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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
As previously mentioned, 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.
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, and the way the module installs in
the socket. Another difference is that 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" 70-Pin SIMM
5-1/4" 168-Pin DIMM
Comparison of a 72-pin SIMM and a 168-pin DIMM.
SO DIMMS
A type of memory commonly used in notebook computers is called SO DIMM or
Small Outline DIMM. The principal difference between a SO DIMM and a DIMM is
that the SO DIMM, because it is intended for use in notebook computers, is
significantly smaller than the standard DIMM. The 72-pin SO DIMM is 32 bits wide
and the 144-pin SO DIMM is 64 bits wide.
2.35" 72-pin SO DIMM 2.66" 144-Pin SO DIMM
Comparison of a 72-pin SO DIMM and a 144-pin SO DIMM.
RIMMS AND SO-RIMMS
RIMM is the trademarked name for a Direct Rambus memory module. RIMMs look
similar to DIMMs, but have a different pin count. RIMMs transfer data in 16-bit chunks.
The faster access and transfer speed generates more heat. An aluminum sheath, called
a heat spreader, covers the module to protect the chips from overheating.
A 184-pin Direct Rambus RIMM shown with heat spreaders pulled away.
An SO-RIMM looks similar to an SO DIMM, but it uses Rambus technology.
A 160-pin SO-RIMM module.
FLASH MEMORY
Flash memory is a solid-state, non-volatile, rewritable memory that functions like
RAM and a hard disk drive combined. Flash memory stores bits of electronic data
in memory cells, just like DRAM, but it also works like a hard-disk drive in that
when the power is turned off, the data remains in memory. Because of its high
speed, durability, and low voltage requirements, flash memory is ideal for use
in many applications - such as digital cameras, cell phones, printers, handheld
computers, pagers, and audio recorders.
PC CARD AND CREDIT CARD MEMORY
Before SO DIMMs became popular, most notebook memory was developed using
proprietary designs. It is always more cost-effective for a system manufacturer to use
standard components, and at one point, it became popular to use the same "credit
card" like packaging for memory that is used on PC Cards today. Because the modules
looked like PC Cards, many people thought the memory cards were the same as PC
Cards, and could fit into PC Card slots. At the time, this memory was described as
"Credit Card Memory" because the form factor was the approximate size of a credit
card. Because of its compact form factor, credit card memory was ideal for notebook
applications where space is limited.
PC Cards use an input/output protocol that used to be referred
to as PCMCIA (Personal Computer Memory Card International Association). This standard
is designed for attaching input/output devices such as network adapters, fax/modems,
or hard drives to notebook computers. Because PC Card memory resembles the types of
cards designed for use in a notebook computer's PC Card slot, some people have
mistakenly thought that the memory modules could be used in the PC Card slot. To
date, RAM has not been packaged on a PCMCIA card because the technology doesn't
allow the processor to communicate quickly enough with memory. Currently, the most
common type of memory on PC Card modules is Flash memory.
On the surface, credit card memory does not resemble a typical memory module
configuration. However, on the inside you will find standard TSOP memory chips.
This section presents the most common memory technologies used for main memory:
This road map offers an overview of the evolution of memory.
| YEAR INTRODUCED |
TECHNOLOGY |
SPEED LIMIT |
| 1987 |
FPM |
50ns |
| 1995 |
EDO |
50ns |
| 1997 |
PC66 SDRAM |
66MHz |
| 1998 |
PC100 SDRAM |
100MHz |
| 1999 |
RDRAM |
800MHz |
| 1999/2000 |
PC133 SRAM |
133MHz (VCM option) |
| 2000 |
DDR SDRAM |
266MHz |
MAJOR CHIP TECHNOLOGIES
It's usually pretty easy to tell memory module form factors apart because of physical
differences. Most module form factors can support various memory technologies so,
it's possible for two modules to appear to be the same when, in fact, they're not. For
example, a 168-pin DIMM can be used for EDO, Synchronous DRAM, or some
other type of memory. The only way to tell precisely what kind of memory a module
contains is to interpret the marking on the chips. Each DRAM chip manufacturer has
different markings and part numbers to identify the chip technology.
