CMSC216 HW08: Review for Exam 2

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                            HW 08 QUESTIONS
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Write your answers to the questions below directly in this text file to
prepare for the associated quiz. Credit for the HW is earned by
completing the associated online quiz on Gradescope.


PROBLEM 1: Movement Mistakes
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  Analyze the files `posneg_main.c' and `posneg_asm.s'.  The C code uses
  a function in assembly and the assembly function has a common bug in
  it.


A
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  Compile the files together and run the resulting program. Make sure
  that you understand how to run a `gcc' command to compile the two
  files, C and assembly code, to produce an executable.

  After running the resulting program, explain why the output appears
  strange based on the local variables defined in `posneg_main.c' and
  the purpose of the function in `posneg_asm.s'.


B
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  Analyze the code in `posneg_asm.s' carefully and compare the `movX /
  cmpX' instructions used in the first few lines against the types of
  variables in the `posneg_main.c' code.  You may want to step into this
  function in GDB to look at the register values after the `movX'
  instruction. Find a bug in this sequence and describe why it causes
  the loaded value to appear negative.


C
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  Fix the bug in `posneg_asm.s' and paste your corrected code below.


PROBLEM 2: Convert C to Assembly
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  Convert the C function in the file `col_check.c' to x86-64
  assembly. Note that the parameter is a packed struct, not a pointer to
  a struct.

  ,----
  | typedef struct{
  |   int cur;            // current value in collatz sequence
  |   int step;           // step number in collatz sequence
  | } colinfo_t;
  | // |       | Byte |   Byte | Packed |
  | // | Field | Size | Offset |   Bits |
  | // |-------+------+--------+--------|
  | // | cur   |    4 |     +0 |   0-31 |
  | // | step  |    4 |     +4 |  32-64 |
  | 
  | int col_check(colinfo_t info){
  |   // Analyzes values in the packed struct arg
  |   // info to detect errors in it. An int
  |   // comprised of set error bits is
  |   // returned. Bit 0: cur field was 0 or
  |   // below, Bit 1: step was negative, Bit 2:
  |   // cur was 1 but step is negative.
  | 
  |   int cur = info.cur;
  |   int step = info.step;
  |   int errs = 0;
  |   if(cur <= 0){
  |     errs = errs | 0x1; // 0b0001
  |   }
  |   if(step < 0){
  |     errs = errs | 0x2; // 0b0010
  |   }
  |   if(cur==1 && step < 0){
  |     errs = errs | 0x4; // 0b0100
  |   }
  |   
  |   return errs;
  | }
  `----


PROBLEM 3: Valgrind Debugging Assembly
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A
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  Study the C file `badstack.c' which presents a small `main()' function
  which calls `inc_larger()'.  Compile this file and run the resulting
  executable.

  After you are comfortable with the C version, examine similar
  `badstack_asm.s' file which encodes the same two functions and
  algorithm but has bugs.  Compile and run this file and show the
  results.


B
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  While there is output produced by `badstack_asm.s' in most cases, it
  will not complete successfully.  To gain insight into what is
  happening, recompile the program with Debugging information by passing
  an additional flag to `gcc'.  Then run the program under Valgrind to
  show more information on the nature of the problem. Show the output
  from Valgrind below.


C
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  Analyze the Valgrind output carefully. It should contain one major
  error. Copy and paste the output that pertains to the error below and
  describe what you think might be going on.  Relate anything you see in
  the output to the values for variables that show up in
  `badstack_asm.s', particularly anything that is labeled as a 'Invalid
  Address' is relevant. As a hint, the problem lies with data that is in
  the stack.


D
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  Use your knowledge of what has happened to repair the buggy `main()'
  function in `badstack_asm.s'.  Describe the changes required below.
  After fixing the code, compile and run it to make sure that it
  performs correctly and identically to `badstack.c' does. You will
  likely need to make changes towards the beginning of `main()' and just
  prior to its `ret' instruction.

  Hint: You may also compile `badstack.c' to assembly code and examine
  the first 5-8 instructions in `main()' to see how it manipulates the
  stack to make space for local variables. Keep in mind that you should
  compile with the `-Og' option to optimize for debugging and that the
  setup generated by the compiler is somewhat more complex than required
  in the hand-coded `badstack_asm.s'.  You may also examine
  complementary stack manipulations near `ret' at the end of `main()'.