Short for Single In-line Memory Module, SIMM is a memory module developed by Wang laboratories in 1983. The SIMM circuit board that holds six to nine memory chips per board, the ninth chip usually an error checking chip (parity or non parity). The SIMM was used with computers using a 486, early Intel Pentium, and compatible processors. However, because the Pentium is 64-bit and a SIMM is only 32-bits wide, they must be installed two at a time when used with any 64-bit processor.
Today, the SIMM is rarely used and have have been replaced by DIMMs. Below is a graphic illustration of a 4MB SIMM as well as a diagram pointing out the important features of a SIMM.