Chapter 4 Computer Hardware

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Chapter 4Computer Hardware

• Information Technology in Theory
• By Pelin Aksoy and Laura DeNardis

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Objectives

• Identify some important historical milestones in
  the development of computers
• Understand logic gates and how computers use them
  to process information
• Identify the fundamental components of a computer
• Understand how computer performance depends on
  factors such as processor speed, chip set, bus
  width, bus speed, number of CPUs, and instruction
  set

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Objectives (continued)

• Examine future trends in computing and societal
  issues related to these advances
• Understand the physical principles of data
  storage and the difference between mechanical,
  magnetic, optical, and electronic storage

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A Brief History of Computers

• Computing devices trace back at least to the
  Abacus
• Mechanical calculation tools in the 1600s
  included William Oughtreds slide rule,
  BlaisePascals arithmetic machine, and Leibnizs
  step reckoner
• Calculation tools of the 1800s included Charles
  Babbages Difference and Analytical engines
• Augusta Ada Byron, the Countess of Lovelace,
  worked with Babbage in programming the analytical
  engine
• Herman Hollerith developed the tabulating machine
  to tabulate the 1890 US census data using punch
  cards

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A Brief History of Computers (continued)

• The punch card concept is attributed to Joseph
  Marie Jacquard
• The 1940s spurred big advancements in computing
• John Vincent Atanasoff and Clifford Berry at the
  Iowa State College built the ABC
• Konrad Zuse built the Z3
• Howard Aiken built the Mark I, which was
  programmed by Grace Murray Hopper
• British scientists built the Colossus
• John Mauchly and J. Presper Eckert built the
  ENIAC

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A Brief History of Computers (continued)

• The transistor and integrated circuit enable
  smaller computers compared to vacuum tubes
• Popular Electronics ran a feature story about a
  home do-it-yourself computer kit called the
  Altair
• Radio Shack began to sell the TRS-80 home
  computer, and Apple Computer Inc. introduced its
  own line of home computers
• These advances, and the introduction of IBMs
  personal computer (PC) in 1981, contributed to
  the explosion of computers in all facets of
  modern life

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A Brief History of Computers (continued)

• A variety of social and economic factors, along
  with advances in microprocessors and distributed
  networks, led to the introduction of mobile,
  wireless, and handheld computers

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Digital Logic

• A computer has one or more main chips called the
  microprocessor, which performs most of the
  processing, and several other supporting chips
• Integrated circuits such as microprocessors are
  built by combining transistors together to create
  logic gates
• These basic building blocks can be combined to
  create successively larger building blocks, which
  may subsequently be combined to create complex
  circuitry and packaged with other components
  within a carrier to create integrated circuits
  such as microprocessors

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Digital Logic (continued)
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Digital Logic (continued)

• Integrated circuits are built using the following
  types of logic gates
• The difference between these logic gates is
  essentially the way the transistors within them
  are connected to each other
• NOT gate (inverter)
• AND gate
• NAND (not AND) gate
• OR gate
• NOR (not OR) gate
• Exclusive OR (XOR) gate
• XNOR (exclusive NOR) gate

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Digital Logic (continued)
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Digital Logic (continued)
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Digital Logic (continued)
Combining log gates to create a 1-bit binary adder
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Digital Logic (continued)
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Fundamental Components of a Computer
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Fundamental Components of a Computer (continued)

• Computer hardware is any physical device
  associated with a computer, such as a keyboard or
  monitor
• Besides the instructions that users issue through
  input hardware devices, computers also receive
  instructions via the software stored in its
  memory
• Software is traditionally defined as a series of
  instructions written by computer programmers in a
  language that people can understand and that the
  computer can translate into binary
• The computer must interpret and then execute each
  software instruction, again through the use of
  its transistors

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Input/Output Devices

• Input and output devices are hardware components
  that interface a computer to the outside world
• Examples of input devices include
• Mice
• Trackballs
• Microphones
• Web cams
• Styluses
• Touch pads
• Pointing sticks
• Joysticks
• Scanners

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Input/Output Devices (continued)

• Examples of output devices include
• Printers
• Plotters
• Speakers
• Projectors
• Some I/O devices, such as keyboards and monitors,
  are integral parts of computers
• However, note that not all computers have input
  and output devices connected to them
• For example, some computers can interface to a
  user over a network and do not need their own I/O
  equipment

