Single In-line Memory
Module
A
single in-line memory module (SIMM) attaches to the main circuit board
(motherboard) of a computer to accommodate memory microprocessors. SIMMs
require less space than conventional memory-mounting hardware.
Computer Memory, a mechanism that stores data
for use by a computer. In a computer all data consist of numbers. A computer
stores a number into a specific location in memory and later fetches the value.
Most memories represent data with the binary number system. In the binary
number system, numbers are represented by sequences of the two binary digits 0
and 1, which are called bits (see Number Systems). In a computer, the
two possible values of a bit correspond to the on and off states of the
computer's electronic circuitry.
In memory, bits are grouped together so they
can represent larger values. A group of eight bits is called a byte and can
represent decimal numbers ranging from 0 to 255. The particular sequence of
bits in the byte encodes a unit of information, such as a keyboard character.
One byte typically represents a single character such as a number, letter, or
symbol. Most computers operate by manipulating groups of 2, 4, or 8 bytes called
words.
Memory capacity is usually quantified in terms of
kilobytes, megabytes, and gigabytes. Although the prefixes kilo-, mega-,
and giga-, are taken from the metric system, they have a slightly
different meaning when applied to computer memories. In the metric system,
kilo- means 1 thousand; mega-, 1 million; and giga-, 1 billion. When applied to
computer memory, however, the prefixes are measured as powers of two, with
kilo- meaning 2 raised to the 10th power, or 1,024; mega- meaning 2 raised to the
20th power, or 1,048,576; and giga- meaning 2 raised to the 30th power, or
1,073,741,824. Thus, a kilobyte is 1,024 bytes and a megabyte is 1,048,576
bytes. It is easier to remember that a kilobyte is approximately 1,000 bytes, a
megabyte is approximately 1 million bytes, and a gigabyte is approximately 1
billion bytes.
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II
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HOW MEMORY WORKS
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Computer memory may be divided into two broad
categories known as internal memory and external memory. Internal memory
operates at the highest speed and can be accessed directly by the central
processing unit (CPU)—the main electronic circuitry within a computer that
processes information. Internal memory is contained on computer chips and uses
electronic circuits to store information (see Microprocessor). External
memory consists of storage on peripheral devices that are slower than internal
memories but offer lower cost and the ability to hold data after the computer’s
power has been turned off. External memory uses inexpensive mass-storage
devices such as magnetic hard drives. See also Information Storage and
Retrieval.
Internal memory is also known as random access
memory (RAM) or read-only memory (ROM). Information stored in RAM can be
accessed in any order, and may be erased or written over. Information stored in
ROM may also be random-access, in that it may be accessed in any order, but the
information recorded on ROM is usually permanent and cannot be erased or
written over.
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A
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Internal RAM
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Random access memory is also called main memory
because it is the primary memory that the CPU uses when processing information.
The electronic circuits used to construct this main internal RAM can be
classified as dynamic RAM (DRAM), synchronized dynamic RAM (SDRAM), or static
RAM (SRAM). DRAM, SDRAM, and SRAM all involve different ways of using
transistors and capacitors to store data. In DRAM or SDRAM, the circuit for
each bit consists of a transistor, which acts as a switch, and a capacitor, a
device that can store a charge. To store the binary value 1 in a bit, DRAM places
an electric charge on the capacitor. To store the binary value 0, DRAM removes
all electric charge from the capacitor. The transistor is used to switch the
charge onto the capacitor. When it is turned on, the transistor acts like a
closed switch that allows electric current to flow into the capacitor and build
up a charge. The transistor is then turned off, meaning that it acts like an
open switch, leaving the charge on the capacitor. To store a 0, the charge is
drained from the capacitor while the transistor is on, and then the transistor
is turned off, leaving the capacitor uncharged. To read a value in a DRAM bit
location, a detector circuit determines whether a charge is present or absent
on the relevant capacitor.
DRAM is called dynamic because it is continually
refreshed. The memory chips themselves cannot hold values over long periods of
time. Because capacitors are imperfect, the charge slowly leaks out of them,
which results in loss of the stored data. Thus, a DRAM memory system contains
additional circuitry that periodically reads and rewrites each data value. This
replaces the charge on the capacitors, a process known as refreshing memory.
The major difference between SDRAM and DRAM arises from the way in which
refresh circuitry is created. DRAM contains separate, independent circuitry to
refresh memory. The refresh circuitry in SDRAM is synchronized to use the same
hardware clock as the CPU. The hardware clock sends a constant stream of pulses
through the CPU’s circuitry. Synchronizing the refresh circuitry with the
hardware clock results in less duplication of electronics and better access
coordination between the CPU and the refresh circuits.
In SRAM, the circuit for a bit consists of
multiple transistors that hold the stored value without the need for refresh.
