessing maximum growth in the DIMM market.
Advantages of DIMM slots
The physical size and application of the computer system decide the ideal number and type of DIMM slots it requires. Generally, most computer motherboards are devised to support 2, 4, 6, or 8 DIMMs. Smaller motherboards have two slots, and high-end motherboards like the ones used in servers have eight slots, while motherboards incorporated into electronics used in daily lives have four slots.
DIMM slots have the following advantages:
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- As DIMM slots are known to have separate contacts on each side of the circuit board, the size of the data path doubles when compared to SIMMs.
- DIMM buffers control signals and addresses from the CPU. Owing to this buffering, there is a significant reduction in memory workloads.
- Thanks to the dual-channel architecture, the CPU can separately access memory on each DIMM. Apart from a dual channel, triple- and quad-channels are also available in the market, used for specific use cases.
- These multi-channel DIMMs support interleaving functionality, thereby reducing memory latency problems. Interleaving implies the spreading of data in alternate patterns across memory modules. As a result, the CPU can continue its operation by sequentially accessing these data patterns from memory without waiting for modules to fill up before accessing the next module.
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Characteristics of DIMM
DIMM refers to an array of DRAM or SDRAM ICs mounted on printed circuit boards. Standard DIMMs are 5.5 inches long and 1.18 inches tall. Let’s now understand some of the prominent characteristics of DIMMs.
DIMM Characteristics
1. Management of multiple memory ranks
Today, DIMMs use independent DRAM chips, also called memory ranks. Accessing these multiple ranks simultaneously is a critical task for processors. Hence, DIMM supports interleaving features wherein the processor can perform operations on multiple ranks. For instance, the processor can read from one rank and write on another. Once the operation is completed, the DRAM chips flush out the entire data. This is primarily done to avoid the presence of many ranks in a single channel which can cause ‘pipeline stalls’ and delay instruction execution.
2. Support for different RAM types
Variants of DIMMs support RAMs that belong to the DDR category. This includes DDR, DDR2, DDR3, DDR4, and DDR5. Each DDR RAM type has a distinct buffer size, operating voltage, and transfer rate. With time, different versions of DDR were launched to support specific needs. The latest version of DDR supported by DIMM is DDR5.
DDR5 is known to enhance system performance, add data integrity functions, lower operating voltage, and double the data transfer rate compared to DDR4. In 2018, JEDEC, an independent standardization body headquartered in the U.S., released the specifications for commercial DDR5. Moreover, it is said that JEDEC has its eyes on potential new standards for non-volatile DIMM (i.e., NVDIMM), which can help in data retention in case of an unexpected event such as a power cut-off, system crash, or normal shutdown. Such features were not observed in the original RAM versions.
3. Error detection and correction (ECC)
DIMMs are capable of implementing error detection and correction by employing ECC DIMMs. Here, DIMMs tend to use extra bits apart from data bits for ECC. Although several ECC methods exist, the commonly used ECC scheme is ‘Single Error Correct, Double Error Detect (SECDED),’ which employs an extra 9th bit for every byte (64-bit word) of data.
4. Different form factors
Although modern DIMMs have a standard size known as form factor, older DIMMs came in 1.5 and 1.7 inches sizes. Later, the development of rack-mounted servers led to the pushing of DIMMs in narrow spaces. As a result, the DIMM sockets had to be tilted at a certain angle to accommodate the memory modules. To eliminate the need for such angled sockets, new standards were introduced, which revealed a ‘low profile’ height of 1.2 inches for DIMMs.
As servers reduced in size, the sockets were again forced to tilt to accommodate the low profile form factor. This led to the emergence of the Very Low Profile (VLP) form factor, with a height of about 0.72 inches. Other popular form factors include SO-DIMM, Mini-DIMM, and VLP Mini-DIMM.
5. Varying speeds
Each DIMM type has its own clock frequency, speed, and bus to manage data, address, and control lines. As a result, each DIMM has varying data transfer rates. For instance, let’s look at the data transfer rates of different DIMMs:
| DIMM type | Chip | Memory clock | Transfer rate | Voltage |
| SDR SDRAM DIMMs | SDR-133 | 133 MHz | 133 MT/s | 3.3 V |
| DDR SDRAM (DDR1) DIMMs | DDR-400 | 200 MHz | 400 MT/s | 2.5 V |
| DDR2 SDRAM DIMMs | DDR2-1066 | 533 MHz | 1066 MT/s | 1.8 V |
| DDR3 SDRAM DIMMs | DDR3-2400 | 1200 MHz | 2400 MT/s | 1.5 V |
| DDR4 SDRAM DIMMs | DDR4-3200 | 1600 MHz | 3200 MT/s | 1.2 V |
(Note: Here, MT/s implies MegaTransfers (or million transfers) per second, which measures the effective data rate or speed.)