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Central Processing Unit

• The most crucial component of any computer is a
  chip called the microprocessor or central
  processing unit (CPU)
• The microprocessor controls the major
  functionality of the computer and is often called
  the brains of the computer
• The microprocessors many tasks include fetching
  a wealth of software instructions, interpreting
  and executing these instructions, and storing and
  outputting results

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Central Processing Unit (continued)
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Central Processing Unit (continued)

• Key component of the CPU is the control unit,
  where instructions are interpreted and control
  signals are generated
• Based on the result of the interpretation, the
  control unit supplies crucial control signals to
  other parts of the computer
• If the instruction involves the addition of two
  numbers, then the control unit issues control
  signals to a unit called the ALU, which is
  responsible for arithmetic and logical operations

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Central Processing Unit (continued)

• The registers within the CPU are small areas for
  temporarily storing information, such as
  instructions that are supplied to the control
  unit and the results of addition and comparison
  operations implemented by the ALU
• Software instructions that the CPU interprets and
  data that it processes are fetched from a module
  called main memory that resides outside the CPU
• Unlike main memory, cache memory is internal to
  the CPU

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Central Processing Unit (continued)

• Cache memory holds frequently used instructions
  and data
• Because some software instructions and data are
  needed more often than others by the CPU, they
  can be stored inside the cache memory and
  retrieved when necessary instead of having to be
  fetched from the distant main memory every time
  they are needed
• Saving frequently used instructions and data in
  close proximity saves considerable time and
  energy

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Main Memory

• Main memory of a computer consists of two types
  of memory random access memory (RAM) and
  read-only memory (ROM)
• RAM is much more prominent it retains its
  contents as long as power is supplied to the
  computer and loses its contents when the power is
  switched off
• RAM is also called temporary memory or volatile
  memory
• ROM, on the other hand, retains its contents even
  after power to the computer

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Main Memory (continued)

• Essentially, two types of RAM are used to store
  information as long as power is supplied to the
  computer static RAM (SRAM) and dynamic RAM
  (DRAM)
• The contents of dynamic RAM must be refreshed
  several times per second, because the electrical
  charges representing the bits of DRAM leak out
  and must be replenished
• There is no need to refresh the contents of SRAM
• DRAMs include transistors and other electronic
  components called capacitors that store
  electricity, whereas SRAMs are based on
  transistors

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Main Memory (continued)

• Although capacitors can store electricity, they
  cannot do so indefinitely
• Electricity eventually leaks out of them, which
  is why it is necessary to refresh the contents of
  DRAM
• Memory chips that correspond to the main memory
  are mostly DRAM
• Cache memory within the CPU is typically SRAM
• Due to fundamental differences in how the two
  types of RAM are constructed, they differ in
  speed, size, and cost

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Main Memory (continued)

• It is faster to save and retrieve the contents of
  SRAM, but SRAM is also more expensive and larger
  than DRAM
• Because cache memory has to be fast, it is better
  to use SRAM to build cache memory
• DRAM is used within the main memory because it is
  the main holding area of the computer and must
  have a large holding capacity
• Main memory also must be cheap to minimize the
  cost of the computer

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Main Memory (continued)

• RAM chips retain ones and zeros that correspond
  to instructions and data within small areas
  called cells
• Each RAM cell can hold one bit of information,
  regardless of whether it is DRAM or SRAM
• Cells within RAM chips are arranged in rows and
  columns
• Saving information to RAM is referred to as
  writing to RAM, and retrieving RAM contents is
  called reading the RAM
• Information is written to and read from the RAM
  by addressing the RAM cells using addresses

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Main Memory (continued)
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Main Memory (continued)
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Main Memory (continued)

• Although a major part of the instructions that
  the computer executes are supplied from RAM, the
  computer also relies on some operations based on
  instructions stored in ROM chips
• ROM chips and RAM chips physically reside at
  different places within a computer, but together
  they constitute the main memory of the computer
• The BIOS chip is an example of a ROM chip found
  inside a computer

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Main Memory (continued)

• The major variations of ROMs have historically
  included the following
• Programmable ROM (PROM)
• Erasable programmable ROM (EPROM)
• Electrically erasable programmable ROM (EEPROM)
• A device that is similar to EEPROM is flash memory

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Storage

• Storage and retrieval of digital information is
  one of the most critical components of an IT
  system
• Without storage, we would have no Web servers,
  electronic banking, e-mail, or almost any other
  digital application
• Storage technologies are discussed later in the
  chapter

