The SIMM, for Single Inline Memory Module, was the first popular modularized memory for consumer computers. The single part is because, unlike older 'DIP' memory expansions where each memory chip was separate, the SIMM design packaged them all together on a PCB for easy handling.

Single is also a bit of a misnomer because depending on the width of the bus they are used on, you may have to add them 2, 4 to even 8 at a time, because the bit width of the memory bank must match the width of the bus it's connected to.

There are two primary styles of SIMM, 30-pin (available in 256kb, 512kb, 1mb, 2mb, 3mb, 4mb and rarely 16mb sizes) and the 72-pin (ranging in size from 2mb, 4mb, 8mb, 16mb, 24mb, 32mb, 64mb and very rare 128mb).

30-pin SIMMs are 8-bits wide and were first used in late 8086 models, where they were installed one per bank, and as late as some early Pentium boards, where 8 were required in every bank.

72-pin SIMMs are 32-bits wide and are used in many 386s and 486s (One per bank) up to Pentium MMX and some Pentium II boards (2 per bank).

Some SIMMS are also be what are called 'parity' SIMMs, meaning that have an extra parity bit per 8-bit block for error checking; this means that a 30-pin parity SIMM is in actually 9-bits wide while a 72-pin parity SIMM is 36-bits wide.

SIMMS have, in general, been largely supplanted by DIMMs and RIMMs.

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