access memory (RAM) is the best known form of computer memory.
RAM is considered "random access" because you can access any
memory cell directly if you know the row and column that intersect at
The opposite of RAM is serial access memory (SAM). SAM stores data as
a series of memory cells that can only be accessed sequentially (like
a cassette tape). If the data is not in the current location, each memory
cell is checked until the needed data is found. SAM works very well
for memory buffers, where the data is normally stored in the order in
which it will be used (a good example is the texture buffer memory on
a video card). RAM data, on the other hand, can be accessed in any order.
In this article, you'll learn all about what RAM is, what kind you should
buy and how to install it.
Similar to a microprocessor, a memory chip is an integrated circuit
(IC) made of millions of transistors and capacitors. In the most common
form of computer memory, dynamic random access memory (DRAM), a transistor
and a capacitor are paired to create a memory cell, which represents
a single bit of data. The capacitor holds the bit of information --
a 0 or a 1 (see How Bits and Bytes Work for information on bits). The
transistor acts as a switch that lets the control circuitry on the memory
chip read the capacitor or change its state.
A capacitor is like a small bucket that is able to store electrons.
To store a 1 in the memory cell, the bucket is filled with electrons.
To store a 0, it is emptied. The problem with the capacitor's bucket
is that it has a leak. In a matter of a few milliseconds a full bucket
becomes empty. Therefore, for dynamic memory to work, either the CPU
or the memory controller has to come along and recharge all of the capacitors
holding a 1 before they discharge. To do this, the memory controller
reads the memory and then writes it right back. This refresh operation
happens automatically thousands of times per second.
Memory is made up of bits arranged in a two-dimensional
In this figure, red cells represent 1s and white cells represent 0s.
In the animation, a column is selected and then rows are charged to
write data into the specific column.
DRAM works by sending a charge through the appropriate column (CAS)
to activate the transistor at each bit in the column. When writing,
the row lines contain the state the capacitor should take on. When reading,
the sense-amplifier determines the level of charge in the capacitor.
If it is more than 50 percent, it reads it as a 1; otherwise it reads
it as a 0. The counter tracks the refresh sequence based on which rows
have been accessed in what order. The length of time necessary to do
all this is so short that it is expressed in nanoseconds (billionths
of a second). A memory chip rating of 70ns means that it takes 70 nanoseconds
to completely read and recharge each cell.
Memory cells alone would be worthless without some way to get information
in and out of them. So the memory cells have a whole support infrastructure
of other specialized circuits. These circuits perform functions such
· Identifying each row and column (row address select and column
· Keeping track of the refresh sequence (counter)
· Reading and restoring the signal from a cell (sense amplifier)
· Telling a cell whether it should take a charge or not (write
Other functions of the memory controller include a series of tasks that
include identifying the type, speed and amount of memory and checking
Static RAM uses a completely different technology. In static RAM, a
form of flip-flop holds each bit of memory (see How Boolean Logic Works
for details on flip-flops). A flip-flop for a memory cell takes four
or six transistors along with some wiring, but never has to be refreshed.
This makes static RAM significantly faster than dynamic RAM. However,
because it has more parts, a static memory cell takes up a lot more
space on a chip than a dynamic memory cell. Therefore, you get less
memory per chip, and that makes static RAM a lot more expensive.
So static RAM is fast and expensive, and dynamic RAM is less expensive
and slower. So static RAM is used to create the CPU's speed-sensitive
cache, while dynamic RAM forms the larger system RAM space.
Memory chips in desktop computers originally used a pin configuration
called dual inline package (DIP). This pin configuration could be soldered
into holes on the computer's motherboard or plugged into a socket that
was soldered on the motherboard. This method worked fine when computers
typically operated on a couple of megabytes or less of RAM, but as the
need for memory grew, the number of chips needing space on the motherboard
The solution was to place the memory chips, along with all of the support
components, on a separate printed circuit board (PCB) that could then
be plugged into a special connector (memory bank) on the motherboard.
