Project 9: Disassembling C on Windows Part 2 (15 pts. + 10 extra credit)

What You Need

Purpose

You will write a small C programs using arithmetic statements, compile it, and examine it in the IDA Pro disassembler to learn what it looks like in assembly language.

Starting Visual Studio Express

Click Start, "VS Express 2013 for Desktop".

Making a New C Program

From the "Visual Studio Express 2012" menu, click FILE, "New Project...".

In the "New Project" window, on the left, expand the "Visual C++" container.

Click Win32.

In the center pane, accept the default selection of "Win32 Console Application".

At the bottom of the "New Project" window, type a Name of YOURNAME-9a, replacing "YOURNAME" with your own name. Do not use any spaces in the name.

In the "Location" line, click the Browse button and navigate to a folder you have permission to save files in, such as your desktop.

Click the "Select folder" button.

In the "New Project" window, click OK.

A box opens, titled "Welcome to the Win32 Application Wizard".

Click Next. In the next screen, accept the default settings and click Finish.

A window opens, showing a simple C program.

Modify this program to match the code shown in text and the image below.

Do not use the literal string "YOURNAME"--replace it with your own name.

// YOURNAME-9a.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"

int _tmain(int argc, _TCHAR* argv[])
{
int i=10;
int j=2;
int k;
i = i + 2;
k = i / j;
printf("YOURNAME-9a: %d %d %d\n", i, j, k);
return 0;
}

Compiling your Program

Click BUILD, "Build Solution".

You should see the message "Build: 1 succeeded" at the bottom of the window. If you see errors, you need to correct them and re-compile the program.

Running your Program

Click DEBUG, "Start Without Debugging".

A Command Prompt window opens, showing the output of "YOURNAME-9a: 12 2 6", as shown below:

Disassembling the EXE

Click in the Command Prompt window, and press Enter to close it.

Minimize the Visual Studio Express window.

Start IDA Pro Free.

In the "About" box, click OK.

Agree to the license.

Close the Help window.

In the "Welcome to IDA!" box, click the New button.

In the "New disassembly database" box, double-click "PE Executable".

In the "Select PE Executable to disassemble" box, navigate to the folder you used to save your program in Visual Studio Express, probably your desktop.

Double-click the "YOURNAME-9a" folder.

Double-click the Debug folder.

Double-click the YOURNAME-9a.exe file.

In the "PE Executable file loading Wizard", click Next, Next, Finish.

A box appears, saying this file was linked with debug information.

Click Yes

IDA Pro loads the file. As before, the graph mode doesn't show the interesting part of the program.

Expand the Strings. Double-click "YOURNAME-9a %d %d %d\n".

The location containing the string appears.

To the right of "YOURNAME-9a" there is a "DATA XREF" comment. To the right of the "XREF", double-click "wmain".

Now the assembly code that performs the task you wrote in C appears, as shown below.

Find the commands listed below, and see how they work. The explanations refer to the C code added to the figure below in the box with green shading.

ASM CodeExplanationC Code
 
mov [ebp+var_8], 0Ah    Put the number 10 into a local variable (i)

    int i=10;
mov [ebp+var_14], 2    Put the number 2 into a local variable (j)

    int j=2;
mov eax, [ebp+var_8]    Put i into eax
add eax, 2    Add 2 to eax
    i = i + 2;
mov [ebp+var_8], eax    Put eax (the result) into a local variable (i)

mov eax, [ebp+var_8]    Put i into eax
cdq    Convert double to quad (required for division)
    k = i / j;
idiv [ebp+var_14]    Divide the value in eax by a local variable (j)
mov [ebp+var_20], eax    Put eax (the result) into a local variable (k)

Saving the Screen Image

Make sure you can see the commands listed above, and YOURNAME at the bottom.

On your keyboard, press the PrntScrn key.

Click Start, type in PAINT, and open Paint.

Press Ctrl+V to paste in the image of your desktop.

YOU MUST SUBMIT WHOLE-DESKTOP IMAGES TO GET FULL CREDIT.

Save the image with a filename of "Proj 9a from YOUR NAME".

CHALLENGE: 10 Pts. Extra Credit

Modify the C program to use multiplication and subtraction, compile it and disassemble it, producing the assembly code shown below.

It must show these features, as labelled in the image above:

Turning in Your Project

Email the images to: cnit.126sam@gmail.com with a subject line of Proj 9 From Your Name, replacing Your Name with your own first and last name. Send a Cc to yourself.

Last Modified: 9-22-14 3:56 pm