15 September

1
15 September

• How Computers Work
• Other Forms of Data

2
Current events

• Court Rules Against Pennsylvania Law That Curbs
  Child-Pornography Sites
• law forced Internet service providers to block
  customers' access to Web sites thought to be
  distributing child pornography
• http//www.nytimes.com/2004/09/11/technology/11fil
  ter.html?th

3
Review Integers

• Binary
• Stored in words (32 bits) or half-words (16 bits)
• Twos complement used to store negative numbers
• Max positive integer 2n-1 - 1

4
Questions?
5
Using Integers

• Not always whole numbers
• Invisible decimal points
• Can be used to represent fractional units
• 1 1 cent
• 1 1 ml
• Using integers avoids rounding errors

6
Floating Point Numbers

• Represent much larger number
• Scientific notation
• 1.452 x 1017
• Mantissa exponent
• Three Parts
• Sign Bit
• Exponent
• Mantissa

7
From Microsoft Calculator

• Understanding Extended Precision
• Extended Precision, a feature of Calculator,
  means that all operations are accurate to at
  least 32 digits.
• Calculator also stores rational numbers as
  fractions to retain accuracy. For example, 1/3 is
  stored as 1/3, rather than .333.
• However, errors accumulate during repeated
  operations on irrational numbers.

8
Dates

• Lots of formats for dates
• Some store month, day, year
• Some store a day and year
• Some store a single number
• Different origins
• Different amount of space to store

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Lots of Calendars
10
Excel

• Microsoft Excel stores dates as sequential
  numbers.
• January 1, 1900 is 1
• January 1, 2008 is 39448 (39,448 days after
  January 1, 1900)
• Stores times as decimal fractions because time is
  considered a portion of a day.
• Except, on the Macintosh, 1 January 1, 1904!
• Rules on interpreting 2 digit years

11
Y2K

• What was the problem?
• What caused the problem?
• Was the problem limited to our calendar?
• How did people solve the problem?
• How would you have solved the problem?

12
Text

• Need to represent characters
• All data in the computer is stored as _____
• Letter a question of interpretation
• Also includes some control new line, tab,
• Not font the appearance is referred to as a
  glyph
• Representations
• ASCII, EBCDIC, UNICODE,
• ASCII simplest
• Most applications moving to UNICODE

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Valuable Characteristics

• Collating sequence
• Simple upper/lower case
• Support of all languages
• Other?

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ASCII

• Some encodings
• Space X20 (2016)
• 2 X32 9 X39
• A X41 Z X5A
• a X61 z X7A
• http//www.webopedia.com/quick_ref/asciicode.asp

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ASCII collating

• Space X20, 2 X32, A X41, a X61 z
  X7A
• For collating sequence, where should punctuation
  be?
• General rule
• space lt punctuation lt number lt letter
• Most common punctuation X21 X2F
• How to deal with case?
• Collating sequence is case insensitive
• Need to force to uppercase

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Uppercase algorithm

• A (4116) 0100 0001 a (6116) 0110 0001 Z
  (5A16) 0101 1010 z (7A16) 0111 1010
• To uppercase a letter, simply turn off the bit
  xx0x xxxx

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Multiple languages

• Used to have complete different set of encodings
  for each language, called code pages
• Unicode is fundamentally 2-bytes
• Preserves the ASCII values
• Doubles the size of files
• www.unicode.org

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Formatted Documents

• Not all formatting information is included in the
  character information
• We already mentioned font
• Two ways to handle formatting
• WYSIWYG (What You See Is What You Get)
• Word DOC
• PDF (Adobe)
• Formatting language
• HTML used for web

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Graphics

• Two fundamental methods of storing data
• Bit maps individual dots to be displayed
• Vector graphics lines to be displayed
• When using bit graphics, need intelligent way to
  store
• Compression techniques
• Long strings of the same value
• Minor changes from picture to picture
• JPEG, MPEG,

20
Coming Attractions

• Next week we start talking about how computers
  have affected society
• Assignment
• Scan references (94, 95, 00)
• See course home page
• Identify key impacts at different levels (for
  discussion not handing in)
• Personal
• Community
• National
• Global
• Future

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Take Home Quiz

• When?
• Handed out on the 20th
• Due the 22nd
• Will have time for questions on Monday
• Intended to be a 30 minute test
• Not timed