CSCI-235 Micro-Computers in Science

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CSCI-235Micro-Computers in Science

• Hardware Design
• Part I

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• Electricity and Switches
• Modern computers are powered by electricity,
  using electrical signals to store and manipulate
  information
• The components of a computer require electrical
  power to carry out their assigned task
• Electricity generates the light that shines
  through a computer screen, illuminating the
  individual pixels that make up images and letters
• Electricity runs the motor that spins the
  hard-drive disk, allowing information to be
  accessed
• Main memory and CPU employ electrical signals to
  store and manipulate data
• Bit patterns are represented by the presence or
  absence of electrical current along a wire

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• Switches
• The most basic tool for controlling the flow of
  electricity is a switch
• A switch can be flipped to connect or disconnect
  two wires, thus regulating the flow of
  electricity between them

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• Transistors
• A transistor is a solid piece of metal attached
  to a wire that serves as a switch by
  alternatively conducting or resisting electricity
• Solid-state switches either permit or block
  current flow
• A control input causes state change

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• A large number of transistors, as well as the
  electrical conducting paths that connect them,
  can be printed photographically on a wafer of
  silicon to produce a device known as an
  integrated circuit or, more commonly, a chip
• At current technology levels, 25 million or more
  transistors can fit into a space only 1cm2
• Transistors can be combined to form a circuit,
  which controls the flow of electricity in order
  to produce a particular behavior

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• The production of integrated circuits is one of
  the most complex engineering processes in the
  world
• Transistors on chips can be as small as .065
  microns (roughly 1/1,500th the width of human
  hair)
• Since a hair or dust particle can damage
  circuitry during manufacture, chips are created
  in climate-controlled "clean rooms"

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• Gate
• The term gate suggests a simple circuit that
  controls the flow of electricity
• In the case of a NOT gate, the flow of
  electricity is manipulated so that the output
  signal is always opposite of the input signal
• We can think of a gate as computing a function of
  binary values
• 0 represents no current 1 represents current
• symbol to the left (triangle w/ circle) used to
  denote NOT gate
• truth table to right describes mapping of input
  to output

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• Many other simple circuits can be defined to
  perform useful tasks
• AND gate produces voltage on its output wire if
  both input wires carry voltage
• OR gate produces voltage on its output wire if
  either input wire carries voltage
• AND, OR, and NOT gates can be combined to
  construct all the circuitry required to store and
  manipulate information within a computer

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• Boolean Logic and Gates
• Boolean logic describes operations on true/false
  values
• True/false maps easily onto bistable environment
• Boolean logic operations on electronic signals
  can be built out of transistors
• Boolean operations
• a AND b
• True only when a is true and b is true
• a OR b
• True when a is true, b is true, or both are true
• NOT a
• True when a is false and vice versa

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• Boolean expressions
• Constructed by combining together Boolean
  operations
• Example (a AND b) OR ((NOT b) AND (NOT a))
• Truth tables capture the output/value of a
  Boolean expression
• A column for each input plus the output
• A row for each combination of input values
• Example (a AND b) OR ((NOT b) and (NOT a))

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• Gates

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• Abstraction in hardware design
• Map hardware devices to Boolean logic
• Design more complex devices in terms of logic,
  not electronics
• Conversion from logic to hardware design can be
  automated

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• Circuits
• A circuit is a collection of logic gates
• Transforms a set of binary inputs into a set of
  binary outputs
• Values of the outputs depend only on the current
  values of the inputs