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Interconnection System

• The physical system that connects I/O devices,
  main memory, CPU, storage, and other components
  is called the interconnection system
• This system is actually a set of wires they are
  often grouped together as a set of parallel wires
  that transfer signals corresponding to data,
  instructions, and control information in an
  arrangement called a parallel bus
• Some connections do not rely on a set of parallel
  wires
• Instead, they use a main connection that
  transfers bits one after the other, or serially,
  from one point to another
• Such connections are called a serial bus

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Interconnection System (continued)
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Interconnection System (continued)

• The number of parallel lines within a parallel
  bus and the transfer speed of bits across each
  line dictate how many overall bits the bus can
  carry
• The larger the bus width, the quicker the bits
  can reach their destination an example would be
  the delivery of bits from the main memory to the
  CPU

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Interconnection System (continued)

• Types of computer buses include the following
• System busA parallel bus that connects the CPU
  and main memory the system bus is also called
  the front side bus
• Peripheral Component Interconnect (PCI) bus and
  PCI Express (PCI-E) busThe PCI bus and its newer
  and faster version, PCI-E bus, connect expansion
  cards such as network interface cards and sound
  cards/adapters
• Accelerated Graphics Port (AGP) busThis bus
  connects expansion cards called graphics
  cards/adapters to the CPU this circuitry
  supports computer graphics capabilities

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Interconnection System (continued)

• Expansion cards are connected to the CPU by buses
  
• They are attached to the buses by being plugged
  into special components called expansion slots on
  the computers motherboard

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Factors That Affect Computer Performance

• Some of the key factors that affect computer
  performance are shown in the following list
• Word length
• Bus width and bus speed
• Memory size and memory access speed
• Processor speed
• Instruction set
• Number of CPUs
• Chip set

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Word Length

• Word length is expressed in terms of bits and
  corresponds to the maximum number of bits of
  information that a computer can process at one
  time
• The larger the word length is, the faster the
  computer
• Engineers are constantly striving to design
  computers with larger word lengths to achieve
  high performance

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Word Length (continued)
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Bus Width and Bus Speed

• Another factor that affects performance is the
  bus size, or parallel bus width
• Like word length, bus size is measured in terms
  of bits
• A computer with a large bus width can carry more
  bits at a time between computer components, such
  as between main memory and the CPU, making it
  faster than a computer with a small bus width
• Because the buses carry important instructions,
  data, addresses, and control signals, the speed
  you can gain by using a larger bus width is
  important

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Bus Width and Bus Speed (continued)

• The bus size determines how many physical wires
  are constructed within the bus
• The more wires that designers can incorporate
  within a bus, the larger the bus width becomes,
  meaning that the bus can carry more bits at one
  time
• Bus speed, which is measured in hertz (Hz), also
  affects computing performance
• Even if large bus widths can enable large number
  of bits to move between computer components in
  parallel, each bus line must also carry bits as
  quickly as possible

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Memory Size and Memory Access Speed

• Computers need large amounts of memory for high
  performance and multitasking
• Because every program occupies some amount of
  space in RAM, a large amount of RAM is essential
  to operate with multiple programs
• Furthermore, RAM size significantly affects
  computer speed because information is frequently
  written to and erased from RAM
• If there is insufficient RAM space, the computer
  must frequently resort to using its hard disk,
  which results in diminished performance

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Memory Size and Memory Access Speed (continued)

• Other than RAM, the size of the cache memory
  (expressed in bytes) is also a consideration
• The more cache memory the CPU has, the better the
  CPU performs
• The speed at which RAM and cache memory contents
  are written and retrieved helps to determine
  performance
• The higher the read/write speed, the faster the
  computer is
• Typical RAM access speeds are on the order of
  nanoseconds (ns), or billionths of a second

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Processor Speed

• The on and off switching of transistors within
  the integrated circuits of a computer is managed
  by a central digital electrical signal called a
  clock
• Computer clock speeds are measured in frequency
  units called hertz
• Just as multipliers are used in the IT world to
  express large numbers of bits, multipliers are
  used to express large frequency values
• 1 kHz 1000 Hz
• 1 MHz 1,000,000 Hz
• 1 GHz 1,000,000,000 Hz

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Processor Speed (continued)

• The higher the clock frequency, the faster the
  computer usually is
• Typical clock frequencies for modern computers
  range from hundreds of megahertz to a few
  gigahertz

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Processor Speed (continued)
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Instruction Set

