CSC231 Homework 7 2014

--D. Thiebaut (talk) 10:33, 11 November 2014 (EST)

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This assignment is due on 11/18/14, at 11:55 p.m.

Preparation

• You may want to do some reading about the NEG and NOT instructions

• http://home.myfairpoint.net/fbkotler/nasmdocc.html#section-A.4.189
• http://cs.smith.edu/~thiebaut/ArtOfAssembly/CH06/CH06-2.html#HEADING2-313

Problem #1

• Use the 231Lib.asm library as inspiration and write a function called _printInt that prints a 32-bit 2-s complement number in decimal on the screen.
• Your function should work similarly to _printDec, in that the number to print is passed through eax, and not through the stack! For example, if we store 0xFFFFFFFF in eax and call your function, it should print -1 on the screen.
• Your function should not modify any of the registers, including eax.
• Your function should be made global in your program file, so that the function can be called by a test program.
• Your function can call _printDec to do some of its printing.
• Submit your function in a program called Hw7_1.asm. Make sure that your Hw7_1.asm program does not contain a _start label, otherwise the linker will be confused when it finds two global labels with the same name, in your program, and in the test program.

Example Test Program

          section .text

          extern  _printInt
_start:
          mov     eax, 0
          call    _printInt         ; prints 0
  
          mov     eax, 0xff
          call    _printInt         ; prints 255

          mov     eax, -1
          call    _printInt         ; prints -1

          mov     eax, 0xffffffff
          call    _printInt         ; prints -1
          
          mov     eax, 1
          mov     ebx, 0
          int     0x80

Problem #2

• Write a function called _printInt16 that prints a 16-bit 2's complement number on the screen.
• Your function should work similarly to _printInt, but the number it prints is the number passed in ax (not eax).
• Your function should not modify any of the registers, including ax or the upper part of eax.
• Your function should be made global so that a test program can call it.
• Your function can call your _printInt function (see previous problem) to do some of its printing.
• Submit your function to Moodle in a program called Hw7_2.asm. Also, as with Program 1, make sure that your Hw7_2.asm program does not contain a _start label.

Example Test Program

          section .text

          extern  printInt16
_start:
          mov     ax, 0
          call    _printInt16         ; prints 0
  
          mov     ax, 0xff
          call    _printInt16         ; prints 255

          mov     eax, 0x0000ffff
          call    _printInt16         ; prints -1

          mov     eax, 0xffff0001
          call    _printInt16         ; prints 1
          
          mov     eax, 1
          mov     ebx, 0
          int     0x80

Problem #3

Please answer the multiple-choice questions in Moodle, Section Homework 7 Problem 3.

Problem #4

• Use emacs to create a file that you will call hw7.cpp
• Store the following C++ program in this file:

#include <iostream>

using namespace std;

main() {
  unsigned int x, lastx;

  // print 4 blank lines
  cout << endl << endl << endl << endl;

  // initialize x and show its first value
  x = 3;
  cout << "x = " << x << endl;

  // loop for-ever... 
  while ( 1 ) { 
    lastx = x;
    x = x * 2;
    // if the value of x is ever less than or equal to lastx, stop the loop
    if ( x <= lastx ) {
      cout << "last x = " << lastx << endl;
      cout << "x = " << x << endl;
      break;
    }
  }
  cout << endl << endl << endl << endl;
}

• Compile the program as follows:

 g++ hw7.cpp -o hw7

• Run the program as follows:

 ./hw7

The following questions should be answered in Moodle, Section Homework 7, Problem 4.

Question 1

Why does the loop stop?

Question 2

From your observation the output, how many bits does C++ use to store integers (the variable x)?

Question 3

Remove the word unsigned in front of int, compile and run the program again. From your observation the output, how many bits do you know C++ use for regular (not unsigned) ints?

Question 4

Replace "int" by "char" in the declaration of x and lastx. Change the way x and lastx are printed by typecasting them to int first:

   cout << "x = " << (int) x << endl;
   ...
   cout << "last x = " << (int) lastx << endl;
   cout << "x = " << (int) x << endl;

Compile and run the program again. From your observation the output, how many bits are used by C++ to store characters?