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Types of DIMM
Typically, DIMMs are segregated based on buffer size and RAM type. Let’s look at each type in detail.
Types of DIMM
1. Unbuffered DIMMs (UDIMMs)
These DIMMs are suitable for desktops and laptops as they are less expensive and tend to have greater speeds. The instructions are directly communicated between the memory controller (located in the CPU) and the memory module.
2. Fully-buffered DIMMs (FB-DIMMs)
These are typically used in server and workstation systems that demand larger memory capacity. FB-DIMMs rely on an advanced memory buffer (AMB) present between the memory controller and memory module to increase reliability, ensure signal integrity, speed up error detection, and reduce soft error. Here, soft error refers to an error in signal due to a defect, design, construction, or broken component.
The AMB bus is divided into two parts: 14-bit for reading and 10-bit for writing. This implies that both read and write operations can happen simultaneously, increasing performance.
3. Registered DIMMs (RDIMMs)
RDIMMS are also referred to as buffered memory and are suitable for servers and systems that demand robustness and stability. Such DIMMs have memory registers positioned between the memory controller and memory. Here, the memory controller does not directly access the DRAM. Instead, it buffers commands, addresses, and clock cycles and then directs instructions to specific memory registers. With such an operating mechanism, the instructions require one CPU cycle longer than usual. However, buffering of commands reduces the load on the memory controller.
4. Load-reduced DIMMs (LR-DIMMs)
LR-DIMM uses an isolation memory buffer (iMB) to buffer data, data signals, and addresses to reduce strain on CPU’s the memory controller. The iMB chip separates the DRAM chips that are a part of DIMM from the memory controller. Consequently, the memory controller cannot view the DRAM chips but directly access iMB chips instead. The buffered memory then performs all read and write operations. As a result, the system’s capacity and speed get a boost.
5. Small outline-DIMM (SO-DIMM)
As stated earlier, the standard DIMM stick is around 5.5 inches long. But, the small outline dual in-line memory module (SO-DIMM) is half this length, i.e., 2.74 inches. These DIMMs are 1.2 inches tall but are used in a very low profile (VLP) format that’s around 0.8 inches tall. SO-DIMM is suited for portable devices like laptops and tablets. DDR4 SO-DIMM has 260 pins, while DDR4 DIMM has 288 pins. Standard DIMMs are used in PCs and servers, while VLP DIMM is specially used for applications with space requirements like blade servers.
6. MicroDIMM
Lastly, a microDIMM refers to a memory module smaller than SO-DIMM. Its configurations include 144-pin SDRAM and 172-pin DDR. Such DIMMs are commonly observed in laptop PCs.
Let’s now discuss different DIMMs based on the RAM type:
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- SDRAM DIMM: These RAMs are synchronized with the clock speeds of the underlying microprocessor. As a result, there is a substantial increase in the number of instructions executed by the processor per unit time.
- SDR (Single data rate) DIMM: SDR refers to a configuration where the data packet is transferred only once per clock cycle. These DIMMs are typically used across Intel devices that tend to transfer data on the rising edge or falling edge of the clock signal.
- DDR (Double data rate) DIMM: DDR refers to a configuration where a data packet is transferred twice per clock cycle. DDR achieves a faster data rate by reading data on both the rising and falling edge of the clock signal.
- DDR2 DIMM: In DDR2 configuration, the bus has double the speed of memory cells. Hence, data transfer happens four times faster per memory cell cycle.
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Takeaway
DIMMs have become an inevitable part of the computing world. Personal and professional computers we use in our daily lives play a major role in driving the DIMM market. Additionally, DIMM’s critical applications also come from the aerospace and defense sector, where rugged RAMs are employed to withstand changing temperatures, pressure, and stress over a period of time. Several manufacturers design and develop DIMMs based on specific applications, needs, and related challenges.
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