A SIMM is an older type of memory module used for RAM in personal computers. SIMM stands for Single In-line Memory Module. It differs from a DIMM (the most predominant form of memory module today) in that the contacts on a SIMM are redundant on both sides of the module.

It is the successor of the SIPP (Single In-line Pin Package) which was used in 80286 systems. Its 30 pins often bent or broke during installation. That's why shortly after contact plates were used in favor of pins.

Memory modules evolved from the need of saving motherboard space and effortless memory expansion. Instead of plugging 8 or 9 single DRAM chips, you could now simply add one memory module to increase the memory of the computer.

The first variant of SIMMs has 30 pins and provides 8 bits of data (9 bits in parity versions).

The second variant of SIMMs - also called PS/2 after the IBM line of computers in which it was first used - has 72 pins and provides 32 bits of data (36 bits in parity versions).

Due to the differing data bus widths of the memory modules and some processors, sometimes several modules must be installed in pairs to fill a memory bank. For instance, on 80386 or 80486 systems (data bus width of 32 bits) either four 30-pin-SIMMs or one 72-pin-SIMM are required for one memory bank. On Pentium systems (data bus width of 64 bits) two 72-pin-SIMMs are required.

To install a SIMM, the module must be placed in the socket at an angle, then rotated (angled) into position. To remove one, the two metal or plastic clips at each end must be pulled to the side, then the chip must be tilted back and pulled out.

RAM technologies used on SIMMs include EDO and FPM.

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