CSC231 Lab 4: Debugger

--D. Thiebaut 14:03, 24 September 2010 (UTC)

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This lab 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:

a          dd          3
b          dd          5
result     dd          0

The code section, something like this:

          nop
          nop
          mov          eax, dword[a]
          add          eax, dword[b]
          mov          dword[result], eax

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

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

Test

Run your program.

What happens?

Why?

The Debugger

Start the Debugger

Start the debugger and instruct it to open the program simpleAdd

         ddd simpleAdd &

Configuration

You need to do this step only once, the very first time you use the debugger.

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. 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.

Quiz

• Create a new program in your account called la4.asm, with the code below

;;; lab4.asm
;;; YourName
;;; 
;;; this program should be assembled, linked, and debugged
;;; with ddd to figure out the contents of msg1 at the different
;;; phases of the program: phase1, phase2, phase3, phase4,
;;; and phase5
;;;
;;; to assemble and run:
;;;
;;;     nasm -f elf -F  stabs lab4_mystery.asm
;;;     ld -melf_i386 -o lab4_mystery lab4_mystery.o
;;;     ./lab4_mystery
;;; -------------------------------------------------------------------
EXIT    equ             1
WRITE   equ             4
STDOUT  equ             1
 
      	;; ------------------------------------------------------------
	;; data areas
	;; ------------------------------------------------------------
 
	section	.data
msg0	db      10, 10, 10, 10, 10
msg1	db	"                               "
	db      10, 10, 10, 10, 10
MSGLEN  equ     $-msg0
	
	;; ------------------------------------------------------------
	;; code area
	;; ------------------------------------------------------------
 
	section	.text
	global	_start
 
_start:
	nop
	nop
	nop
phase1:	
	mov	eax, 0x6c6c6468
	mov	ebx, 0x6064216e  
	mov	ecx, 0x6c687400  
	mov	edx, 0x13070e09  

phase2:	
	mov	dword [msg1], eax
	mov	dword [msg1+4], ebx
	mov	dword [msg1+8], ecx
	mov	dword [msg1+12], edx
	
phase3:	or	dword [msg1], 256
	xor	dword [msg1+4], 0x01010101
	or	dword [msg1+8], 'r'
phase4:	
	or	word [msg1+12], 0x6060
	or	word [msg1+14], 0x6060

phase5:	
	mov	eax, WRITE
	mov	ebx, STDOUT
	mov	ecx, msg0
	mov	edx, MSGLEN
	int	0x80
 
  
	;; exit()
 
	mov	eax,EXIT
	mov	ebx, 0
	int	0x80		; final system call

• Assemble, link, run and/or debug the program with ddd and answer the following questions on a sheet of paper.

Question 1

How many spaces are in msg1 (do not count, use ddd to answer the question!)

Question 2

what is the contents in 4 data registers (eax, ebx, ecx, and edx) in decimal at the beginning of phase2? In Ascii? (you may want to use this Ascii table to answer this part.

Question 3

what is the contents of msg0 in memory, at the beginning of Phase3 (before Phase3 starts), in hex?

Question 4

Same question, but this time express msg0 in ascii?

Question 5

What gets printed by the program?