A single in-line memory modules ( SIMM short ) (English for: einzelreihiger memory module ) is a design of memory modules has been developed primarily for use as a memory in computers. SIMMs were built for use with Fast Page Mode DRAM ( FPM) and later Extended Data Output RAM ( EDO RAM) and are now found only in devices such as printers or Cisco routers are used. Compared to the at home computers and even at 80286 - each and 80386 systems widely used memory chips allow SIMMs significantly simplified assembly, a smaller footprint, higher reliability and lower costs. In later systems 80386 and especially 80486 SIMMs systems are widely used. In the PC they have been since about 1993 displaced by PS/2-SIMM-Module. The transition took place in the late 80486 systems. On motherboards from the transition period, there are some both four SIMM slots as well as two or three PS/2-SIMM-Steckplätze. The last 80486 motherboards have only two to four PS/2-SIMM-Steckplätze.


Single Inline Memory Modules come in three types of steps:

  • For 8-bit data bus width initially a module with 30 contact pins is defined. In PC systems with 80286 - or 80386SX processors a 16-bit wide memory interface is realized with these modules typically by getting two parallel driven 8 -bit modules are installed in pairs in the system. Computer with 32-bit wide memory bus, such as the 80486 system, require four such modules for each memory bank. The module is approximately 90 mm ​​wide and 18 mm high. Thus, it can not be inserted the wrong way round, it is provided on one side with a recess ( See picture).
  • Common only in the FPM version, otherwise identical electrically, work SIPP modules ( Single Inline Pin Package). Instead of the contact surfaces of these are provided with pins for fitting in socket connectors. Because there is no mechanical coding in this version, they can also be the wrong way round or offset installed and thus destroyed when switching on the computer. Frequently, but not always, SIPP modules could be used after desoldering the leads in SIMM sockets. In the other direction, there are adapters that allow SIMM modules could be used in SIPP slots.
  • For 32 -bit data width later PS/2-SIMM-Modul with 72 contact pins is developed which allows a considerable saving of space. In the transition period could be employed with the help of a so-called Simmshuttle four or eight SIMM modules in a PS/2-Steckplatz.

Unlike DIMM modules that are pushed with the exception of SO- DIMMs always perpendicular to the motherboard SIMM modules are easily inserted obliquely without forcing it into the socket and secured by folding over a spring latch.

Memory sizes

30 -pin SIM and SIPP modules are common in 256 KB, 1 MB and 4 MB variants, 16 - MB - variants are not widely used because of their high price at that time. The 72 -pin PS/2-SIMMs are available in sizes from 1 MB to 128 MB per module, wherein the sizes of 4 MB, 8 MB, 16 MB and 32 MB are the most widespread.