CS2200 Presentation 2a

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CS2200Presentation 2a

• Pre-assessment Results

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Results

• Number of students who took the assessment

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Question

• What do you want to do?

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Question 1
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Question

• What do you know?

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Question 2

• Write a function in C called swap that will swap
  two ints
• It will be called like this
• int a 42
• int b 78
• / Call to swap goes here /
• printf("d d\n", a, b)
• The output would be
• 78 42
• Write swap here

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Solution

• void swap(int x, int y)
• int t
• t x
• x y
• y t
• / Call... /
• swap(a, b)

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Results Question 2 (Swap)
Lots of people don't know how pointers work!
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Question 3

• What does a "make" do? Your answer should include
  three major items.

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Solution

• The make program generates a sequence of commands
  for execution by the Unix shell
• Make uses a table of dependencies input by the
  programmer
• Make automatically updates files for the user
• When a change is made, make creates proper files
  with a minimum of effort
• Make has a macro facility
• name string CCgcc
• (name) (CC)

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Wrong answers?

• Make compiles your program
• Make cleans up your directory
• Make turns in your assignment
• Make is a preprocessor
• Make is like a batch file
• Makes your life faster and easier
• Touches files
• Slices, dices, liquefies a bass!

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Results

• Question 3 (Make)Wide variety of answers from
  blank to pretty close.Many answers based on
  perceptions gained from use.

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Question 4

• Write 42 in binary and in hexadecimal
• Write 42.25 in binary
• (Not IEEE Floating Point)

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Solution

• 4210 1010102
• 4210 2A16 0x2A
• 42.2510 101010.012

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Results
Obsolete

• Question 4 (Binary/Hex numbers)

a
b
c
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Question 4a Write 42 in Binary
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Question 4b Write 42 in Hexadecimal
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Question 4c Write 42.25 in Binary (Not IEEE
Floating Point)
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Question 5

• Given a full adder as a building block
• Inputs
• A B
• ---------
• Carry out -- -- Carry in
• ----------
• Output

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Question 5

• Given a full adder as a building block

Inputs
A
B
Carry out
Carry in
Output
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What does it do?

• 5 Adds A, B and Carry in

Inputs
A
B
Carry out
Carry in
Output
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What does it do?

• A, B and Carry in Carry out Output
• 3 zeros 0 0
• One 1 0 1
• Two 1's 1 0
• Three 1's 1 1

Inputs
A
B
Carry out
Carry in
Output
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Make an 4 bit adder
A0
A1
A2
A3
B0
B1
B2
B3
0
C0
C1
C2
C3
C A B
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How do we subtract?i.e. A - B

• Take the 2's complement of B and add!!!
• A B 1

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Complement?
B
x o r
C
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Can it subtract?
A0
A1
A2
A3
B0
B1
B2
B3
C0
C1
C2
C3
Control Calculation 0 C A B 1 C
A - B
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Overflow?
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Recall
N 3
Number Stored
Number Represented
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Consider

• 3 bit numbers
• Max allowed 3 (011)
• Min allowed -4 (100)
• 2 -2 2 -2
• 3 3 -3 -3
• 5 1 -1 -5
• 010 110 010 110
• 011 011 101 101
• 0101 1001 0111 1011
• Bad Ok Ok Bad

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Overflow?
A0
A1
A2
A3
B0
B1
B2
B3
C0
C1
C2
C3
Control Calculation 0 C A B 1 C
A - B
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Results

• Question 5 (Simple Arithmetic unit)

Drew 4 boxes
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Question 6

• What do you suppose this does
• load r1, 2
• load r2, 3
• add r3, r1, r2
• store r3, answer
• answer word 0

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One possibility...

• load r1, 2 Put 2 in R1
• load r2, 3 Put 3 in R2
• add r3, r1, r2 Add R1 and R2 result R3
• store r3, answer Store R3 in location
• answer
• answer word 0 Variable

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Results
Obsolete

• Question 6 (Pseudo assembly code)

Some confusion about memory use. Indication that
very rigid ideas exist as toassembler syntax.
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Question 6 (Pseudo assembly code)
-Some confusion about memory use. -Memory
Address Value -Indication that very rigid ideas
exist as to assembler syntax.
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Question 7

• What do you think about this
• Plan createNewSelectionNode(
• Cond condition,
• char relation)
• Plan newNode
• newNode(Plan)malloc(sizeof(Plan))
• newNode-gtopSELECTION
• newNode-gtcondParams0condition
• newNode-gttableP1NULL
• newNode-gttableP2NULL
• newNode-gttable1relation
• free(newNode)
• return newNode

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Solution

• Where do you think that this question came from?
• What's the really big error?
• What will happen?
• How likely?
• What's the other evil sin?
• What might happen?
• How likely?

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Misconceptions

• free() returns NULL
• free doesn't return anything!
• free() causes an immediate core dump?
• No! The free statement is perfectly valid!
• Hardly anyone mentioned checking the return value
  from malloc!!!
• 4 people caught the malloc but not the free!
• Casting return value from malloc is not illegal!
  Dates back to days before void pointers were
  invented. In fact, it might still be a good idea!

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Results
Obsolete

• Question 7 (Bad C Function)Note Wide variety
  of interpretations ofexactly what were
  implications of problem.

Excellent Identified errors
consequences Good Identified errors Fair
Identified free error
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Results
Excellent Identified errors
consequences Good Identified errors Fair
Identified free error

• Question 7 (Bad C Function)Note Wide variety
  of interpretations of exactly what were
  implications of problem.

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Conclusions

• Many of you need to quickly get the prerequisite
  knowledge for this course
• You will be designing control systems for digital
  logic circuits and implementing them in C!!!
• You need to know pointers, structs, queues,
  stacks, etc. really well

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Recommendation 1

• Buy (don't borrow) Kernighan Ritchie The C
  Programming Language
• Put the book under your pillow
• Crawl under your pillow with a laptop and a case
  of Surge
• Read the book. Work all the problems!!!

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Recommendation 2

• Review your 2030 stuff
• Use Appendix B in Patterson Hennessy
• Buy Code by Charles Petzold
• Follow same instructions re pillow and Surge

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Questions?
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