• A computers CPU is typically based on a complex
  instruction set computer (CISC) architecture or a
  reduced instruction set computer (RISC)
  architecture
• The difference between them is the number of
  clock cycles it takes to execute a single
  instruction
• Some computer architectures require more cycles
  to execute the same instruction than others

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Number of CPUs

• Incorporating more than one microprocessor
  increases a computers processing power and speed
  because it can use more transistors and perform
  multitasking
• Some computers have dual-core processors and
  quad-core processors
• Besides multicore computers, multiprocessor
  computers are also available

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Chip Set

• The term chip set refers to a group of chips that
  support the microprocessor by controlling the
  flow of information between it and other
  components, such as the memory chips, graphics
  and sound cards, disk drives, and I/O devices
• Various manufacturers produce different types of
  chip sets, which can significantly affect
  computer performance, due to the limitations that
  chip sets may impose on memory size, number of
  processors, and bus speed

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Inside a Typical Computer

• Some essential components of a modern computer
  include
• The motherboard
• DVD drive
• Hard drive
• Graphics card
• Sound card
• Network card
• Ports
• Power supply
• Cooling system

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Inside a Typical Computer (continued)
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The Motherboard

• The motherboard is a printed circuit board
  (PCB)a planar structure made of resin or other
  materials that contains the microprocessor and
  many other chips
• It also includes the chip set and components that
  support the operation of a computer, such as
  capacitors and mechanical switches
• A PCB supports and interconnects components
  through metallic traces called buses printed on
  the board

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The Motherboard (continued)

• Other components on the motherboard include
• The BIOS chip
• The chip set
• Slots for connecting RAM modules to the
  microprocessor
• Default circuitry for supporting sound and
  graphics capabilities
• Expansion slots for connecting external graphics
  cards, sound cards, and network cards
• Connectors for attaching hard drives, CD drives,
  and other disk drives to the motherboard
• Ports for connecting I/O devices such as the
  keyboard and mouse

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The Motherboard (continued)

• Boards that carry the RAM chips are called memory
  modules, and are connected to the microprocessor
  through memory slots on the motherboard
• Modern motherboards also come with extra memory
  slots to allow users to expand the RAM for faster
  processing

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The Motherboard (continued)

• Circuitry for supporting enhanced graphics may
  also be incorporated on separate PCBs called
  graphics cards or graphics adapters
• These types of external graphics adapters connect
  to the motherboard via a special expansion slot,
  such as the AGP slot
• Sound adapters enable a computer to output
  high-quality sound
• Like graphics adapters, external sound adapters
  can be connected to the motherboard via one of
  its expansion slots, such as the PCI/PCI-E slots

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The Motherboard (continued)

• Computer networking is all around us, so
  motherboards also come with circuitry that
  supports networking
• Alternatively, network adapter cards may be
  plugged into expansion slots on the motherboard
• The motherboard also contains connectors and
  ports for hard drives, I/O devices such as the
  keyboard and mouse, and other devices

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The Motherboard (continued)
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Drives

• Drives are devices that can read and write large
  amounts of information to and from various types
  of magnetic, optical, or electrical devices, such
  as hard disks, CDs, DVDs, and flash memory
• If drives are installed in the computer case,
  they are called internal drives
• If they are connected to the computer externally,
  they are called external drives
• These drives attach to the motherboard using its
  specifically reserved connectors

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Drives (continued)

• One common type of hard drive is the Advanced
  Technology Attachment (ATA) drive, which is also
  called a parallel ATA drive, an Integrated Device
  Electronics (IDE) drive, or an Enhanced IDE
  (EIDE) drive
• These drives include an integrated component
  called a controller, and are attached to the
  motherboard via special connectors
• Other hard drives include serial ATA (SATA) and
  small computer system interface (SCSI) drives,
  which also attach to the motherboard via special
  connectors

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Drives (continued)

• SATA connections have two advantages over
  parallel ATA they can transfer data at a faster
  rate between the motherboard and the drive, and
  they are more physically compact
• SCSI drives are faster than ATA drives, but
  because they are more complex, SCSI drives are
  not a standard feature of most computers
• While internal hard drives connect to the
  motherboard with special connectors, external
  hard drives attach to the motherboard via
  Universal Serial Bus (USB) or FireWire ports
  outside the computer casing

