Contents:

• Overview
• Practice Problems
• Problems to be Submitted

Topic: The Intel x86 ISA, x86 Assembly Programming, Conditionals (if-then-else), and Loops (while, for, and do-while)
Related Reading: Chapter 3 and class notes

Practice problems from the textbook (answers are at the end of the chapter):

• Practice Problems 3.16-3.18, on pp. 195-196.
• Practice Problem 3.20, on p. 199.
• Practice Problem 3.21, on pp. 201-202.
• Practice Problem 3.22, on p. 202.
• Practice Problem 3.23, on p. 205.
• Practice Problem 3.31, on p. 224.
• Practice Problem 3.33, on pp. 228-229.
• Practice Problem 3.34, on pp. 231-232.
• Practice Problem 3.35, on pp. 233.
• Practice Problem 3.36, on pp. 234.
• Practice Problem 3.38, on pp. 238.

When you turn in your assignment, you must include a signed cover sheet (PDF version) with your assignment (you're assignment will not be graded without a completed cover sheet).

You are allowed to submit your assignment via email, but if you choose to do so, you must bring a hardcopy of your assignment along with a completed cover sheet to the instructor at the next class. (Note: Do not email the instructor any .zip file attachments, as SLU's email may not accept these emails; i.e. the instructor may not receive your email.)

1. (5 points)

   Given the following C code:

            if (a - 3 > b)
   	     c = a + 7;
   	 else if (b > (c & 7))
                c = b >> 3;
   	 else
   	     c = -1;
   

   Given the equivalent Intel x86 assembly code.

   Assume that: variables a, b, and c are in registers %eax, %ebx, and %ecx, respectively.

2. (5 points)

   Given the following assembly program, hw4_prob2.s, determine the equivalent C code corresponding to the procedure hw4_prob2(), and fill in the blanks below:

   Note 1: These blanks may include assignment statements and/or other conditional (if-then-else) statements. Also, you may not need to use all the blanks (in the conditional's true and false code bodies).

   Note 2: This file can be downloaded along with a Makefile in hw4_code.zip.

       int hw4_prob2 (int x, int y)
       {
           int result = ________________________________________;
   
           if (( x ___________________________) || ( ___________________________))
           {
               _________________________________________________;		// one or more of these lines will start with:   result = 
   
               _________________________________________________;
   
               _________________________________________________;
   
               _________________________________________________;
   
               _________________________________________________;
           }
           else
           {
               _________________________________________________;		// one or more of these lines will start with:   result = 
   
               _________________________________________________;
   
               _________________________________________________;
   
               _________________________________________________;
   
               _________________________________________________;
           }
   
           return result;
       }
   

3. (5 points)

   Problem 3.56 at the end of Chapter 3 (p. 296).

4. (5 points)

   Given the following x86 assembly code that sums the numbers from 1 to N:

         movl  $0, %eax			# %eax:  sum = 0
         movl  $1, %ecx			# %ecx:  i = 1
   
         LOOP:
   	 addl  %ecx, %eax
            addl  $1, %ecx
   
            cmpl  %edi, %ecx
            jle   LOOP
   
         LOOP_EXIT:
   	 ...
   

   Note: The variables of i, N, and sum are stored in registers %ecx, %edi, and %eax, respectively.

   1. What one line would you need to add and/or change in order to sum the following number sequence?
             1 4 7 10 13 16 19 22 25 28 ...

   2. What two lines (of the original code) would you need to add and/or change in order to compute the factorial of N?

   3. What one line lines (of the original code) would you need to add and/or change in order to compute the sum of squares from 1 to N?
             i.e. 1^2 + 2^2 + 3^2 + 4^2 + 5^2 + ...
                 = 1 + 4 + 9 + 16 + 25 + ...

   4. Add a conditional to the loop that checks if the number is even or odd, and negates the value if it's odd. In essence, this will generate the sum:        -1 + 2 - 3 + 4 - 5 + 6 - 7 + 8 - 9 + ...

      Hint: What one bit in a binary integer representation indicates whether the number is even or odd? And what bit-wise Boolean operation can you use to isolate that number?

5. (5 points)

   Using the same C program for the Fibonacci sequence as on Exam #1:

         a = 0;			 // F(0)
         b = 1;			 // F(1)
   
         for (i = 2; i < n; i++)
         {
             c = b + a;		 // F(i)
             a = b;
             b = c;
         }
   

   Give the equivalent sequence of assembly instructions for computing the Nth value of the Fibonacci sequence

   1. First, give the equivalent assembly in the form of a WHILE or FOR loop.
   2. Then, give the equivalent assembly in the form of a (converted) DO-WHILE loop.