Most of these chips use a small outline J-lead (SOJ) pin configuration,
but quite a few manufacturers use the thin small outline package (TSOP)
configuration as well. The key difference between these newer pin types
and the original DIP configuration is that SOJ and TSOP chips are surface-mounted
to the PCB. In other words, the pins are soldered directly to the surface
of the board, not inserted in holes or sockets.
Memory chips are normally only available as part of a card called a
module. You've probably seen memory listed as 8x32 or 4x16. These numbers
represent the number of the chips multiplied by the capacity of each
individual chip, which is measured in megabits (Mb), or one million
bits. Take the result and divide it by eight to get the number of megabytes
on that module. For example, 4x32 means that the module has four 32-megabit
chips. Multiply 4 by 32 and you get 128 megabits. Since we know that
a byte has 8 bits, we need to divide our result of 128 by 8. Our result
is 16 megabytes!
The type of board and connector used for RAM in desktop computers has
evolved over the past few years. The first types were proprietary, meaning
that different computer manufacturers developed memory boards that would
only work with their specific systems. Then came SIMM, which stands
for single in-line memory module. This memory board used a 30-pin connector
and was about 3.5 x .75 inches in size (about 9 x 2 cm). In most computers,
you had to install SIMMs in pairs of equal capacity and speed. This
is because the width of the bus is more than a single SIMM. For example,
you would install two 8-megabyte (MB) SIMMs to get 16 megabytes total
RAM. Each SIMM could send 8 bits of data at one time, while the system
bus could handle 16 bits at a time. Later SIMM boards, slightly larger
at 4.25 x 1 inch (about 11 x 2.5 cm), used a 72-pin connector for increased
bandwidth and allowed for up to 256 MB of RAM.
From the top: SIMM, DIMM and SODIMM memory modules
As processors grew in speed and bandwidth capability, the industry adopted
a new standard in dual in-line memory module (DIMM). With a whopping
168-pin or 184-pin connector and a size of 5.4 x 1 inch (about 14 x
2.5 cm), DIMMs range in capacity from 8 MB to 1 GB per module and can
be installed singly instead of in pairs. Most PC memory modules operate
at 2.5 volts, while Mac systems typically use 3.3 volts. Another standard,
Rambus in-line memory module (RIMM), is comparable in size and pin configuration
to DIMM but uses a special memory bus to greatly increase speed.
Many brands of notebook computers use proprietary memory modules, but
several manufacturers use RAM based on the small outline dual in-line
memory module (SODIMM) configuration. SODIMM cards are small, about
2 x 1 inch (5 x 2.5 cm), and have 144 or 200 pins. Capacity ranges from
16 MB to 1 GB per module. An interesting fact about the Apple iMac desktop
computer is that it uses SODIMMs instead of the traditional DIMMs.
Most memory available today is highly reliable. Most systems simply
have the memory controller check for errors at start-up and rely on
that. Memory chips with built-in error-checking typically use a method
known as parity to check for errors. Parity chips have an extra bit
for every 8 bits of data. The way parity works is simple. Let's look
at even parity first.
When the 8 bits in a byte receive data, the chip adds up the total number
of 1s. If the total number of 1s is odd, the parity bit is set to 1.
If the total is even, the parity bit is set to 0. When the data is read
back out of the bits, the total is added up again and compared to the
parity bit. If the total is odd and the parity bit is 1, then the data
is assumed to be valid and is sent to the CPU. But if the total is odd
and the parity bit is 0, the chip knows that there is an error somewhere
in the 8 bits and dumps the data. Odd parity works the same way, but
the parity bit is set to 1 when the total number of 1s in the byte are
The problem with parity is that it discovers errors but does nothing
to correct them. If a byte of data does not match its parity bit, then
the data are discarded and the system tries again. Computers in critical
positions need a higher level of fault tolerance. High-end servers often
have a form of error-checking known as error-correction code (ECC).
Like parity, ECC uses additional bits to monitor the data in each byte.
