Memory chips are generally packaged into small plastic or ceramic dual inline packages (DIPs) which are themselves assembled into a memory module. The single inline memory module or SIMM is a small circuit board designed to accommodate surface-mount memory chips. SIMMs use less board space and are more compact than previous memory-mounting hardware.

Memory modules evolved from the need of saving motherboard space and effortless memory expansion. Instead of plugging in 8 or 9 single DRAM DIP chips, only one additional memory module was needed to increase the memory of the computer. The SIMM was the successor of SIPP memory (Single In-line Pin Package), which was used in 80286 systems. SIPP’s 30 pins often bent or broke during installation, leading to a SIMM’s contact plate design.

By the early 1990s the original 30-pin SIMM had been superseded by the 72-pin variety. These supported 32-bit data paths, and were originally used with 32-bit CPUs.

A typical motherboard of the time offered four SIMM sockets capable of taking either single-sided or double-sided SIMMs with module sizes of 4, 8, 16, 32 or even 64MB. With the introduction of the Pentium processor in 1993, the width of the data bus was increased to 64-bits. When 32-bit SIMMs were used with these processors, they had to be installed in pairs, with each pair of modules making up a memory bank. The CPU communicated with the bank of memory as one logical unit.