Chapters 914 Exam 3 Review

1
Chapters 9-14Exam 3 Review
2
Exam 3 Information

• Tuesday to FridayBe sure to check the testing
  centers hours!
• Closed book, closed note, no time limit
• No calculators or electronic devices are allowed
• Covers chapters 9-14
• 50 multiple choice questions (1-25 1 point, 26-50
  3 points)
• You may NOT write on exam! But only the bubble
  sheet will be graded

3
Suggestions on Studying for an Exam

• Make sure you understand these review slides
• Review your homework
• Make sure you understand your mistakes
• If you guessed on anything, go back and see why
  your guess was right or wrong
• Skim through the all the lecture slides
• Slow down and take a close look when you get to a
  topic with which you arent comfortable
• Reread the relevant paragraphs in the book, if
  necessary
• Read the review topics posted online
• Look at relevant lecture slides if you arent
  completely confident you understand the topic

4
Suggestions on Studying for an Exam

• As you review, ask yourself questions
• Consider scenarios or ideas not discussed in
  class
• Dont just read try to understand
• If you find something you dont understand, take
  the time to figure it out!
• Look at other sources or examples
• Try not to move on without resolving your
  questions
• Ask the TAs or instructor about questions you
  cannot resolve or topics you dont understand

5
Test Taking Strategy

• Take your time!
• Theres no time limit on these exams
• Read each question carefully and completely
• Dont be mislead by a partially correct answer
• The correct answer may be further down the list
• If you read too quickly you might miss keywords
• Double-check any question that involves
  calculation
• If possible, solve the problem a different way

6
Exam 3 Review

• What is the purpose of the trap and interrupt
  vector tables?
• They contain the starting addresses of the trap
  and interrupt service routines
• How does the TRAP instruction compare with the
  JSR instruction?
• TRAP gets its target address from the trap vector
  table, whereas JSR uses PCoffset to calculate
  its target address
• TRAP loads the PC with an OS address, JSR loads
  PC with user-space address
• The programmer/assembler does not need to know
  the target address for a trap service routine
  it just needs its number. The programmer/assembler
  must know the target address for a subroutine to
  be called with JSR

7
Exam 3 Review

• What is the purpose of the LC-3 TRAP instruction?
• For calling OS service routines for common system
  operations (e.g., I/O)
• What precautions must be taken regarding R7 in
  assembly code that uses TRAP or other
  subroutines?
• A subroutine must properly save and restore R7 if
  it going to be modified (e.g., by making another
  subroutine call)
• What is the difference between caller-save and
  callee-save?
• Caller-save Code saves needed registers before
  calling a subroutine, restores them after
  subroutine returns
• Callee-save Subroutine saves registers before
  using them, restores them before returning

8
Exam 3 Review

• What is polling?
• A method of I/O where a loop is executed to
  constantly check a device status register until
  the device is ready
• What is interrupt driven I/O?
• A method of I/O where an I/O device informs the
  CPU that it is ready via an interrupt signal,
  causing the CPU to interrupt what its other
  processing and service the device
• What are the advantages/disadvantages of each?
• Polling is very simple and requires little code,
  but is often inefficient because it keeps the
  processor busy
• Interrupt driven I/O is more complicated and
  requires more code, but is usually much more
  efficient (especially for slow I/O devices)
  because the CPU can be doing other processing
  while waiting for an I/O device

9
Exam 3 Review

• What are the KBSR, KBDR, DSR, and DDR?
• Keyboard Status Register
• Keyboard Data Register
• Display Status Register
• Display Data Register

10
Exam 3 Review

• What is the MCR?
• Machine Control Register
• What is the MCR used for?
• MSB is ANDed with clock
• Can be used to halt the machine
• Whats the common name for a LIFO data structure
• A stack
• How does a LIFO data structure behave
• Last In First Out
• The last item pushed will be the next item popped

11
Exam 3 Review

• What is the stack pointer?
• R6 on the LC-3
• Always points to top of run-time stack
• Write the push and pop operations for the LC-3
• PUSH ADD R6, R6, -1 STR R0, R6, 0
• POP LDR R0, R6, 0 ADD R6, R6, 1
• What is stack overflow?
• When too many items are pushed onto the stack
• What is stack underflow?
• When too many items are popped off the stack

12
Exam 3 Review

• What are the advantages of high-level languages
  as compared to low-level?
• Allows us to give symbolic names to values
• ISA independent
• More expressive do more with less code by
  providing commonly used operators, data types,
  and control structures
• More readable, easier/faster to write and debug
• Provides safeguards, such as checking data type
  usage and checking for common errors
• Provides commonly used operators