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Computer Ports

• Peripheral devices may be connected to a computer
  through different types of ports that vary
  depending on the type of computer
• These ports in turn connect to the motherboard
  itself
• Some examples of ports include serial ports,
  parallel ports, video ports, S-video ports, USB
  ports, FireWire ports, sound ports, keyboard
  ports, mouse ports, network ports, and telephone
  ports

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Computer Ports (continued)
65
Power Supply

• A power supply is included within the computer
  casing (or externally) to convert the electricity
  levels from the wall outlet into levels that the
  computer can use
• Besides power supplies, other components such as
  surge protection systems and uninterruptible
  power supplies (UPS) may be employed to safeguard
  the computer against sudden surges of electricity
  caused by lightning strikes, power outages, and
  other phenomena

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Cooling System

• A significant amount of heat is generated by the
  many electrical components in a computer, and
  problems can arise if these components are not
  sufficiently cooled
• The chip that generates the most heat is
  typically the microprocessor
• One popular technique to cool the chip is to
  install a metallic component called a heat sink
• Another way to conduct heat away from the chip is
  to install a fan on the microprocessor or pass
  cryogenic materials or other liquids inside tubes
  in contact with the microprocessor

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Cooling System (continued)
68
Types of Computers and Their Applications

• Types of computers include
• Desktop computers
• Laptops
• Handhelds
• Mainframes
• Supercomputers
• Servers
• Some of these computers may function standalone
  or as servers and clients
• Some types of servers include e-mail servers,
  file servers, and Web servers

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Types of Computers and Their Applications
(continued)

• Servers usually have more than one processor,
  considerable memory, and extra hardware
• One popular type of server is called a blade
  server
• Blade servers can be controlled in a computer
  network through a remote access card installed in
  the server casing
• They have the advantage of saving a significant
  amount of space they can be housed in remote
  locations such as data centers
• These servers generate significant amounts of
  heat and must be kept properly cooled

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Types of Computers and Their Applications
(continued)

• Thin clients are computers that do not have the
  regular hardware typically found on other
  computers
• They do not have a hard disk or other disk
  drives, and they have a scaled-down version of an
  operating system stored on a special memory chip
• Thin clients rely on other computers over a
  network to perform most of their processing
  functions
• Their major advantage is lower cost due to their
  lack of hardware and low power consumption

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Types of Computers and Their Applications
(continued)
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The Future of Computers

• Although transistors have continued to shrink
  over the years in accordance with Moores Law,
  the trend will not hold true indefinitely
• Similarly, wires that connect transistors to each
  other continue to diminish in size
• This creates a problem as well, because very thin
  wires can cause a large loss of energy and limit
  the speed by which signals travel over the wires
  that constitute computer buses, thereby limiting
  the computers speed

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The Future of Computers (continued)

• Alternative technologies are underway, some of
  which include
• Optical computing
• Optical interconnects
• Nanotechnology
• Carbon nanotubes
• Quantum computing
• Biological computing

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Storage Technologies

• Storage technologies are critical for businesses
  to archive and access data, for governments to
  store important political information, and for
  libraries to store electronic information
• Storage schemes initiated with paper punch cards
  and paper tapes
• Magnetic tapes and disks were introduced later
• Optical and electronic media were introduced last

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Magnetic Storage

• To record bits, a plastic tape, plastic disk, or
  ceramic disk is coated with a ferromagnetic
  materiala unique material that can be magnetized
• The recording material is positioned close to a
  writing head that magnetizes small sections of
  the medium in one of two directions corresponding
  to ones and zeros
• A hard disk stores information using the same
  principle as magnetic tape, but there are major
  differences in their recording formats

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Magnetic Storage (continued)
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Magnetic Storage (continued)

• A hard disk stores bits on a circular disk (also
  called a platter) made of highly polished glass
  or another material
• A typical computer hard disk has many of these
  circular disks stacked on top of each other
• Each disk stores information on circular tracks
  and wedge-like sectors

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Magnetic Storage (continued)
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Magnetic Storage (continued)

• In the case of magnetic tape, information is
  accessed serially (sequential access), whereas a
  magnetic disk can provide random access to data
• Hard disks are limited mainly by their complex
  and sensitive mechanics for spinning, writing,
  and reading data
• Floppy disks are external storage devices that
  are not widely used anymore except for file
  backups in some legacy systems
• Floppies are conceptually similar to hard disks,
  but they have a much lower storage capacity

