CSC231 DDD Short Tutorial

--D. Thiebaut (talk) 20:25, 4 April 2017 (EDT)

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This tutorial will introduce you to the ddd debugger, and simple steps you can take to debug assembly programs.

The Target Program

Create a simple program that adds the contents of two variables and stores the result in a third one.

The data section should be something like this:

                section .data
a		dd	3
b		dd	5
sum		dd	0
table		dd	6, 7, 8
	
	
                section .text
                global  _start
_start:
		nop			; we put nops to 
		nop			; set a breakpoint first

		mov	eax, dword[a]
		add	eax, dword[b] 	
		mov	dword[sum], eax

		mov	ebx, dword[table]
		add	ebx, dword[table+4]
		mov	dword[table+8], ebx
	
;;; exit                                                                                                                                    
                mov     ebx, 0
                mov     eax, 1
                int     0x80

Save your program as demoDDD.asm, and generate the executable as follows:

         nasm -f elf -F stabs demoDDD.asm
         ld -melf_i386  -o demoDDD demoDDD.o

Test

Run your program...

Do you see anything?

Should you?

You probably figured out that since we are not printing anything, it's impossible to know if it computed what we expected.

The debugger will allow us to figure it out!

The DDD Debugger

Start the Debugger

Start the debugger and instruct it to open the program demoDDD. At the prompt, type:

         ddd  demoDDD  &

The first thing you'll want to do is change a couple default behaviors:

Configuration

Before we can start using DDD, we need to change some of its default settings.

Select "Edit->Preferences" from the main menu. On the window that will appear, open the "Source" tab (top of the window). Then, check the "Display Source Line Numbers" option. Click OK to close the window.

Next, select "Edit->GDB Settings" from the main menu. In the window that pops up, scroll approximately half way down through the list of options, until you find the option "Disassembly flavor". Change its value to "Intel" (Figure 3.2) and click on CLOSE.

Running the Debugger

Just follow the same steps we did in class to execute your program one step at a time, and to verify that the number 8 ends up in the variable result.

1. Click on Status, Registers to see the register window
2. Set a breakpoint on the second NOP instruction by clicking right on the instruction
3. Display the 3 variables: Data, then Memory, then select to display 1 hex double-word at location &a. Same thing for b and result.
4. Run the program by clicking Run in the tall grey window pane. The debugger will execute the first NOP and stop on the second one.
5. Single step the program with Stepi.

Exercise 1

Edit your program so that now it computes the sum of 5 variables, a, b, c, d, and e, and saves the sum in result.

Single step your program and verify that it generates the correct answer.

Exercise 2

Modify your program one more time and make the data section equal to this definition:

Fib        dd          1, 1, 0, 0, 0, 0, 0

And modify the code so that your program will store the sum of the first two double-words into the third double-word, the sum of the second and third double-word in the fourth, and so on. This way your program will compute the first 7 terms of the Fibonacci sequence.

Debug your program and show your instructor that you end up with 1, 1, 2, 3, 5, 8, 13 in memory.