lis508 lecture 1: bits, bytes and characters

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lis508 lecture 1 bits, bytes and characters

• Thomas Krichel
• 2002-09-23

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Structure

• Bits
• Bytes
• Character sets
• Coded character set
• Character endcoding

3
Literature

• Norton new inside the PC chapter 4
• http//www.danbbs.dk/erikoest/bb_terms.htm
• http//wwwinfo.cern.ch/asdoc/WWW/publications/ictp
  99/ictp99N2705.html
• http//www.cl.cam.ac.uk/mgk25/unicode.html

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Information

• Information is best understood as what it takes
  to answer a question.
• The simplest question has a yes or no answer.
  Therefore a bit is the natural measure of
  information.
• Term first used by John Turkey in 1946.
• Concatenation of binary digit.

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Usage of bits

• Computers are sometimes classified by
• The number of bits they can process at one time
  i.e. the register size. Larger registers make a
  computer run faster.
• The number of bits they use to represent
  addresses i.e. address size. A larger address
  size allows to run larger programs.
• Graphics are also often described by the number
  of bits used to represent each dot.

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Many bits

• The first chips used to process 8 bits at a time.
  It become customary to refer to them as a byte.
• Larger units are
• Kilo byte is 2 power 10 bytes
• Mega bytes is 2 power 20 bytes
• Giga bytes is 2 power 30 bytes
• Tera byte is 2 power 40 bytes
• From ancient Greek words for "thousand", "large",
  "giant", and "monster", respectively. Terms date
  back to the French revolution.

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More than a monster

• In 1975, the General Conference of Weights and
  Measures (CGPM), based at Sèvres near Paris,
  agreed to add peta- (P) and exa- (E)
• Petabyte is 2 power 50 bytes
• Exabyte in 2 power 60
• Nowadays they are followed by yottabyte (70) and
  zettabyte (80)

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Hex numbers

• A byte is often represented by two hex numbers.
• Each hex number can encode 16 values
• Written 0 to 9, then A B C D E F. F is 15.
• Here, prefixed with 0x
• Use Microsoft calculator with scientific notation
  to convert.

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decimal/binary numbers

• 0 0
• 1 1
• 2 10
• 3 11
• 4 100
• 5 101
• 6 110
• 7 111

• 8 1000
• 9 1001
• 10 1010
• 11 1011
• 12 1100
• 13 1101
• 14 1110
• 15 1111

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Characters

• Much of the information processed by computers is
  in the form of characters.
• A character only makes sense for a human user of
  a minimum cultural level.
• A character is not a glyph.
• ligatures

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Representing characters

• Computers don't understand text, they only
  understand numbers. For computers to be able to
  treat text, there must be a correspondence
  between numbers and text characters. Such a
  correspondence is called a coded character set.
• Important examples are
• ASCII
• ISO 8859--1
• cp1252

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ASCII

• American Standard Code for Information
  Interchange
• 7-bit character set. There is no such thing as
  8-bit ASCII
• 95 printable symbols
• 33 control characters (0-31, 127)
• http//www.ccmr.cornell.edu/helpful_data/ascii2.ht
  ml has a list.

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ASCII control codes

• ACK (6, F) used to acknowledge receipt of
  message, NAK (21, U) used to signal non-receipt
• CR (13, M) is the carriage return
• LF (10, J) is the linefeed
• FF (12, L) is the form feed (new page)
• BS (8, H) is the backspace
• DEL (ALT-127) is delete
• ESC () escape
• Different programs use them in different ways, a
  big pain in the a

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ISO-8859-1

• PCs work with bytes, so manufactures were free to
  fill the other 128 characters.
• ISO-8859-1, aka ISO-latin-1, it extends ASCII
  with characters that are used by the western
  European languages.
• It is the default character set of html.
• Positions 128 to 159 are not used.
• Cp1252 fills these with graphic chars.

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Three concepts for characters

• Abstract Character Repertoire the set of
  characters to be encoded, e.g., some alphabet or
  symbol set
• Coded Character Set a mapping from an abstract
  character repertoire to a set of non-negative
  integers
• Character Encoding Scheme a mapping from a
  coded character set to a serialized sequence of
  bytes

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ISO 10646-1

• Defines the Universal Character Set (UCS)
• UCS contains the characters required to represent
  characters used by practically all known
  languages, even the likes of Gurmukhi, Oriya,
  Telugu, Bopomofo, Runic.
• There are proposals for more, like Hieroglyphs
  and Tengwar.
• Note that there are about 6800 known languages.

.
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UCS organization

• ISO 10646 defines formally a 31-bit character
  set. They are represented as 32 bits, i.e. 4
  bytes, or 8 hex chars.
• The canonical form of ISO 10646 uses a
  four-dimensional coding space consisting of 256
  groups. Each group consists of 256 planes with
  each plane containing 256 rows, each having 256
  cells.

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UCS organization

• The first plane (Plane 0x00) of Group (0x00) is
  called the Basic Multilingual Plane (BMP). It has
  been fixed since first publication.
• The subsequent 223 planes (0x01 to 0xDF) of Group
  0x00, as well as planes 0x00 to 0xFF in Groups
  0x01 to 0x5F are reserved for further
  standardization.
• The last 32 planes (0xE0 to 0xFF) of Group 0x00,
  as well as all code positions of 32 groups (0x60
  to 0x7F) are reserved for private use.

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Relationship with legacy sets

• Let U(four hex numbers) denote characters in the
  BMP.
• The UCS characters U0000 to U007F are identical
  to those in ASCII
• The range U0000 to U00FF is identical to ISO
  8859-1 (Latin-1).

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Types of characters in UCS

• Letters
• Base characters
• Ideographic characters
• Combining characters
• Digits
• Extenders

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• Thank you for your attention!