Random access memory (RAM) functions as temporary data storage for the central processing unit (CPU). Common instructions are stored in RAM and can be retrieved in any order to speed up processes. The evolution of RAM technology has resulted in a variety of memory modules. With each new type of memory module, new module slots were designed for motherboards to take advantage of the latest types of RAM.
Single inline memory modules (SIMMs) were the standard type of RAM module used during the 1980s and 1990s. The modules were widely available in 30-pin and 72-pin models with 64-pin models used in proprietary systems. 30-pin SIMMs were available in 256 KB (kilobyte), 1 MB (megabyte), 4 MB and 16 MB capacities while the greater number of pins allowed 72-pin modules to store between 1 MB and 128 MB of data. The later memory modules had a 32-bit data path, so SIMMs of identical capacity were installed in pairs to synchronize with processors using 64-bit data paths.
The term "pin" was used to describe the needle-like contact points of early modules. The original pins often broke or were bent during installation, so they were replaced with more durable flat contact plates.
The memory module slot for SIMM RAM came in varying sizes depending on the number of pins it was meant to receive. 30-pin slots were shorter and typically used in laptops while the 64-pin and 72-pin slots were found on desktop motherboards. SIMM RAM had to be installed at an angle and pushed upright into place. There was a notch on both ends of the memory module that was clipped by arms on opposite ends of the slot.
Dual inline memory modules (DIMMs) are the standard type of RAM used in modern computers. The "dual" part of the name comes from the separate electrical contact points designed on both sides of the module. DIMMs are designed to allow for 64-bit data paths and thus, unlike SIMMs, do not have to be installed in pairs. The multiple contact points and the greater number of pins (between 72 and 240) allow computers with DIMMs to have greater memory capacity and faster access speed than computers with SIMMs.
As with module slots for SIMMs, slots for DIMMs feature clips at opposite ends to hold the memory module in place. Unlike the SIMM slots, DIMM slots allow for straight-froward installation of memory modules. Modules can be pressed straight into the slot and clipped into place. Because there is a wide variety of pin sizes for DIMMs, the slots likewise vary in size.
Rambus inline memory modules (RIMM) was the California-based company Rambus' answer to SIMMs and DIMMs. RIMMs featured faster clock cycles and greater bandwidth than SIMMs and early DIMMs, but suffered from latency, over-heating and high production costs. Also like SIMMs, RIMMs had to be installed in pairs. The arrival of double-rate synchronous dynamic random access memory (DDR SDRAM) DIMMs with far greater bandwidth and clock cycle rates while remaining cost effective eventually knocked RIMM out of competition.
The module slots for RIMMs were physically similar to DIMM slots and the modules were installed in the same way. However, every slot on the board had to hold a RIMM to form a working memory bank. If a computer had three module slots, a dummy module had to be ordered then installed to complete the memory bank. Where a pair of SIMMs could function in a three slot motherboard, RIMMs could not.