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 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  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 In ASCII > In EBCDIC > Copyright © 2008 by Helene G. Kershner

Using Binary Coding Information  Codes are designed so that the ordering of English letters makes sense.  A B C D E F … U V W X Y Z  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  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 < In EBCDIC < X > F In ASCII > In EBCDIC > 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 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 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  EBCDIC does it similarly, by adding a digit at the end. Copyright © 2008 by Helene G. Kershner