The chief advantage of SRAM lies in its speed. A computer can access data in
SRAM more quickly than it can access data in DRAM or SDRAM. However, the SRAM
circuitry draws more power and generates more heat than DRAM or SDRAM. The
circuitry for a SRAM bit is also larger, which means that a SRAM memory chip
holds fewer bits than a DRAM chip of the same size. Therefore, SRAM is used
when access speed is more important than large memory capacity or low power
consumption.
The time it takes the CPU to transfer data to
or from memory is particularly important because it determines the overall
performance of the computer. The time required to read or write one bit is
known as the memory access time. Current DRAM and SDRAM access times are
between 30 and 80 nanoseconds (billionths of a second). SRAM access
times are typically four times faster than DRAM.
The internal RAM on a computer is divided into
locations, each of which has a unique numerical address associated with it. In
some computers a memory address refers directly to a single byte in memory,
while in others, an address specifies a group of four bytes called a word.
Computers also exist in which a word consists of two or eight bytes, or in
which a byte consists of six or ten bits.
When a computer performs an arithmetic operation,
such as addition or multiplication, the numbers used in the operation can be
found in memory. The instruction code that tells the computer which operation
to perform also specifies which memory address or addresses to access. An
address is sent from the CPU to the main memory (RAM) over a set of wires
called an address bus. Control circuits in the memory use the address to select
the bits at the specified location in RAM and send a copy of the data back to
the CPU over another set of wires called a data bus. Inside the CPU, the data
passes through circuits called the data path to the circuits that perform the
arithmetic operation. The exact details depend on the model of the CPU. For
example, some CPUs use an intermediate step in which the data is first loaded
into a high-speed memory device within the CPU called a register.
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B
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Internal ROM
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Read-only memory is the other type of internal
memory. ROM memory is used to store items that the computer needs to execute
when it is first turned on. For example, the ROM memory on a PC contains a
basic set of instructions, called the basic input-output system (BIOS). The PC
uses BIOS to start up the operating system. BIOS is stored on computer chips in
a way that causes the information to remain even when power is turned off.
Information in ROM is usually permanent and cannot
be erased or written over easily. A ROM is permanent if the information cannot
be changed—once the ROM has been created, information can be retrieved but not
changed. Newer technologies allow ROMs to be semi-permanent—that is, the
information can be changed, but it takes several seconds to make the change.
For example, a FLASH memory acts like a ROM because values remain stored in
memory, but the values can be changed.
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C
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External Memory
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External memory can generally be classified as
either magnetic or optical, or a combination called magneto-optical. A magnetic
storage device, such as a computer's hard drive, uses a surface coated with
material that can be magnetized in two possible ways. The surface rotates under
a small electromagnet that magnetizes each spot on the surface to record a 0 or
1. To retrieve data, the surface passes under a sensor that determines whether
the magnetism was set for a 0 or 1. Optical storage devices such as a compact
disc (CD) player use lasers to store and retrieve information from a plastic
disk. Magneto-optical memory devices use a combination of optical storage and
retrieval technology coupled with a magnetic medium.
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C1
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Magnetic Media
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Inside a Computer Hard
Drive
The
inside of a computer hard disk drive consists of four main components. The
round disk platter is usually made of aluminum, glass, or ceramic and is coated
with a magnetic media that contains all the data stored on the hard drive. The
yellow armlike device that extends over the disk platter is known as the head
arm and is the device that reads the information off of the disk platter. The
head arm is attached to the head actuator, which controls the head arm. Not shown
is the chassis which encases and holds all the hard disk drive components.
FOTO-WERBUNG/Phototake
NYC
Memory stored on external magnetic media include
magnetic tape, a hard disk, and a floppy disk. Magnetic tape is a form of
external computer memory used primarily for backup storage. Like the surface on
a magnetic disk, the surface of tape is coated with a material that can be
magnetized. As the tape passes over an electromagnet, individual bits are
magnetically encoded. Computer systems using magnetic tape storage devices
employ machinery similar to that used with analog tape: open-reel tapes,
cassette tapes, and helical-scan tapes (similar to video tape).
Another form of magnetic memory uses a spinning
disk coated with magnetic material. As the disk spins, a sensitive
electromagnetic sensor, called a read-write head, scans across the surface of
the disk, reading and writing magnetic spots in concentric circles called
tracks.
Magnetic disks are classified as either hard or
floppy, depending on the flexibility of the material from which they are made.
A floppy disk is made of flexible plastic with small pieces of a magnetic
material imbedded in its surface. The read-write head touches the surface of
the disk as it scans the floppy. A hard disk is made of a rigid metal, with the
read-write head flying just above its surface on a cushion of air to prevent
wear.
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C2
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Optical Media
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Optical external memory uses a laser to scan a spinning
reflective disk in which the presence or absence of nonreflective pits in the
disk indicates 1s or 0s. This is the same technology employed in the audio CD.
Because its contents are permanently stored on it when it is manufactured, it
is known as compact disc-read only memory (CD-ROM). A variation on the CD,
called compact disc-recordable (CD-R), uses a dye that turns dark when a
stronger laser beam strikes it, and can thus have information written
permanently on it by a computer.
