VAST 20002 Computers and Society

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VAST 200-02 Computers and Society

• Lecture 5 - Building Software
• Spring 2004

Prof. John NestorECE DepartmentLafayette
CollegeEaston, Pennsylvania 18042nestorj_at_lafayet
te.edu
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Roadmap for the Term Major Topics

• Course Introduction
• Computer Systems Overview
• Computer History
• Technology Trends
• The Art of Building Hardware and Software \
• Current Issues
• Looking to the Future

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References - HW and SW Design

• 1. M. Reilly, "Designing an Alpha
  Microprocessor", IEEE Computer, July 1999.
  
• 2. C. Hoare, "The Emperor's Old Clothes (Turing
  Award Lecture)", Communications of the
  ACM,    February 1981.
  
• 3. D. Parnas, "Software Aspects of Strategic
  Defense Systems", Communications of the ACM,
  December 1985.
  
• 4. M. Baer, The New X-Men, Wired, Issue 11.09,
  September 2003(available at http//www.wired.com/
  wired/archive/11.09/xmen.html)
  

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Todays Lecture

• Software Design
• What is software design?
• Is Software harder than hardware?
• Estimating Software Complexity
• Software design methods
• Software Engineering as an emerging field

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General Steps in Software Design

• 1. Requirements Analysis deterimine what problem
  the software will solve.
  
• 2. Design the program and select the tools to
  solve the problem.
  
• 3. Code the program in the language of choice
• 4. Compile into executable code
• 5. Test the program to make sure it works if
  not, return to step 3.
  
• 6. Maintenance - correct bugs after the program
  is in use (and try to avoid creating new ones!)
  

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What Makes Software Hard?

• Software is
• Abstract in nature - invisible and
  unvisualizable1
  
• Arbitrarily complex - often too large for one
  person to comprehend
  
• Arbitrarily changeable
• Arbitrarily fragile
• Specifications change before, during and after
  development
  
• Communications issues between designers and
  programmers
  

1F. W. Brooks, No Silver Bullet, The Mythical
Man Month Anniversary Edition, Addison Wesley,
1995
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Measuring SW Complexity

• Source Lines of Code (SLOC)
• Measures how many lines (statements) in a
  program
  
• Useful as a measure of software complexity
• SOME SLOC Estimates

Sources D. Wheeler, More Than A Gigabuck
Estimating GNU/Linuxs Size, http//www.dewheeler
.com/sloc/ Wikipedia ( wikipedia.org).
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Estimating Software Cost

• Barry Boehms Constructive Cost Model (COCOMO)1
• T cost in person-years
• k - empirical constant
• x - empirical constant (0.1 - 0.25)

1Barry Boehm, Software Engineering Economics
Prentice-Hall, 1981
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Software Cost

• Programmer time is not fungible
• Brooks Law Adding programmers to a late
  project makes it later
  
• Brooks key challenge Conceptual Integrity

1Frederic Brooks, The Mythical Man-Month,
Addison-Wesley, 1975
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Software Design Methods

• Define the steps of creating a large software
  system
  
• Common Models
• Code and change
• Waterfall
• Spiral
• Extreme Programming - a recent proposal

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SW Development - Waterfall Model
Integration
Product Verif.
Product design
System feasibility
Verification
System Test
Detailed Design
Operations Maint.
Verification
Revalidation
Source A Spiral Model of Software Development
and Enhancement, IEEE Computer, May 1988.
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SW Development - Spiral Model
Source A Spiral Model of Software Development
and Enhancement, IEEE Computer, May 1988.
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Extreme Programming

• A recent design method developed by Beck,
  Cunningham, and Jeffries
  
• Key ideas
• Incremental and iterative development
• Continuous testing
• Pair programming
• Onsite customer - encourage interaction between
  user developers
  
• Simplicity
• Feedback

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Software Engineering

• Software development as an engineering
  discipline
  
• Still a young discipline - consider Civil
  Engineering
  
• Fundamentals remain to be discovered
• Engineering failures are still commonplace
• Late release of products
• Bugs in released products
• Software-related disasters

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Roadmap for the Term Major Topics

• Course Introduction
• Computer Systems Overview
• Computer History
• Technology Trends
• The Art of Building Hardware and Software
• Current Issues \
• Looking to the Future

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Outline - Course Overview

• Administrative Details \
• Course Overview and Introduction
• Computer Systems Overview
• General Concepts
• History and Trends
• The Creative Process
• Hardware Design
• Software Design
• Current Issues Computers and Society
• Faults and Failures
• Privacy and Encryption

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The Illusion of Privacy

• 1993 - On the Internet, nobody knows youre a
  dog -- New Yorker cartoon, 1993
  
• 1998 On the Internet, nobody knows youre a
  police officer posing as a 14-year old
  
• 2002 On the Internet, nobody knows youre a
  Carnivore
  
• 2003 On the Internet, nobody knows youre the
  RIAA

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The Internet - A Network of Networks

• Common addressing scheme
• Routers connect networks,
• TCP/IP
• IP Addresses - what identifies computers on the
  internet
  
• Static addresses - permanently assigned
• Dynamic addresses