Memory comes in a variety of sizes and shapes. In
general, it looks like a flat green stick with little black cubes on it.
DRAM (DYNAMIC RANDOM ACCESS MEMORY)
DRAM is the most common form of RAM. It's called "dynamic" RAM because it can
only hold data for a short period of time and must be refreshed periodically.
Most memory chips have black or chrome coating, or packaging, to
protect their circuitry.
SIMMS
The term SIMM stands for Single In-Line Memory Module. With
SIMMs, memory chips are soldered onto a modular printed circuit board (PCB),
which inserts into a socket on the system board.
The first SIMMs transferred 8 bits of data at a time. Later, as CPUs began to
read data in 32-bit chunks, a wider SIMM was developed, which could supply 32
bits of data at a time. The easiest way to differentiate between these two
different kinds of SIMMs was by the number of pins, or connectors. The earlier
modules had 30 pins and the later modules had 72 pins. Thus, they became
commonly referred to as 30-pin SIMMs and 72-pin SIMMs.
Another important difference between 30-pin and 72-pin SIMMs is that 72-pin
SIMMs are 3/4 of an inch (about 1.9 centimeters) longer than the 30-pin SIMMs
and have a notch in the lower middle of the PCB. The graphic below compares
the two types of SIMMs and indicates their data widths.
4-1/4"
72-Pin SIMM
3-1/2"
30-Pin SIMM
DIMMS
Dual In-line Memory Modules, or DIMMs, closely resemble SIMMs. Like
SIMMs, most DIMMs install vertically into expansion sockets. The principal
difference between the two is that on a SIMM, pins on opposite sides of the
board are "tied together" to form one electrical contact; on a DIMM, opposing
pins remain electrically isolated to form two separate contacts.
168-pin DIMMs transfer 64 bits of data at a time and are typically used in
computer configurations that support a 64-bit or wider memory bus.
Some of the
physical differences between 168-pin DIMMs and 72-pin SIMMs include:
- the length of module, the number of notches
on the module
- the way the module installs in the socket
- many 72-pin SIMMs install
at a slight angle, whereas 168-pin DIMMs install straight into the memory
socket and remain completely vertical in relation to the system motherboard.
The illustration below compares a 168-pin DIMM to a 72-pin SIMM.
4-1/4"
72-Pin SIMM
5-1/4"
168-Pin DIMM
SIMM
DIMM
SODIMM
All about Memory
