SIMMs

SIMMs, or single inline memory modules, are small circuit boards that contain DRAM chips and plug into dedicated sockets on a motherboard. The term “single inline” refers to the memory data lines being the same on both sides of the module, a design later superseded by DIMMs, which use dual inline data paths. SIMMs were widely used in personal computers and servers before DIMMs became the dominant form factor.

Two common physical formats were used for SIMMs: 30-pin and 72-pin. The 30-pin form factor appeared first and was typical of older systems with smaller memory bandwidth, while the 72-pin format provided a wider data path and higher capacities and became standard in many PCs during the 1990s. SIMMs came in various memory technologies and error-checking options, including FPM and EDO RAM, with both parity (or ECC in some cases) and non-parity variants. Capacities for 72-pin modules typically ranged from a few megabytes up to several dozen megabytes per module, with higher densities appearing later in the era.

Compatibility depended on the motherboard’s supported type, speed, and capacity. SIMMs were installed in banks or sets that matched the system’s memory controller requirements, and some boards required modules to be paired or aligned to maintain the proper bus width and timing. They were not typically hot-swappable and required powering down the machine for installation or removal.

Today SIMMs are largely obsolete, replaced by DIMMs and newer memory technologies. They remain of historical interest for vintage systems and early-era computers, where they played a key role in expanding system memory during the 1990s.