SIMM (single in-line memory module), invented and patented by Wang Laboratories, is a memory module containing random access memory. The contacts on a SIMM module are redundant on both sides, differentiating it from a dual in-line memory module (DIMM), presently the most popular form of memory module. Occasionally several modules must be installed in identical pairs or groups of four to fill a memory bank due to mismatching widths of both the memory modules and some processors’ data busses. All SIMMs were standardized through the JEDEC JESD-21C standard.
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Wang invented what was to become the basic memory module, now known as a SIMM in 1983. The original modules had pins (later removed) and were built upon ceramic (later updated to standard PCB material). The earliest SIMM sockets were traditional push-type sockets, which were soon superceded by ZIF sockets. These were inserted and rotated until locked in place.
Nearly all early PC motherboards used socketed DIP chips. When 286-based IBM XT/286 was introduced, it boasted larger amounts of memory, evolving modules to save motherboard space and easing memory expansion. Just a single additional memory module was required to increase the memory of a computer instead of eight or nine single DIP DRAM chips. Some 286-based computers used SIPP memory (single in-line pin package) that utilized pins that often bent or broke during installation, which was rapidly replaced by SIMMS which used contact plates instead of pins.
Variant I SIMMs, configured with 30 pins and yielding 9 bits of data were utilized in 286, 386, 486, Macintosh Plus, Macintosh II, Quadra,Atari STE and Wang VS systems. Variant II SIMMs had 72 pins and yielded 32 bits (36 bits in parity versions) of data. These first appeared on the IBM PS/2, 486, Pentium, Pentium Pro and some Pentium II systems. 72-pin SIMMs had succeeded 30-pin SIMMs by the mid 90s. UNIX workstations, Macintosh IIfx (proprietary non-standard SIMMs with 64 pins), and other non-IBM PC computers may utilize proprietary non-standard SIMMs.
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