SIMM

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Image:SIMMs.jpg A SIMM, or single in-line memory module, is a type of memory module used for random access memory in personal computers. 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.

Most early PC motherboards (8088 based PCs and XTs) used socketed DIP chips. With the introduction of 80286 based PC/ATs, which could use larger amounts of memory, memory modules evolved from the need of saving motherboard space and to ease 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. A few 80286-based computers used (often non-standard) memory modules like SIPP memory (single in-line pin package). SIPP's 30 pins often bent or broke during installation, which is why they were quickly replaced by SIMMs which used contact plates rather than pins.

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).

Macintosh IIx and Macintosh IIfx has non-standard 64-pin SIMMs.

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.

SIMM is standardised under JEDEC JESD-21C standard.

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