Some History

A SIP - though not a memory chip - this is some kind of amplifier

Over time, memory has come in various shapes and sizes. Suffice to say the main reason that you need to buy the right format is that it won’t fit if you don’t!

A DIP chip

A DIP chip

Back in the mists of time, memory came as loose chips, normally in what are called DIPs or SIPs  – Dual in Line Package or Single in Line Package – which were plugged either directly on to the motherboard or into a large board plugged into an ISA slot or the like. On some machines (for example the Atari ST) the DIPs had to be soldered directly to the board which was a laborious and error prone process.

Later, manufacturers decided that the ability to easily add and remove memory would be beneficial for both them and their users – it would allow the manufacturers to assemble the machines more quickly and offer more configurations, and would permit users to easily add more memory later.

A 30 pin SIMM

Therefore, after a bit of faffing about with modules such as SIPPs (not to be confused with SIPs),  a simple standard for a “module” that would support a common interface was adopted – the 30 pin SIMM – the Single Inline Memory Module.

A 72 pin SIMM

As time went on the memory busses on computers became wider and more than one SIMM was required to support the amount of memory the computer could see. By the time of the 486, four SIMMs at a time would have to be installed, and with the advent of x8 chips, it was becoming silly to have so many SIMMs on a motherboard. Therefore the 72pin SIMM was invented. This, in effect, is four 30-pin SIMMs rolled into one, and supports a width of 32 or 36 bits rather than 8 or 9 (see previous post on parity and ECC).

Around about the same time that the industry moved to SDRAM from EDO, a similar move was made to move to 64 bit widths, mainly due to the larger numbers of ancilliary (non-data) connections required to talk to SDRAM. This required a move to a Dual Inline design as fitting 168 pins along the bottom of the module at 5V would have required a memory module nearly seven inches across!

Over time, more pins have been added as the voltage has dropped and more separation has been required between data signals, but essentially the format has remained similar.

What’s the difference between a SIMM and a DIMM?

A DIMM

A single inline module, although having pins on both sides of the module, effectively only has one set of pins as the two sides are connected together, whereas a DIMM has totally separate sets of pins on the two sides and hence can fit twice as many pins in the same space.

So what’s a SODIMM then?

DDR2 SODIMM

A SODIMM or Small Outline DIMM is just a smaller form factor of memory made to fit in confined spaces. In order to save on space, it doesn’t have the pins to support functions like ECC and parity and tends to have smaller gaps between the pins.

Over time, SODIMMs have been available in 144pin (SDRAM), 200pin (DDR/DDR2) and 204pin (DDR3) formats, though other, specialist types like 100pin (Printer DIMMs) and 172pin “MicroDIMMs” (even smaller) also exist.

And a RIMM?

A RIMM is just a trade name for a specific type of DIMM which carries RAMBUS memory. There’s nothing special about them except that they are totally incompatible with any other kind of memory.

So what are the notches for?

Most DIMMs have a notch or notches in the bottom between the pins. These serve three purposes:

1. To prevent the wrong memory being plugged into a slot where it could damage the computer or the memory.

2. To identify the memory technology. On 168 and 240pin DIMMs and 200pin SODIMMs, the notch identifies the type of memory – EDO or SDRAM for 168pin, and DDR3, DDR2 or DDR for the 240pin and 240pin parts.

3. To identify the voltage on 168pin DIMMs and 144pin SODIMMs as this changed between 5V and 3.3V without changing technologies. On newer parts, voltage changes were made at the same time as the technology changed.

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