FAST PAGE MODE (FPM)
At one time, FPM was the most common form of DRAM found in computers. In
fact, it was so common that people simply called it "DRAM," leaving off the "FPM".
FPM offered an advantage over earlier memory technologies because it enabled
faster access to data located within the same row.
EXTENDED DATA OUT (EDO)
In 1995, EDO became the next memory innovation. It was similar to FPM, but with
a slight modification that allowed consecutive memory accesses to occur much
faster. This meant the memory controller could save time by cutting out a few steps
in the addressing process. EDO enabled the CPU to access memory 10 to 15%
faster than with FPM.
SYNCHRONOUS DRAM (SDRAM)
In late 1996, SDRAM began to appear in systems. Unlike previous technologies,
SDRAM is designed to synchronize itself with the timing of the CPU. This enables
the memory controller to know the exact clock cycle when the requested data will
be ready, so the CPU no longer has to wait between memory accesses. SDRAM
chips also take advantage of interleaving and burst mode functions, which make
memory retrieval even faster. SDRAM modules come in several different speeds so
as to synchronize to the clock speeds of the systems they'll be used in. For example,
PC66 SDRAM runs at 66MHz, PC100 SDRAM runs at 100MHz, PC133 SDRAM
runs at 133MHz, and so on. Faster SDRAM speeds such as 200MHz and 266MHz
are currently in development.
DOUBLE DATA RATE SYNCHRONOUS DRAM (DDR SDRAM)
DDR SDRAM, is a next-generation SDRAM technology. It allows the memory chip to
perform transactions on both the rising and falling edges of the clock cycle. For
example, with DDR SDRAM, a 100 or 133MHz memory bus clock rate yields an
effective data rate of 200MHz or 266MHz. Systems using DDR SDRAM are expected
to ship at the end of the year 2000.
DIRECT RAMBUS
Direct Rambus is a new DRAM architecture and interface standard that challenges
traditional main memory designs. Direct Rambus technology is extraordinarily fast
compared to older memory technologies. It transfers data at speeds up to 800MHz
over a narrow 16-bit bus called a Direct Rambus Channel. This high-speed clock
rate is possible due to a feature called "double clocked," which allows operations to
occur on both the rising and falling edges of the clock cycle. Also, each memory
device on an RDRAM module provides up to 1.6 gigabytes per second of bandwidth -
twice the bandwidth available with current 100MHz SDRAM.
In addition to chip technologies designed for use in main memory, there are also
specialty memory technologies that have been developed for video applications.
MEMORY TECHNOLOGIES FOR VIDEO OR GRAPHICS PROCESSING
VIDEO RAM (VRAM)
VRAM is a video version of FPM technology. VRAM typically has two ports instead
of one, which allows the memory to allocate one channel to refreshing the screen
while the other is focused on changing the images on the screen. This works much
more efficiently than regular DRAM when it comes to video applications. However,
since video memory chips are used in much lower quantities than main memory
chips, they tend to be more expensive. So, a system designer may choose to use
regular DRAM in a video subsystem, depending on whether cost or performance is
the design objective.
WINDOW RAM (WRAM)
WRAM is another type of dual-ported memory also used in graphics-intensive
systems. It differs slightly from VRAM in that its dedicated display port is smaller
and it supports EDO features.
SYNCHRONOUS GRAPHICS RAM (SGRAM)
SGRAM is a video-specific extension of SDRAM that includes graphics-specific
read/write features. SGRAM also allows data to be retrieved and modified in blocks,
instead of individually. This reduces the number of reads and writes that memory
must perform and increases the performance of the graphics controller by making
the process more efficient.
BASE RAMBUS AND CONCURRENT RAMBUS
Before it even became a contender for main memory, Rambus technology was
actually used in video memory. The current Rambus main memory technology is
called Direct Rambus. Two earlier forms of Rambus are Base Rambus and Concurrent
Rambus. These forms of Rambus have been used in specialty video applications in
some workstations and video game systems like Nintendo 64 for several years.
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