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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
240-pin DIMM: DDR2 400/533/667/800 DDR-2 SDRAM
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 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 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
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. 144-pin and 200-pin modules are the most common SO DIMMs today.
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.
MicroDIMM (Micro Dual In-Line Memory Module)
Smaller than an SO DIMM, MicroDIMMs are primarily used in sub-notebook computers. MicroDIMMs are available in 144-pin SDRAM, 172-pin DDR and 214-pin DDR2.
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.
Flash memory is available in many different form factors, including: CompactFlash, Secure Digital, SmartMedia, MultiMedia and USB Memory
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.
TECHNOLOGY TIMELINE
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 |
| 2001 |
DDR SDRAM |
333MHz |
| 2002 |
DDR SDRAM |
434MHz |
| 2003 |
DDR SDRAM |
500MHz |
| 2004 |
DDR2 SDRAM |
533MHz |
| 2005 |
DDR2 SDRAM |
800MHz |
| 2006 |
DDR2 SDRAM |
667 - 800MHz |
| 2007 |
DDR3 SDRAM |
1066 - 1333MHz |
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.
Click here for more on DDR.
Click here for more on DDR.
DOUBLE DATA RATE 2 SYNCHRONOUS DRAM (DDR2 SDRAM)
DDR2 is the second generation of Double Data Rate (DDR) SDRAM memory. It is an evolution of DDR memory technology that delivers higher
speeds (up to 800 MHz), lower power consumption and heat dissipation. It is an ideal memory solution for bandwidth hungry systems and
the lower power consumption is a perfect match for today's mobile users.
Click here for more on DDR2.
DOUBLE DATA RATE 3 SYNCHRONOUS DRAM (DDR3 SDRAM)
DDR3 is the third generation of Double Data Rate (DDR) SDRAM memory. Similar to DDR2, it is a continuing evolution of DDR memory technology that
delivers higher speeds (up to 1600 MHz), lower power consumption and heat dissipation. It is an ideal memory solution for bandwidth hungry systems
equipped with dual and quad core processors and the lower power consumption is a perfect match for both server and mobile platforms. DDR3 modules
will be available in the second half of 2007.
DIRECT RAMBUS
Direct Rambus is a 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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