Using Binary
Coding Information
Remember
 Bit = 0 or 1, Binary Digit
 Byte = the number of bits used to represent letters,
numbers and special characters such as $ # , / &.
 Word = number of bytes a computer can process at
one time by the CPU.
So,
Bits form Bytes and Bytes form Words.
Copyright © 2008 by Helene G. Kershner
Using Binary
Coding Information
Two common formats for coding letters, numbers and
special characters are:
 ASCII -- American Standard Code for Information Interchange
 7 bit code
 Originally used on non-IBM systems
 Basis of most currently used systems
 EBCDIC -- Extended Binary Coded Decimal Interchange
Code
 8 bit code
 Originally used in IBM mainframes
Copyright © 2008 by Helene G. Kershner
Using Binary
Coding Information
Coding in EBCDIC and ASCII:
Letters
 We know certain facts about letters
They have an implied ordering
This ordering must be maintained when letters are translated
into 0’s and 1’s inside the computer
Numbers
Numbers clearly have a mathematical order
Numerical ordering must also be maintained when
represented as 0’s and 1’s inside the computer
Copyright © 2008 by Helene G. Kershner
Using Binary
Coding Information
 Codes are organized so that mathematics makes sense
 They are ordered so that each subsequent number is larger than
the one before.
7
> 5
> 011 0101
0111 > 1111 0110
In ASCII
011 0111
In EBCDIC
1111
Copyright © 2008 by Helene G. Kershner
Using Binary
Coding Information
 Codes are designed so that the ordering of English
letters makes sense.
ABCDEF… UVWXYZ
 Over time we consider this ordering to be fixed
 The implication is that we can compare letters and that certain
letters appear before others in the alphabet.
 For Example: A < F and X > F
 ASCII and EBCDIC code letters so that this concept remains
valid.
Copyright © 2008 by Helene G. Kershner
Using Binary
Coding Information

Codes are designed so that the ordering of English
letters makes sense.
A < F
In ASCII
100 0001 < 100 0110
In EBCDIC 1100 0001 < 1100 0110
X>F
In ASCII
101 1000 > 100 0110
In EBCDIC 1110 0111 > 1100 0110
Copyright © 2008 by Helene G. Kershner
Using Binary
Coding Information
 Codes are designed so that the ordering of English letters makes
sense.
 At the time of its creation, it was argued that ASCII was a more rational
coding scheme based on the way it codes letters.
 Both ASCII and EBCDIC are limiting
 Both can code all of English and most Romance languages.
 As computer use has expanded both became incomplete
 ASCII is still the foundation for text based email on most computers
 UNICODE
 Designed to provide a single coding system for every character in every
natural language
 The Internet uses UNICODE
Copyright © 2008 by Helene G. Kershner
Using Binary
Coding Information
 Error Checking
 Transmission errors are a fact of computer life
 Think of the game of Telephone
 Power interruptions happen
 Life happens
 GIGO
 What can be done to reduce or repair errors in
transmitted characters?
Copyright © 2008 by Helene G. Kershner
Using Binary
Coding Information
 Parity
–
 In common English, Parity means two things are equal
How could this be applied to error checking?
 With computers it applies to the special bit added to a
byte so assist computer systems in reducing errors.
Copyright © 2008 by Helene G. Kershner
Using Binary
Coding Information
Odd Parity
 Look at byte, attach either a 0 or a 1 to force the byte
to be odd
 Examine the byte coded to mean Z
In ASCII
101 1010
Count the number of 1s, there are 4, 4 is even, the
parity bit is set to one (1) to make the string of bits
odd.
The byte is coded 1 101 1010
EBCDIC does it similarly, by adding a digit at the end.
Copyright © 2008 by Helene G. Kershner