A SIMM (single in-line memory module) is a type of memory module used in computers from the early 1980s to the early 2000s. It is a printed circuit board on which has random-access memory attached to one or both sides.^[1] It differs from a dual in-line memory module (DIMM), the most predominant form of memory module since the late 1990s, in that the contacts on a SIMM are redundant on both sides of the module. SIMMs were standardised under the JEDEC JESD-21C standard.

Most early PC motherboards (8088-based PCs, XTs, and early ATs) used socketed DIP chips for DRAM. As computer memory capacities grew, memory modules were used to save motherboard space and ease memory expansion. Instead of plugging in eight or nine single DIP chips, only one additional memory module was needed to increase the memory of the computer.

SIMMs were invented in 1983 by James E. Clayton^[2] at Wang Laboratories with subsequent patents granted in 1987.^{[3] [4]} Wang Laboratories litigated both patents against multiple companies.^{[5][6][7][8][9]} The original memory modules were built upon ceramic substrates with 64K Hitachi "flip chip" parts and had pins, i.e. single in-line package (SIP) packaging.^[2] SIMMs using pins are usually called SIP or SIPP memory modules to distinguish them from the more common modules using edge connectors.

The first variant of SIMMs has 30 pins and provides 8 bits of data (plus a 9th error-detection bit in parity SIMMs). They were used in AT-compatible (286-based, e.g., Wang APC^[10]), 386-based, 486-based, Macintosh Plus, Macintosh II, Quadra, Atari STE microcomputers, Wang VS minicomputers and Roland electronic samplers.

The second variant of SIMMs has 72 pins and provides 32 bits of data (36 bits in parity and ECC versions). These appeared first in the early 1990s in later models of the IBM PS/2, and later in systems based on the 486, Pentium, Pentium Pro, early Pentium II, and contemporary/competing chips of other brands. By the mid-90s, 72-pin SIMMs had replaced 30-pin SIMMs in new-build computers, and were starting to themselves be replaced by DIMMs.

Non-IBM PC computers such as UNIX workstations may use proprietary non-standard SIMMs. The Macintosh IIfx uses proprietary non-standard SIMMs with 64 pins.

DRAM technologies used in SIMMs include FPM (Fast Page Mode memory, used in all 30-pin and early 72-pin modules), and the higher-performance EDO DRAM (used in later 72-pin modules).

Due to the differing data bus widths of the memory modules and some processors, sometimes several modules must be installed in identical pairs or in identical groups of four to fill a memory bank. The rule of thumb is a 286, 386SX, 68000 or low-end 68020 / 68030 (e.g. Atari Falcon, Mac LC) system (using a 16 bit wide data bus) would require two 30-pin SIMMs for a memory bank. On 386DX, 486, and full-spec 68020 through 68060 (e.g. Atari TT, Amiga 4000, Mac II) systems (32 bit data bus), 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. However, some Pentium systems have support for a "half bank mode", in which the data bus would be shortened to only 32 bits to allow operation of a single SIMM. Conversely, some 386 and 486 systems use what is known as "memory interleaving", which requires twice as many SIMMs and effectively doubles the bandwidth.

The earliest SIMM sockets were conventional push-type sockets. These were soon replaced by ZIF sockets in which the SIMM was inserted at an angle, then tilted into an upright position. To remove one, the two metal or plastic clips at each end must be pulled to the side, then the SIMM must be tilted back and pulled out (low-profile sockets reversed this convention somewhat, like SODIMMs - the modules are inserted at a "high" angle, then pushed down to become more flush with the motherboard). The earlier sockets used plastic retainer clips which were found to break, so steel clips replaced them.

Some SIMMs support presence detect (PD). Connections are made to some of the pins that encode the capacity and speed of the SIMM, so that compatible equipment can detect the properties of the SIMM. PD SIMMs can be used in equipment which does not support PD; the information is ignored. Standard SIMMs can easily be converted to support PD by fitting jumpers, if the SIMMs have solder pads to do so, or by soldering wires on.^[11]

Standard sizes: 256 KB, 1 MB, 4 MB, 16 MB

30-pin SIMMS have 12 address lines, which can provide a total of 24 address bits. With an 8 bit data width, this leads to an absolute maximum capacity of 16 MB for both parity and non-parity modules (the additional redundancy bit chip usually does not contribute to the usable capacity).

^* Pins 26, 28 and 29 are not connected on non-parity SIMMs.

Standard sizes: 1 MB, 2 MB, 4 MB, 8 MB, 16 MB, 32 MB, 64 MB, 128 MB (the standard also defines 3.3 V modules with additional address lines and up to 2 GB)

With 12 address lines, which can provide a total of 24 address bits, two ranks of chips, and 32 bit data output, the absolute maximum capacity is 2²⁷ = 128 MB.

^* Pins 35, 36, 37 and 38 are not connected on non-parity SIMMs. ^[12]

^†/RAS1 and /RAS3 are only used on two-rank SIMMS: 2, 8, 32, and 128 MB.

^# These lines are only defined on 3.3V modules.

^x Presence Detect signals are detailed in JEDEC Standard.

• Dual in-line package (DIP)
• Single in-line package (SIP)
• Zig-zag in-line package (ZIP)
• Dual in-line memory module (DIMM)