What is a dual in-line memory module (DIMM)?

DIMMs connect to a computer’s motherboard through a double-sided pin connection, enabling a native 64-bit data path throughput that is inherently faster and more efficient than previous types of RAM data transfer hardware, such as single in-line memory modules (SIMM).

DIMMs are available in a variety of configurations and form factors, most of which are standardized through the Joint Electron Device Engineering Council (JEDEC) to fit into typical DIMM slots–with personal computers (PC) typically requiring a standard 133.35 mm (5.25 in) DIMM and laptops requiring a smaller 67.6 mm (2.66 in) small outline dual in-line memory module (SO-DIMM). In addition to the component’s physical dimensions, DIMMs are also available in a wide range of different types of RAM.

While most modern workstations use DIMM memory chips, the specific type of DIMM best suited for any given computer is dependent on the physical constraints of the hardware and its intended application.

Essentially, a DIMM is a type of RAM module that uses a specific type of pin connector to add multiple RAM chips to a computer system in such a way that efficiently increases central processing unit (CPU), data transfer and throughput speeds without increasing power consumption. Computer systems use RAM to temporarily store data that is currently being used to perform real-time operations. Demanding applications, such as rendering digital video or online gaming, require a lot of RAM. Computer systems with insufficient RAM run slowly or time out.

Generally, fast, more expensive forms of data storage, like RAM, are referred to as memory, while stable, cheaper storage hardware or components are referred to as storage. Computers use storage to hold most data, especially things like application files, documents and/or media that may not be currently needed. Computers use memory, or RAM, to access and manage data and files that are relevant or necessary to moment-to-moment activities and functions.

Most RAM is considered to be a volatile form of memory because it requires constant electricity to store data and will lose all stored data should the system lose power. That’s why computers use non-volatile forms of memory that don’t require constant power, such as solid-state hard drives, for long-term storage.

The two main types of RAM are static random access memory (SRAM) and dynamic random access memory (DRAM). Developed in the early 1960s, SRAM technology uses transistors to store data, which is fast and effective but bulky and expensive. However, in 1968, IBM researcher Robert Dennard made one of modern computing’s most significant breakthroughs when he invented what would become the first DRAM chips developed by Intel in 1970—an innovation that so tremendously increased RAM functionality its impact is still felt today. While SRAM-type memory cells are still used for some select purposes, DRAM has become so dominant as to be nearly synonymous with RAM, although there are also many sub-categories of DRAM chips, as well.

The main innovation of a dual in-line memory module (DIMM) as compared to a single in-line memory modules (SIMM) is the double-sided pin connector.

With a SIMM, RAM chips are shorted together and only pass data through one side of the module. DIMM RAM, however, can achieve a double data rate of throughput by utilizing connector pins on either side of the module.

As the maximum data storage offered by a SIMM is 32-bit per clock cycle, SIMM modules are used in pairs to achieve a standard 64-bit data path transfer rate, with a 5-volt voltage consumption per SIMM. SIMMs offer 4 MB to 64 MB of data storage. As stated, SIMMs have connectors on only one side of the circuit board.

By doubling the number of connectors, DIMMs effectively double the capacity of SIMMs, requiring only 3.3 volts. This innovation does require a specialized DIMM slot in the computer’s motherboard, as DIMM is not backward compatible with SIMM slots. However, DIMM style memory has become the go-to solution for adding memory to most modern computer systems, as a single DIMM unit 32 MB to 1 GB of storage with greater energy efficiency.

In addition to the signature double-sided pin connector, most modern units share a number of beneficial qualities that make DIMM well-suited for many various types of computing.

Apart from size, speed, and capacity, DIMM varieties are also differentiated by unique functional features of the DIMM itself, as well as by the type of RAM chips utilized.

• Unbuffered DIMMs (UDIMMs): As the name implies, unbuffered DIMMs have no memory buffer and operate by communicating directly with the memory controller located in the CPU. UDIMMs are known for cost-effective speed and are frequently used in desktop and laptop computers.
• Fully buffered DIMMs (FB-DIMMs): Unlike UDIMMs, FB-DIMMs feature an advanced memory buffer (AMB) to facilitate communication between the memory module and the memory controller. The AMB bus divides operations into two parts—reading and writing—and can perform both functions simultaneously for better performance. FB-DIMMs offer improved reliability, signal integrity and error detection speeds, making them a favored choice for servers and workstations that demand larger memory capacity.
• Registered DIMMs (RDIMMs): Named for the additional memory registers positioned between the memory controller and the memory module, RDIMMs are also referred to as buffered memory and are well-suited for servers and other systems requiring robust stability. RDIMMs buffer commands, addresses and clock cycles from the CPU and direct instructions to specific memory registers, reducing the load on the memory controller.
• Load-reduced DIMMS (LR-DIMMs): Another sub-category of buffered DIMM, LR-DIMMs feature an isolation memory buffer (iMB) to reduce CPU strain and achieve improved speeds and capacity by separating the DIMM’s DRAM chips from the main CPU. Instead of communicating with the DRAM directly, the memory controller sends instructions to the iMB chip, then the buffered memory performs all operations.

• Synchronous dynamic RAM (SDRAM)/Single data rate (SDR): The term SDR SDRAM is often shortened to SDRAM, as these types of RAM are synonymous. SDRAM synchronizes operations with the underlying microprocessor’s clock speed, resulting in substantial increases in the DIMMs' capacity for executable instructions per clock unit time. While asynchronous DRAM memory responds to CPU input immediately, SDRAM waits for the clock signal before executing instructions. This method, known as “pipelining,” allows SDRAM to receive (read) new orders before the previous instructions have been fully completed (write). As a result, the CPU can process overlapping orders simultaneously, executing one read and one write function per clock cycle—resulting in higher overall CPU transfer and performance rates.
• Double data rate (DDR): DDR SDRAM functions like SDR SDRAM, with twice the speed. DDR SDRAM processes two read and two write instructions per clock cycle and also functions at a lower standard voltage—2.5 volts vs 3.3 volts.
• Double data rate 2 (DDR2): An improvement on DDR SDRAM, this type of RAM also makes two read and write functions per clock cycle, but with support for higher clock speeds which results in faster performance. Whereas standard DDR ROM modules max out at 200 MHz, DDR2 memory can achieve 533 MHz, the additional bonus of only requiring 1.8 volts.
• Double data rate 3 (DDR3): The next evolution of DDR2, DDR3 uses advanced signal processing for improved reliability, memory capacity, and lower power consumption (1.5 volts).
• Double data rate 4 (DDR4): A further improvement over DDR3, improvements to signal processing grant DDR4 even better capacity, performance, and lower power consumption (1.2 volts) with higher clock speeds up to 1600 MHz.

Compared to a SIMM, the dual-channel DIMM architecture makes dual in-line memory modules twice as functional as their predecessors.

Furthermore, DIMMs offer many current-generation advantages, making DIMMs the go-to solution for most modern computing systems, which are designed with DIMM slots to support two, four, six or eight individual DIMMs. DIMM buffers help process CPU signals to reduce memory workloads while the dual-channel design allows for the spreading of data across memory modules for fast interleaving of multiple requests. For especially demanding use cases, triple- and quad-channel DIMMs are also available. From personal computing to demanding data centers, advanced DIMM solutions enable cutting-edge computing.