What’s a Computer?
Analog, Digital does it matter?
• Technically there are two categories of
computers, analog or digital.
• Designs reflect two different ways to look at or
analyze the world.
Copyright © 2008 by Helene G. Kershner
What’s a Computer?
Analog, Digital does it matter?
• Analog device – operates with measurements that are
continuous such a voltage, temperature and rotation.
– Speedometer in your car
– Traditional thermometer
– Classic clock
• An Analog computer uses analog measurements in it’s
calculations. Analog measurements are “physical
phenomena such as electrical, mechanical, or hydraulic
quantities to model the problem being solved.”
(http://en.wikipedia.org/wiki/Analog_computer)
– Very fast
– Somewhat inaccurate
– Difficult to replicate results, can you spin a wheel at exactly the
same rate twice in a row?
Copyright © 2008 by Helene G. Kershner
What’s a Computer?
Analog, Digital does it matter?
• This device is a
Harmonium. It is a
mechanical, analog
computer designed
analyze sine waves used
in Fournier Analysis.
What’s a Computer?
Analog, Digital does it matter?
•
Digital Computer –
–
–
–
Information is represented by counting
This is what we think of as a computer.
All information used by the machine is in one of two
states ON (1) or Off (0).
•
Things we can count include:
–
–
–
•
•
Change from a dollar
Number of students registered in CSE 111
Tuition in dollars
Highly flexible
Easy to replicate
Copyright © 2008 by Helene G. Kershner
What’s a Computer?
Analog, Digital does it matter?
• Moving from Analog to Digital
• Digital Age
– Clock
• Hands moving in a circle  digits on a screen
– Telephone
• 10 digits in a circle, distance from beginning was used to represent
the digit  push a button
• Voice as a sound wave  voice as a pattern of pulses
– Television and Movies
• Pictures as a series of light waves  pictures as a pattern of dots
(pixels).
• It is all about speed and ability to replicate results
Copyright © 2008 by Helene G. Kershner
What’s a Computer?
The Binary Machine
• Computers “speak” a very simple language
–
–
–
–
–
Two digits – 0 and 1
Don’t really understand the data or instructions they are given
Know how to “follow” them -- circuit path
The 1 state, current is present -- ON
The 0 state, current is absent – OFF
• Binary -- The entire language of mathematics can be
converted into a system that just uses 0s and 1s.
Copyright © 2008 by Helene G. Kershner
What’s a Computer?
The Binary Machine
• Computers use Binary
– (0,1)
– Base 2
• People do not – we use Decimal
– (0,1,2,3,4,5,6,7,8,9)
– Base 10
– Perhaps this is because we have 10 finger and 10 toes
• Modern computers take in decimal number and letters
– Translate them into 0s and 1s
– Do whatever they do – “Magic”
– Give us the results in a way human understand
Copyright © 2008 by Helene G. Kershner
What’s a Computer?
The Binary Machine
•
In decimal every number is represented as the
digits 0-9
In binary every number is represented as the
digits 0-1
•
–
Decimal to Binary conversion
Decimal Binary
-0-1-2-3-4-5-6-7-8-9-
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
Copyright © 2008 by Helene G. Kershner
What’s a Computer?
The Binary Machine
• For people in the know –
- 0 - 0000
- 1 - 0001
- 2 - 0010
http://www.thinkgeek.com/tshirts/frustrations/5aa9/zoom/
Copyright © 2008 by Helene G. Kershner
What’s a Computer?
The Binary Machine
• BIT for Binary Digit -- each 0 or 1 in the binary system
– Single bit is not overly useful
• BYTE – a group of bits (usually 8)
– Each byte represents one character of data
• Numbers, letters, special characters (%, $, # etc.)
• Letter, numbers and symbols are the form we use to represent
information.
• WORD – the number of bits that can be processed at one time
by the central processing unit (the “brain”) of the computer.
– Early computers – Apple II – 8 bit Words
– Modern machines – can process up to 64 bits/word
Copyright © 2008 by Helene G. Kershner
What’s a Computer?
The Binary Machine
• Computers are described by amount of processing
memory they contain
– Usually described in terms of bytes (K ~ 1000 bytes)
• Megabyte or megs [MB]= ~1000K or ~1 million bytes
• Gigabyte or gigs [GB] = ~1000MB or ~1 billion bytes
• Terabyte = ~1000GB or ~1 trillion bytes
• The notation has gained common usage
– Earn 100K
– House is priced at 450K
Copyright © 2008 by Helene G. Kershner
What’s a computer?
• For many a computer is a
“black box”!
– Incredible machine that
works mysteriously
– How is of little
consequence.
– Magic?
Copyright © 2008 by Helene G. Kershner
What’s a computer?
Basic parts
Input & Output
Input
Central Processing
Unit
Brains
Memory
Output
Input & Output
computer.howstuffworks.com/cd.htm, staples.com, dell.com
Copyright © 2008 by Helene G. Kershner
What’s a computer?
Basic parts
• Every computer is made up of the following components:
– A Central Processing Unit
• Arithmetic & Logic Unit
• Control Unit
– Memory
• Primary Memory (Usually called RAM)
• Secondary Memory or Mass Storage (Disk drives)
– Input and Output Devices
Copyright © 2008 by Helene G. Kershner
What’s a computer?
Processing Unit
Brains of the Machine
• Central Processing Unit -– Arithmetic and Logic Unit (ALU)
– Control Unit
• Associated Primary Memory
Working together these are the core or any computer.
Size doesn’t matter!
Copyright © 2008 by Helene G. Kershner
What’s a computer?
Central Processing Unit
• Arithmetic & Logic Unit
– As the name implies performs two functions:
– Arithmetic
• Responsible for performing the arithmetic operations of
addition, subtraction, multiplication and division
– Logic
• Makes simple “decisions” by comparing alternatives and
choosing between them
– Less than, greater than, equal to
– AND, OR and NOT
Copyright © 2008 by Helene G. Kershner
What’s a computer?
Control Unit
• The arithmetic and logic unit is responsible for
mathematical calculations and comparisons.
• The Control Unit is the computer’s internal coordinator
– Traffic Cop
– Sends out electronic signals directing the computer to perform
specific tasks such as:
•
•
•
•
Move data between memory and the CPU
Activating the ALU
Receiving data
Sending information to an output device
– The control unit manages the flow of data throughout the machine
based on the instructions it receives from programs.
Copyright © 2008 by Helene G. Kershner
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
 ASCII exists NOT because it makes more sense (which it
does) but because it was an economic necessity.
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
In ASCII
In EBCDIC
011 0111 > 011 0101
1111 0111 > 1111 0110
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 a special bit added to each byte
before it is transmitted to another computer to allow the
receiving machine to check on the accuracy of the
transmission.
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