13
Exam 3 Review

• What are the differences between an interpreter
  and a compiler?
• Compiler creates an executable image once,
  whereas interpreter must interpret the program
  each time it is run
• Compiled code runs faster
• Interpreted programs are platform independent
• Interpreted code is often easier to debug and
  develop

14
Exam 3 Review

• What are the three components of a compiler
• Preprocessor
• Compiler (and assembler)
• Linker
• In which order are the compilers components run?
• Preprocessor ? Compiler (and assembler) ? Linker
• What does the preprocessor do?
• Preprocesses your C code, making insertions,
  deletions, and substitutions
• What does the compiler do?
• Converts the preprocessed code into a binary
  object module
• What does the linker do?
• Links the binary object file with libraries
  and/or other object files to create the final
  executable image

15
Exam 3 Review

• What are the essential components of a C program?
• include ltstdio.hgtint globalVarint
  main() int localVar 1 printf(d and d
  makes d\n, globalVar, localVar,
  globalVarlocalVar)
• Where does a C program start?
• The main() function

• Preprocessor Directives
• Variable declarations
• The main() function (must be present)
• Statements (ending in or )
• Note the syntax!

16
Exam 3 Review

• What are valid C identifier names?
• Must start with letter or underscore, not with
  number
• Cannot use C operator symbols (, -, /,
  , gt, etc.)
• Cannot use C reserved words (for, if,
  while, etc.)
• What are the differences between a local and
  global variables?
• Local variables are not initialized by default,
  global variables initialized to 0
• Local variables are stored on the stack (pointed
  to by the frame pointer, R5), global variables
  stored in global data section (pointed to by R4)
• Local variables have limited scope (from the
  variable declaration to the following ),
  global variables can be used anywhere in the
  program

17
Exam 3 Review

• How big of a number can be stored in a variable
  of type int?
• Depends on ISA!
• Whats the difference between int and signed int?
• They are the same
• Which can represent more values, an int or an
  unsigned int?
• The ranges are different, but they both represent
  the same number of values (2n where n is the
  number of bits)
• E.g., -32,768 to 32,767 compared to 0 to 65,535
  (both 16-bit)
• Or in general -2n-1 to 2n-1-1 compared 0 to
  2n-1-1
• Which is bigger, a char or int?
• Usually int, but depends on ISA
• On the LC-3, they are the same!

18
Exam 3 Review

• Whats the difference between bitwise and logical
  operators in C?
• Bitwise operators operate on individual bits
  using Boolean operators (hence the name bitwise)
• Logical operators treat the operands as Boolean
  values and produce a Boolean result (0 or 1)
• Example x 6 y 5
• x y ? 01102 01012 ? 01002 ? 4
• x y ? true true ? true ? 1

19
Exam 3 Review

• What is operator precedence?
• Determines the order in which operations are
  executed in an expression
• See the operator precedence table for details
• How is variable scope determined?
• Based on where the variable is declared
• Local variables are declared within a function,
  and scope is from variable declaration to
• Global variables are declared outside any
  function, scope is global

20
Exam 3 Review

• What does the following code do?

START add r6, r6, -2 str r2, r6, 0 str r3,
r6, 1 and r0, r0, 0 and r3, r3, 0 add r2,
r2, 0 brzp L02 not r2, r2 add r2, r2,
1 add r3, r3, -1 L02 add r2, r2,
0 brz L04 add r2, r2, -1 add r0, r0,
r1 brnzp L02 L04 add r3, r3, 0 brz L06 not r0
, r0 add r0, r0, 1 L06 ldr r2, r6,
0 ldr r3, r6, 1 add r6, r6, 2 ret
21
Exam 3 Review

• What does the following code do?

START add r6, r6, -2 Store r2, r3 on the
stack str r2, r6, 0 NOTE r0 isnt stored on
the stack since str r3, r6, 1 a value
gets returned in r0 and r0, r0, 0 Clear
r0,r3 and r3, r3, 0 add r2, r2, 0 Test r2,
if negative then invert r2 and brzp L02 set
flag in r3 not r2, r2 add r2, r2, 1 add r3,
r3, -1 L02 add r2, r2, 0 Test r2, if zero
then skip to the end brz L04 else perform
multiplication loop add r2, r2, -1 add r0, r0,
r1 brnzp L02 L04 add r3, r3, 0 Test r3, if
flag set then invert mult result brz L06 not r0,
r0 add r0, r0, 1 L06 ldr r2, r6, 0
Restore r2,r3 from the stack ldr r3, r6,
1 add r6, r6, 2 Clean up the stack
pointer ret
22
Exam 3 Review

• What does the following code output?

include ltstdio.hgt int x int main() int y
x int x 15 int i 0 for (i 0 i
lt 4 i) int x i printf("d d\n",
x, y) printf("d d\n", x, y)
0 0 1 0 2 0 3 0 15 0