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C3
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Magneto-Optical Media
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Magneto-optical (MO) devices write data to a disk with
the help of a laser beam and a magnetic write-head. To write data to the disk,
the laser focuses on a spot on the surface of the disk heating it up slightly.
This allows the magnetic write-head to change the physical orientation of small
grains of magnetic material (actually tiny crystals) on the surface of the
disk. These tiny crystals reflect light differently depending on their
orientation. By aligning the crystals in one direction a 0 can be stored, while
aligning the crystals in the opposite direction stores a 1. Another, separate,
low-power laser is used to read data from the disk in a way similar to a
standard CD-ROM. The advantage of MO disks over CD-ROMs is that they can be
read and written to. They are, however, more expensive than CD-ROMs and are
used mostly in industrial applications. MO devices are not popular consumer
products.
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D
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Cache Memory
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CPU speeds continue to increase much more rapidly
than memory access times decrease. The result is a growing gap in performance
between the CPU and its main RAM memory. To compensate for the growing
difference in speeds, engineers add layers of cache memory between the CPU and
the main memory. A cache consists of a small, high-speed memory system that
holds recently used values. When the CPU makes a request to fetch or store a
memory value, the CPU sends the request to the cache. If the item is already
present in the cache, the cache can honor the request quickly because the cache
operates at higher speed than main memory. For example, if the CPU needs to add
two numbers, retrieving the values from the cache can take less than one-tenth
as long as retrieving the values from main memory. However, because the cache
is smaller than main memory, not all values can fit in the cache at one time.
Therefore, if the requested item is not in the cache, the cache must fetch the
item from main memory.
Cache cannot replace conventional RAM because cache
is much more expensive and consumes more power. However, research has shown
that even a small cache that can store only 1 percent of the data stored in
main memory still provides a significant speedup for memory access. Therefore,
most computers include a small, external memory cache attached to their RAM.
More important, multiple caches can be arranged in a hierarchy to lower memory
access times even further. In addition, most CPUs now have a cache on the CPU
chip itself. The on-chip internal cache is smaller than the external cache,
which is smaller than RAM. The advantage of the on-chip cache is that once a
data item has been fetched from the external cache, the CPU can use the item
without having to wait for an external cache access.
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III
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DEVELOPMENTS AND
LIMITATIONS
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Since the inception of computer memory, the
capacity of both internal and external memory devices has grown steadily at a
rate that leads to a quadrupling in size every three years. Computer industry
analysts expect this rapid rate of growth to continue unimpeded. Computer
engineers consider it possible to make multigigabyte memory chips and disks
capable of storing a terabyte (one trillion bytes) of memory.
Some computer engineers are concerned that the
silicon-based memory chips are approaching a limit in the amount of data they
can hold. However, it is expected that transistors can be made at least four
times smaller before inherent limits of physics make further reductions
difficult. Engineers also expect that the external dimensions of memory chips
will increase by a factor of four, meaning that larger amounts of memory will
fit on a single chip. Current memory chips use only a single layer of
circuitry, but researchers are working on ways to stack multiple layers onto
one chip. Once all of these approaches are exhausted, RAM memory may reach a
limit. Researchers, however, are also exploring more exotic technologies with
the potential to provide even more capacity, including the use of biotechnology
to produce memories out of living cells. The memory in a computer is composed
of many memory chips. While current memory chips contain megabytes of RAM,
future chips will likely have gigabytes of RAM on a single chip. To add to RAM,
computer users can purchase memory cards that each contain many memory chips.
In addition, future computers will likely have advanced data transfer
capabilities and additional caches that enable the CPU to access memory faster.
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IV
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HISTORY
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Early electronic computers in the late 1940s and early
1950s used cathode ray tubes (CRT), similar to a computer display screen, to
store data. The coating on a CRT remains lit for a short time after an electron
beam strikes it. Thus, a pattern of dots could be written on the CRT,
representing 1s and 0s, and then be read back for a short time before fading.
Like DRAM, CRT storage had to be periodically refreshed to retain its contents.
A typical CRT held 128 bytes, and the entire memory of such a computer was
usually 4 kilobytes.
International Business Machines Corporation (IBM) developed
magnetic core memory in the early 1950s. Magnetic core (often just called
“core”) memory consisted of tiny rings of magnetic material woven into meshes
of thin wires. When the computer sent a current through a pair of wires, the
ring at their intersection became magnetized either clockwise or
counterclockwise (corresponding to a 0 or a 1), depending on the direction of
the current. Computer manufacturers first used core memory in production
computers in the 1960s, at about the same time that they began to replace
vacuum tubes with transistors. Magnetic core memory was used through most of
the 1960s and into the 1970s.
The next step in the development of
computer memory came with the introduction of integrated circuits, which
enabled multiple transistors to be placed on one chip. Computer scientists
developed the first such memory when they constructed an experimental
supercomputer called Illiac-IV in the late 1960s. Integrated circuit memory
quickly displaced core and has been the dominant technology for internal memory
ever since.