SIMM

30- (top) and 72-pin (bottom) SIMMs. Early 30-pin modules commonly had either 256 KB or 1 MB of memory.

A SIMM, or single in-line memory module, is a type of memory module containing random access memory used in computers from the early 1980s to the late 1990s. It differs from a dual in-line memory module (DIMM), the most predominant form of memory module today, in that the contacts on a SIMM are redundant on both sides of the module. SIMMs were standardised under the JEDEC JESD-21C standard.

30-pin SIMM, 256kB capacity

Most early PC motherboards (8088-based PCs and XTs) used socketed DIP chips. With the introduction of 286-based PC/ATs, which could use larger amounts of memory, memory modules evolved to save motherboard space and to ease memory expansion. Instead of plugging in eight or nine single DIP DRAM chips, only one additional memory module was needed to increase the memory of the computer. A few 286-based computers used (often non-standard) memory modules like SIPP memory (single in-line pin package). The 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.

SIMMs were invented and patented by Wang Laboratories. One of Wang's scientists, James Clayton, invented what was to become the basic memory module, now known as a SIMM (single in-line memory module) in 1983. The first SIMMs appeared on the PS/2 in the mid 1980s, having been first proposed by Skip Coppola while at IBM. They solved several problems at the time, including shrinking motherboard real estate (they took up much less board space than socketed chips) as well as the effects of rapidly advancing memory capacities (a motherboard would quickly become obsolete based on its sockets for a particular RAM chip capacity) This also allowed the manufacturer (IBM at this time) to source RAM chips from different vendors and in different packaging, yet still allow them to be interchangeable through this intermediate form (the SIMM).

The first variant of SIMMs has 30 pins and provides 8 bits of data (9 bits in parity versions). They were used in 286, 386 and 486 systems.

The second variant of SIMMs has 72 pins and provides 32 bits of data (36 bits in parity versions). These appeared on 486, Pentium, Pentium Pro and even some Pentium II systems. By the mid 90s, 72-pin SIMMs had replaced 30-pin SIMMs.

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

DRAM technologies used in SIMMs include EDO and FPM.

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 general rule of thumb is a 286 or 386SX system (data bus width of 16 bits) would require two 30-pin SIMMs for a memory bank. On 386DX or 486 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. 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 and rotated until it locked into place. 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 SIMM must be tilted back and pulled out. 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^[1].

30-pin SIMMs

30-pin SIMM, 256 KiB capacity

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 MiB.

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

72-pin SIMMs

72-pin EDO DRAM SIMM

Standard sizes: 1 MiB, 2 MiB, 4 MiB, 8 MiB, 16 MiB, 32 MiB, 64 MiB, 128 MiB

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 MiB.

^* Pins 35, 36, 37 and 38 are not connected on non-parity SIMMs. ^[2]

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

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

^x Presence Detect signals are deteiled in JEDEC Standard.

Proprietary SIMMs

GVP 64-pin

Several CPU cards from Great Valley Products for the Commodore Amiga used special 64-pin SIMMs (32 bits wide, 1 or 4 MB, 60 ns).

Apple 64-pin

64-pin were used in Apple Macintosh IIfx computers to allow overlapping read/write cycles (1, 4, 8, 16 MB, 80 ns).^[3]

See also

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

References

1. ^ Making Standard SIMMs Work – Memory Upgrade on the HP LaserJet 6MP/5MP Article on fitting jumpers to add Presence Detect to standard SIMMs
2. ^ JEDEC Standard No. 21-C, Section 4.2.2 "72 pin SIMM DRAM Module Family"
3. ^ Macintosh IIfx

External links

• General SIMM Installation Guide