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Optical Storage

• Types of optical storage devices include
• Compact disc-read only memory (CD-ROM)
• Compact disc-recordable (CD-R)
• Compact disc-rewritable (CD-RW)
• Digital versatile disk-read only memory (DVD-ROM)
• Digital versatile disk-recordable (DVD-R)
• Digital versatile disk-rewritable (DVD-RW)

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Optical Storage (continued)

• Bits on optical disks are stored by creating
  small areas or domains with different reflective
  properties across a long spiral track
• Bits are read by shining laser light on these
  tiny domains and detecting the light reflected
  back from them
• Each domain reflects light in one of two ways,
  enabling the reader to distinguish between a one
  and a zero

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Optical Storage (continued)

• Bits on a CD-ROM are recorded by physically
  creating tiny protrusions, also called lands, on
  the CDs clear plastic surface during the
  manufacturing phase
• Areas without lands remain shallow these areas
  are called pits

83
Optical Storage (continued)
84
Optical Storage (continued)

• CDs can record many types of information
• For a device to properly read and save the
  information stored on a CD, the device must use
  standard formats
• For example, music on an audio CD is recorded
  following the Red Book standard developed by
  Philips and Sony
• DVDs rely on the same principles as CD technology
• DVDs can store much more data than CDs, primarily
  because the size of lands and pits can be
  diminished and spaced in closer proximity, and
  because more than one spiral track may be used to
  store data due to multiple layers

85
Electronic Storage

• Electronic storage, also called semiconductor
  storage, has become an extremely effective
  technology for quick and efficient information
  storage
• Electronic storage lets users save data quickly
  without the relatively lengthy burn process
  required for optical media such as CDs and DVDs
• Electronic storage technologies store bits by
  trapping electricity within an array of
  transistors arranged on a silicon chip

86
Electronic Storage (continued)
87
Comparing Various Types of Storage Media

• Hard disks provide very fast recording and access
  to information and can store large amounts of
  data
• Because they can be written over and over again
  without encountering the limitations of CDs or
  DVDs, hard disks are currently the primary
  storage medium for computers
• Optical disks are portable, lightweight, and can
  store information with greater stability than
  magnetic media
• Optical disks are not affected by magnetic
  fields, so they are considered more robust than
  magnetic devices, which are prone to data loss
  over time
• Electronic storage media are compact, fast, and
  versatile because they do not contain moving
  parts

88
Comparing Various Types of Storage Media
(continued)

• Manufacturers can achieve greater capacity by
  reducing the physical area occupied by each bit
  on the media
• However, each storage technology has a limit to
  its packing density
• On magnetic media, the superparamagnetic effect
  causes random fluctuations toward the magnetic
  field if bits are packed too closely to each
  other
• The ability to pack these bits closely on optical
  media is governed by the wavelength of light and
  the quality of the optics used to focus the laser
  beam onto the tracks

89
Comparing Various Types of Storage Media
(continued)

• Integrated circuit fabrication technology limits
  the density of semiconductor storage devices
  because the size of transistors is limited
• With advancements in chip manufacturing
  techniques, flash memory devices are now
  considered to be at the forefront of portable
  storage technology
• The ultimate limitations of all these storage
  technologies have spurred a large amount of
  research for alternatives, including holographic
  storage and molecular storage

90
Summary

• Transistors can be combined to create logic
  gates, which can be used as the basic building
  blocks for integrated circuits
• A computers fundamental components include
  input/output (I/O) devices, a central processing
  unit, main memory, storage (secondary memory),
  and an interconnection system
• Factors that affect the performance of computers
  include word length, bus width and bus speed,
  memory size and memory access speed, processor
  rate, instruction set, number of processors, and
  chip set

91
Summary (continued)

• Components you can expect to find in a modern
  computer include the motherboard, CD drive, hard
  drive, graphics adapter, sound adapter, network
  adapter, ports, power supply, and a cooling
  system
• Computers come in various types, including
  desktop computers, laptops, handhelds,
  mainframes, supercomputers, servers, and thin
  clients
• Future possibilities for computing include
  nanotechnology, quantum computing, and biological
  computing

92
Summary (continued)

• Magnetic media store digital data by magnetizing
  a ferromagnetic medium in one of two directions
• Optical media store digital data by creating
  domains with different reflectivities
• Electronic storage media save data by storing
  different levels of electrical charge through
  transistors
• Magnetic tapes are classified as serial access
  devices hard disks, floppy disks, CDs, DVDs, and
  electronic memory are classified as random access
  devices
• Possible alternatives for future storage media
  include holographic and molecular storage