Hardware: How Computer Programs Run on Physical Machines

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Hardware How Computer Programs Run on Physical
Machines
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News

• Schedule for the rest of the semester
• Nov 14, 16, 21 circuits and hardware
• Nov 23 No Class (Thanksgiving)
• Nov 28 Internet and WWW
• Nov 30 Artificial Intelligence
• Dec 5 Social Impacts of Computing
• Dec 7 Exam 3
• Dec X Final Exam
• Read chapter 4 of your textbook

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The hardware level of a computer

• Binary numbers are used to encode all forms of
  data, such as numbers and letters
• Electric circuits process and store binary data
• Simple gates can be combined to build circuits
  for any computation
• Boolean logic is a useful abstraction of electric
  circuits

These are the topics well be discussing today
and next week
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The binary number system

• Binary digits (bits) have values 0 and 1
• The position of each digit determines its value
• for example, the binary number 101011 equals
• 1x25 0x24 1x23 0x22 1x21 1x20
• 32 0 8 0 2
  1 4310
• note the similarity with decimal numbers
• 57210 5x102 7x101 2x100

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Binary numbers can represent all types of data

• negative integers reserve the leading bit for
  sign (0positive 1negative)
• letters ASCII codes (using 8 bits1 byte)
• A 01000001, B01000010, C01000011,
  and so on

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Internal vs external forms of data

• Internal External
• 00..01010112 4310
• 10..01010112 -4310
• 01000001001000102 AB

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Calculating in binary

• First practice counting to 2010 in binary
• Practice adding 10112 1012
• Try subtracting 10112 - 1012

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Why use binary, not decimal or ...?

• At the hardware level, data is manipulated and
  stored with electrical circuits
• So, data must correspond to electronic
  quantities, such as voltage and current
• A decimal computer could be built using an
  electrical device with 10 voltage levels
• But, the device would not be reliable!

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Electrical devices with 2 states are the most
reliable

• 2 states which are hard to confuse
• full on - full off
• fully charged - fully discharged
• charged positively - charged negatively
• magnetized - nonmagnetized
• magnetized clockwise - magnetized ccw

These states are separated by a huge energy
barrier.
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A Switch is a 2-state device that can be toggled

• A relay is a switch built with an electromagnet

when the controlling current is 1, the
electromagnet pulls the switch closed, and the
controlled current flows through the switch
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Switches can do it all

• With switches we can implement AND, OR, and NOT
  gates
• With just those types of gates, we can design
  circuits to perform any computation

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Basic electric circuits Computing AND
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Basic electric circuits Computing OR
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Basic electric circuits Computing NOT
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A better switch Transistors

• bi-stable, solid state device
• switching is done electronically, not
  mechanically
• no moving parts therefore, fast, small,
  reliable
• can switch states in about one 10 billionth of a
  second
• about 5 million transistors can fit on a chip 1
  centimeter square. Density is increasing rapidly
  with new technology.

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Summary

• Computers use binary numbers for storage and
  computation because
• all types of data can be stored this way
• 2-state devices are highly reliable
• A switch is a 2-state device that can be toggled
• Switches can implement AND, OR, and NOT gates
• With these gates, we can perform any computation,
  as well see next time!