Single Inline Memory Module (SIMM)
The first implementation of this new packaging was called a Single In-line Memory Module, or SIMM. These packages had a series of DIP chips soldered into a small circuit board with a row of tin or gold contact points on one edge. The first SIMM packaging used 30 pins, with later models using 72 pins. The 30-pin modules supported in 8-bit wide address bus, whereas the 72-pin modules supported a 32-bit wide address bus. Therefore, in a Pentium class system using 72-pin SIMMs, you had to install the memory modules in matching pairs. This is because the Pentium class processor has a 64-bit data bus.
One easy way to Identify a 72-pin SIMM is the notch that separates the two sets of connectors on the bottom edge of the module (see Figure 45 below).
To install a SIMM module, place the bottom edge of the module into the socket at a 45° angle, and rotate the module until is at a right angle to the motherboard. When the module is in place, you will hear a small clicking noise as the two metal spring clips pop into place in front of the module, holding it in place.
Due to the limitations of most chipsets and the overall real estate that a socket takes up on the motherboard, the industry created a new memory packaging technology, called a Dual In-line Memory Module, or DIMM.
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