stands for Single In-line Memory Module. In SIMMs, the pins on the front and
back are electrically connected so that they provide a single line of
communication channel between module and the motherboard. SIMMs generally come
in 30- or 72-pin PCB. These modules have a bandwidth transfer of 8 or 32 bits and were designed
for use in very early PCs usually with up to 486 processors. On Pentium classic and higher processors with 64-bit bus,
32-bit SIMMs should always be installed in pairs. If not, additional memory will
be ignored or the system may not boot at all.
DIMM stands for Dual In-line Memory Module. In a DIMM, the opposing pins remain electrically isolated to form two separate
contacts enabling a 64-bit data transfer. This makes it possible to
install modules individually. Early DIMMs have 72 pins, but commonly
they have 168 pins for desktop PCs and 144 pins for laptops. Very recent processors, however,
require faster transfer rate, hence the advent of DDR DIMMs. These modules have 184 or 200 pins.
(See ff. links for relative information - SDR vs. DDR and
DIMMs and SIMMs are not designed to be interchangeable.
Their sizes are different and they install into different socket types.