The difference is that ECC uses several bits for error checking -- how
many depends on the width of the bus -- instead of one. ECC memory uses
a special algorithm not only to detect single bit errors, but actually
correct them as well. ECC memory will also detect instances when more
than one bit of data in a byte fails. Such failures are very rare, and
they are not correctable, even with ECC.
The majority of computers sold today use nonparity memory chips. These
chips do not provide any type of built-in error checking, but instead
rely on the memory controller for error detection.
Common RAM Types
Static random access memory uses multiple transistors, typically four
to six, for each memory cell but doesn't have a capacitor in each cell.
It is used primarily for cache.
Dynamic random access memory has memory cells with a paired transistor
and capacitor requiring constant refreshing.
Fast page mode dynamic random access memory was the original form of
DRAM. It waits through the entire process of locating a bit of data
by column and row and then reading the bit before it starts on the next
bit. Maximum transfer rate to L2 cache is approximately 176 MBps.
Extended data-out dynamic random access memory does not wait for all
of the processing of the first bit before continuing to the next one.
As soon as the address of the first bit is located, EDO DRAM begins
looking for the next bit. It is about five percent faster than FPM.
Maximum transfer rate to L2 cache is approximately 264 MBps.
Synchronous dynamic random access memory takes advantage of the burst
mode concept to greatly improve performance. It does this by staying
on the row containing the requested bit and moving rapidly through the
columns, reading each bit as it goes. The idea is that most of the time
the data needed by the CPU will be in sequence. SDRAM is about five
percent faster than EDO RAM and is the most common form in desktops
today. Maximum transfer rate to L2 cache is approximately 528 MBps.
Double data rate synchronous dynamic RAM is just like SDRAM except that
is has higher bandwidth, meaning greater speed. Maximum transfer rate
to L2 cache is approximately 1,064 MBps (for DDR SDRAM 133 MHZ).
Rambus dynamic random access memory is a radical departure from the
previous DRAM architecture. Designed by Rambus, RDRAM uses a Rambus
in-line memory module (RIMM), which is similar in size and pin configuration
to a standard DIMM. What makes RDRAM so different is its use of a special
high-speed data bus called the Rambus channel. RDRAM memory chips work
in parallel to achieve a data rate of 800 MHz, or 1,600 MBps.
Credit Card Memory
Credit card memory is a proprietary self-contained DRAM memory module
that plugs into a special slot for use in notebook computers.
PCMCIA Memory Card
Another self-contained DRAM module for notebooks, cards of this type
are not proprietary and should work with any notebook computer whose
system bus matches the memory card's configuration.
CMOS RAM is a term for the small amount of memory used by your computer
and some other devices to remember things like hard disk settings --
see Why does my computer need a battery? for details. This memory uses
a small battery to provide it with the power it needs to maintain the
VideoRAM, also known as multiport dynamic random access memory (MPDRAM),
is a type of RAM used specifically for video adapters or 3-D accelerators.
The "multiport" part comes from the fact that VRAM normally
has two independent access ports instead of one, allowing the CPU and
graphics processor to access the RAM simultaneously. VRAM is located
on the graphics card and comes in a variety of formats, many of which
are proprietary. The amount of VRAM is a determining factor in the resolution
and color depth of the display. VRAM is also used to hold graphics-specific
information such as 3-D geometry data and texture maps. True multiport
VRAM tends to be expensive, so today, many graphics cards use SGRAM
(synchronous graphics RAM) instead. Performance is nearly the same,
but SGRAM is cheaper.
For a comprehensive examination of RAM types, including diagrams and
speed tables, check out the PDF document A Basic Overview of Commonly
Encountered Types of Random Access Memory.
How Much Do You Need?
It's said that you can never have enough money, and the same seems to
hold true for RAM, especially if you do a lot of graphics-intensive
work or gaming. Next to the CPU itself, RAM is the most important factor
in computer performance. If you don't have enough, adding RAM can make
more of a difference than getting a new CPU!
