Computer Organization

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Computer Organization
Project 6 Prof. Jerry Breecher CSCI 140 Spring
2002
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Software Engineering

• Engineering is the art and science of building
  things. Over the years, Engineering has moved
  more from art to technique. People have learned
  what works and have developed practices to build
  things as efficiently as possible.
• A little known fact is that half of all
  cathedrals built in Europe in the Middle Ages
  fell down (mostly during construction.) Those
  cathedrals were state-of-the-art for their time
  they stretched materials and craftsmanship to
  their limits.
• Over the years, weve learned a great deal about
  building things. For instance, the last time a
  major suspension bridge fell down was in the
  1930s. You occasionally hear about buildings
  falling down, but its usually attributed to
  shoddy construction - the builders didnt follow
  the rules.
• In a similar fashion, Software Engineering is the
  craft of building large software programs.

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Software Engineering

• You are ready to hear about this topic, because
  Software Engineering is required whenever you
  have a project to build that you cant keep in
  your head all at one time.
• When you start writing programs, they are usually
  exercises, used as examples of coding techniques.
• By the time you get to upper level courses, you
  are no longer learning programming for the sake
  of learning programming, but have a reason
  associated with that program - youre trying to
  build something. And that something can get
  quite large.
• So its time to start using some of the
  techniques of Software Engineering in order to
  build your projects.

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Software Engineering

• Software Engineering covers a wide range of
  topics
• 1. Figuring out what it is you want to build -
  the requirements of the project.
• 2. Going from the requirements to a functional
  design.
• 3. Taking that high level/functional design and
  turning it into smaller chunks or algorithms.
• 4. Figuring out the steps required to implement
  the algorithms.
• 5. Coding the algorithms in a specific language.
• 6. Testing the code to ensure that it works as
  required.
• 7. Maintaining the code - fixing bugs and adding
  modifications as the requirements change.
• For our purposes, were going to look at Steps 4
  and 5. We implicitly do the other steps, but
  this exercise will not stress those aspects.

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Breaking Down A Project

• Now were going to learn a few rules. Then well
  apply these rules in a very simple example. And
  then well go wild and try the rules on a more
  complicated software example.
• The Rules
• 1. Break down the project into smaller pieces.
• A. There should be no more than four or five
  pieces.
• B. If possible, make those pieces
  approximately equal in size.
• 2. If each of the pieces contains more than one
  action, go to Step 1.
• 3. Stop only when you have a series of pieces
  and sub-pieces that each contain only one action.

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Breaking Down A Project

• An Example
• Break down into single actions, the Project of
  eating lunch in the cafeteria.
• Level 1 - Eating Lunch
• 1. Find a friend to eat lunch with.
• 2. Get your food.
• 3. Eat it.
• Level 2 - 2. Get your food
• 2A. Pay the cashier.
• 2B. Salad bar
• 2C. Pasta Bar
• 2D. Hot food
• 2E. Dessert

Each of the Level 1 items has a Level 2. Picking
just one of these we get
Breaking this down still further
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Breaking Down A Project

• Level 3 - 2A. Pay The Cashier
• 2A1. Find your card
• 2A2. Hand the card to the cashier
• 2A3. Get the card back from the cashier
• 2A4. Say Thank You
• 2A5. Put the card back in your pocket.

Breaking this down still further
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Breaking Down A Project

• Level 4 - 2A1. Find your card

Can you think and talk at the same time
Yes
No
Stop Talking
Think about where your card is.
Reach into your pocket.
This is a flowchart
Is the card there?
No
Yes
Extract Card From Pocket.
A decision box
A descriptive box
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Breaking Down A Project

• Level 4 - 2A1. Find your card
• 2A1a. Can you think and talk at the same time?
  If yes, go to 2A1c.
• 2A1b. Stop talking.
• 2A1c. Think about where your card is.
• 2A1d. Reach into your pocket.
• 2A1e. Is the card there? If no, go to 2A1a.
• 2A1f. Extract card from pocket.

This is a list.
This page and the previous one are identical.
Either a flowchart or a list works for showing
decision making.
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Breaking Down A Project

• Here it is from another perspective!!

