0. Preparations

• To do this project, you must obtain a copy of the prepared files with these commands:

  mkdir ~/cs255/pj6 cp /home/cs255001/Handouts/pj6/* ~/cs255/pj6

1. Euler's Algorithm

I have "converted" the famous Euler algorithm for determining the Greatest Common Divisor (GCD) of two numbers A and B without using a division operation:

    int A, B, GCD;

    // First, make sure A is largest of the two numbers...
    if (B > A)
     { X = A;    // Swap A and B 
       A = B;
       B = X;
     }

    while (A != B)
    {
       while ( B < A )
       {
         A = A - B;
       }

       X = A;    // Swap A and B
       A = B;
       B = X;
    }

    GCD = B;

When the algorithm ends, the variable GCD will contain the Greatest Common Divisor of A and B.

Your program can change the values in A and B; I won't check these values when the program ends. I will only check whether GCD variable contains the correct GCD of A and B at the end of execution.

2. Assignment

Your assignment is to write the above Euler algorithm in ARM assembler language between the main and Stop labels in pj6.s that will determine the GCD of the integer variables A and B.

The variables A, B, and GCD have already been defined in pj6.s - they are all integer variables.

Also, the variables A and B have been initialized with some value. You need to write the Euler Algorithm beginning at the label main. The GCD computed by the algorithm must be stored in the integer variable GCD.

Note:

Your program must work using any value stored in the variables A and B (I will use different values in A and B to test your program !!!) You can assume that A and B are both positive numbers

3. The EGTAPI debug file

I have made an Egtapi startup file (egt.pj6.arm) for use in this project that will display the variables A, B and GCD. You have already made a copy of this debug file in the preparation section

You may need to add more display variables and/or breakpoints to help you debug the assembler program.

To add a variable:

Enter the name of the variable in the variable watch area Select the data type (signed int, signed short, etc) Select the base (binary, decimal or hexadecimal) Then click Add Var

Make sure that:

The variable name is defined as: .global Varname at the start of your program (you will need to add this assembler directive to your program)

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To add a breakpoint (learn more about break points in next section of this handout):

Enter the name of a label in the Break Points panel of Egtapi Click the Add Brkpoint button

You must have defined the label in your program (and compiled it.)

Make sure that:

The label is defined as: .global label at the start of your program (you will need to add this assembler directive to your program)

4. Some Information on Breakpoints

This is your first real assembler programming project; the program is no longer a bunch of move instructions. Your program will surely have bugs that you need to find, so here is a little advice on what to do to find them.

To find bugs in an assembler program, you have to look the the execution of the portion of the program that is in error. First, you have to make the program stop at the place that contains an error - that is done using "break points".

A breakpoint is a location in the program that when the program reaches that location, Egtapi will stop and await your next command. After the program stopped, you can: look at the value of the variables to see if the program has progressed correctly so far.

After examining various things (e.g. variables), you can then choose to continue the execution of the program from the breakpoint with one of these two actions:

• "Run" (the "Run" button is located at the top right corner of the Egtapi window).

  When you click on the "Run" button, Egtapi will run the program until it reaches a break point.

• "Step" (the "Step" button is located at the top right corner of the Egtapi window, it's the left most button on the "bar")

  When you click on the "Step" button, Egtapi will execute one assembler instruction and stop.

  When Egtapi stops, it will show you the current value of all registers and variables that you have requested (in the variable watch window).

The "Step" button is very helpful to find bugs in your program !!

5. How to debug assembler programs

To find bugs in a program, you need to

1. determine what is the correct behavior of your program and then
2. follow the execution of the program to see if it is indeed doing what it is supposed to do.

If the result prodcued by the program differs from what it supposed to do, you found yourself a bug....

Ofcourse, this requires from you that you know what the assembler program is supposed to do. If you don't have a clue about the assembler program is supposed to do, Egtapi will not be much help to you.... - because you do not know what to look for. (Analogy: if you know nothing about radiology, showing you an X-ray picture is useless since you don't know what to look for....)

So to be successful in finding bugs:

• Find out what values of the variables is supposed to be changed to.

• When you detect that the values of the variable are NOT what they should be, STOP.

  Find out what caused the program to stray !

More information on how to find errors in your assembler programs using Egtapi:

I have written an extensive guide on how to use breakpoints in Egtapi here: click here           --- Read Sections 9 and 10 !!!

6. Turn in

Turn in your pj6.s assembler program as pj6 using the turnin command (execute the turnin command while you're inside your cs255 directory)

In other words, open the Terminal in EGTAPI and type in thses 2 commands to turn in pj6:

cd ~/cs255/pj6 /home/cs255001/turnin pj6.s pj6 // If you're in section 1 or /home/cs255002/turnin pj6.s pj6 // If you're in section 2

I want the source (so I can read it). DO NOT turn in the executable or the object code !

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• Extension request
  • To request an extension for pj6, you must execute the following command before the deadline of the homework

    Open the Terminal in EGTAPI and type in this command to request an extenstion for pj6:

    /home/cs255001/req-ext pj6 // If you're in section 1 or /home/cs255002/req-ext pj6 // If you're in section 2

    You request will be successful if you have not exceeded the maximum number of "free" (no-questions-asked) requests allowed