If your system responds slowly or accesses the hard drive constantly,
then you need to add more RAM. If you are running Windows 95/98, you
need a bare minimum of 32 MB, and your computer will work much better
with 64 MB. Windows NT/2000 needs at least 64 MB, and it will take everything
you can throw at it, so you'll probably want 128 MB or more.
Linux works happily on a system with only 4 MB of RAM. If you plan to
add X-Windows or do much serious work, however, you'll probably want
64 MB. Apple Mac OS systems should have a minimum of 32 MB.
The amount of RAM listed for each system above is estimated for normal
usage -- accessing the Internet, word processing, standard home/office
applications and light entertainment. If you do computer-aided design
(CAD), 3-D modeling/animation or heavy data processing, or if you are
a serious gamer, then you will most likely need more RAM. You may also
need more RAM if your computer acts as a server of some sort (Web pages,
database, application, FTP or network).
Another question is how much VRAM you want on your video card. Almost
all cards that you can buy today have at least 8 MB of RAM. This is
normally enough to operate in a typical office environment. You should
probably invest in a 32-MB graphics card if you want to do any of the
· Play realistic games
· Capture and edit video
· Create 3-D graphics
· Work in a high-resolution, full-color environment
· Design full-color illustrations
How to Install RAM
Most of the time, installing RAM is a very simple and straightforward
procedure. The key is to do your research. Here's what you need to know:
· How much RAM you have
· How much RAM you wish to add
· Form factor
· RAM type
· Tools needed
· Where it goes
In the previous section, we discussed how much RAM is needed in most
situations. RAM is usually sold in multiples of 16 megabytes: 16, 32,
64, 128, 256, 512. This means that if you currently have a system with
64 MB RAM and you want at least 100 MB RAM total, then you will probably
need to add another 64 MB module.
Once you know how much RAM you want, check to see what form factor (card
type) you need to buy. You can find this in the manual that came with
your computer, or you can contact the manufacturer. An important thing
to realize is that your options will depend on the design of your computer.
Most computers sold today for normal home/office use have DIMM slots.
High-end systems are moving to RIMM technology, which will eventually
take over in standard desktop computers as well. Since DIMM and RIMM
slots look a lot alike, be very careful to make sure you know which
type your computer uses. Putting the wrong type of card in a slot can
cause damage to your system and ruin the card.
You will also need to know what type of RAM is required. Some computers
require very specific types of RAM to operate. For example, your computer
may only work with 60ns-70ns parity EDO RAM. Most computers are not
quite that restrictive, but they do have limitations. For optimal performance,
the RAM you add to your computer must also match the existing RAM in
speed, parity and type. The most common type available today is SDRAM.
Before you open your computer, check to make sure you won't be voiding
the warranty. Some manufacturers seal the case and request that the
customer have an authorized technician install RAM. If you're set to
open the case, turn off and unplug the computer. Ground yourself by
using an anti-static pad or wrist strap to discharge any static electricity.
Depending on your computer, you may need a screwdriver or nut-driver
to open the case. Many systems sold today come in toolless cases that
use thumbscrews or a simple latch.
To install more RAM, look for memory modules on your computer's motherboard.
At the left is a Macintosh G4 and on the right is a PC.
The actual installation of the memory module does not normally require
any tools. RAM is installed in a series of slots on the motherboard
known as the memory bank. The memory module is notched at one end so
you won't be able to insert it in the wrong direction. For SIMMs and
some DIMMs, you install the module by placing it in the slot at approximately
a 45-degree angle. Then push it forward until it is perpendicular to
the motherboard and the small metal clips at each end snap into place.
If the clips do not catch properly, check to make sure the notch is
at the right end and the card is firmly seated. Many DIMMs do not have
metal clips; they rely on friction to hold them in place. Again, just
make sure the module is firmly seated in the slot.
Once the module is installed, close the case, plug the computer back
in and power it up. When the computer starts the POST, it should automatically
recognize the memory. That's all there is to it!