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Breaking Down A Project

• OK, so that was good practice.
• Lets try the same thing but on a programming
  problem. Youll find that problem at the end of
  this handout. Now its your turn to practice.
• Task N
• Your job will be (eventually) to write a program
  to solve this problem. But were going to
  sub-divide the job into many smaller tasks.
• Task 1
• Write a series of Level 1 steps that break down
  this problem. Remember, there should be no more
  than 4 or 5 steps in this level. Remember also
  you want to make each of the steps about equal in
  weight. This is equivalent to the example we did
  where we ended up with the following
• Level 1 - Eating Lunch
• 1. Find a friend to eat lunch with.
• 2. Get your food.
• 3. Eat it.
• Go no further were going to talk this over as
  a group.

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Breaking Down A Task

• Now that weve talked over the Results of Task 1,
  we are ready to go on.
• Task 2
• Take the first of these high-level steps and
  subdivide IT into Level 2 Steps. (This can be
  either a list or a flowchart.)
• Find a friend in the lab, and talk over your
  solutions to this Level 2 subdivision. Agree on
  the best way of doing this subdivision.
• Go no further were going to talk over this
  result as a group.

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Breaking Down A Task

• Now that weve talked over the Results of Task 2,
  we are ready to go on.
• Task 3
• Are your Level 2 steps small enough? Can you
  take each of those steps and write code? If yes,
  great. But you might find that you need to do a
  low-level breakdown.
• Take one of your Level 2 steps and break it down
  until it is of a size that can be coded these
  are Level 3 steps. (This can be either a list or
  a flowchart.)
• Find a friend in the lab, and talk over your
  solutions to this Level 3 subdivision. Agree on
  the best way of doing this subdivision.
• Go no further were going to talk over this
  result as a group.

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Breaking Down A Task

• Now that weve talked over the Results of Task 3,
  we are ready to go on.
• Task 4
• You will do the rest of this on your own. Do NOT
  work with a partner on the remaining pieces.
• Continue to break down your remaining Level 1
  Steps, each into a series of Level 2 Steps.
• Those Level 2 Steps that are still too big to
  code directly should be divided into Level 3
  Steps.
• You will end up with something like the chart
  below.
• Hand in this final product.

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Representation
There are a number of ways of representing the
complete breakdown of the problem
LIST 1. Step 1 Level 1 Description 2. Step
1A Level 2 Description 3. Step 1B Level 2
Description 4. Step 1C Level 2
Description 5. if input is TRUE, then
goto line 7 6. Step 1C1 Level 3
Description 7. Step 1C2 Level 3
Description 8. Step 1D Level 2
Description 9. Step 2 Level 1 Description 10.
etc.
Flow Chart Step 1 Level 1
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The Problem

• Heres a description of the problem you are to
  solve. You will do this via the method described
  on the previous pages.
• Finding a Square Root
• Computers, of course, can be asked to solve a
  square root function. In C this problem could
  be solved by using the function
• root pow( X, 0.5 )
• So, how does C solve this function? If C is
  running on a CISC processor, theres probably an
  Assembly Level instruction that can do the job.
  In fact, on an Intel processor, this instruction
  is
• SQRTSD Rd, Rs
• Take the square root of the double precision
  number found in Rs, and put the result in Rd.
• On a RISC processor, there is no such
  instruction, so the C Language Runtime Library
  has to do the calculation, using the RISC
  instructions (multiply, divide, etc.) that it has
  available from the RISC language set.
• Whether its done in hardware (CISC) or in
  software (RISC), the calculation is based on a
  mathematical entity called a Taylors Series.

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The Problem

• The Taylors Series

One of the things thats interesting about this
is that it goes on forever!
The Rest Of The Story As you can see, this
series only works for square roots between 0 and
2. To make this work in a general case, you need
to do the following Step 1 Start with the
value you want to evaluate Step 2 Find Y,
the largest square thats less than Z. Step 3
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The Problem

• So The Hardware or Software Performs This
  Calculation
• As you can see, the solution of the Taylors
  Series involves a loop in which each term of the
  series is calculated. These terms each have a
  lot of pieces in them, so theres a bit of
  thinking to do here.
• Your job is to calculate this series.
• First do the original series that will accept
  values from 0 to 2.
• Only when that is successful should you undertake
  The Rest Of The Story.