The memory standard on middle-aged PCs (486s and early Pentiums) is the SIMM. The edge connector on a SIMM has either 30 pins or 72 pins. A SIMM's memory capacity ranges from 1MB to 16MB in either a one-sided or two-sided style, with chips soldered to one or two sides of the board. Because the Pentium processor uses a 64-bit path to memory, 32-bit SIMMs must be installed in pairs.
DIMM is the memory standard on most newer and larger PCs. Because its 64-bit memory matches the 64-bit data path of the Pentium processor, you only need to install one DIMM at a time. In comparison to the SIMM, a DIMM has 168 contact pins as opposed to the 30 and 72 pins of the SIMM.
A DIMM looks just like a SIMM, except that it is slightly larger, has memory chips on both sides, and has about twice as many contacts on its edge connector. See Section 1, 'Components for computer systems' for a photo of a DIMM and other memory.
DIMMs come in different voltages - 3.3V and 5.0V - and as buffered or unbuffered, which yields four possible combinations. A smaller version of the DIMM, the 100-pin Small Outline DIMM (SODIMM), which is used primarily in laptop computers, supports PC100 SDRAM. An even smaller DIMM package, the 144-pin MicroDIMM, which is used in sub-notebook portable computers, supports PC100 SDRAM.