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Ultimate Memory Guide
DIFFERENT KINDS OF MEMORY
Some people like to know a lot about the computer systems they own - or are considering buying - just because. They're like
that. It's what makes them tick. Some people never find out about their systems and like it that way. Still other people - most
of us, in fact - find out about their systems when they have to - when something goes wrong, or when they want to upgrade it.
It's important to note that making a choice about a computer system - and its memory features - will affect the experience and
satisfaction you derive from the system. This chapter is here to make you smarter about memory so that you can get more
out of the system you're purchasing or upgrading.
MODULE FORM FACTORS
The easiest way to categorize memory is by form factor. The form factor of any memory module describes its size and pin
configuration. Most computer systems have memory sockets that can accept only one form factor. Some computer systems
are designed with more than one type of memory socket, allowing a choice between two or more form factors. Such designs
are usually a result of transitional periods in the industry when it's not clear which form factors will gain predominance or be
more available.
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
Comparison of a 30-pin and a 72-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.
DIMMs come in various form factors and are specific to different DRAM technologies.
168-pin DIMM: EDO and PC66/100/133 SDRAM
184-pin DIMM: DDR 200/266/333/400 DDR SDRAM