Tech Juice

Introduction

If learning assembler with the NASM.exe assembler peaks your interest, you might be interested in this extensive tutorial by Paul Carter. Annoyingly, finding a linker on Windows that plays nice with NASM took me a while. The tutorial by P. Carter is not very clear on this matter. I wanted to use the linker link.exe that comes with Visual Studio 2010, because the free DJGPP linker that is mentioned by Mr Carter generates badly formatted .exe files. In this post I will show how to compile and link a Hello World assembler program with nasm.exe and link.exe.

Compiling Assembly Code

Let’s compile and link this Hello World program (courtesy of Ray Toal) in a file named helloworld.asm.

; This is a Win32 console program that writes "Hello, World" on one line and
; then exits.  It needs to be linked with a C library.
 
global  _main
extern  _printf
 
section .text
_main:
push    message
call    _printf
add     esp, 4
ret
message:
db      'Hello, World', 10, 0

As you can see we use the printf() function to print the text “Hello, World”. This function is marked as extern, because it is an imported function (it resides in the C Runtime Library).

The tutorial …

Linking Types

A C or C++ program compiled on Windows almost invariably uses the C Runtime Libary (CRT). This set of functions comes with your default Windows installation and provides some of the most basic operations, read this page for details. Now, remember that the last step of creating an EXE is called linking — the process that combines one or more compiled objects and combines them into a single executable.This post describes how to change your CRT linking method to control the size of your EXE file. There are two ways to link an external library such as a DLL.

• Static linking: All library routines used in the program are copied into the final EXE file. This means there is no dependency on some file existing at run-time.
• Dynamic linking: Both the name of a shareable library and the functions you use from that library are placed in the EXE file. When the executable is loaded, the operating system needs to do some extra work to locate this library and map it into your program’s virtual memory.

There is actually a third, “super-dynamic” way of loading DLL’s : calling a loadLibrary() function in your code. This means that you can load different libraries…

The dumpbin.exe tool is included with Visual Studio, and its a great way to get details on the structure of a Windows Portable Executable or COFF file. For those of you who don’t know: the Portable Executable format is used by Windows for .exe files, and it is based on COFF. You can run it from the Visual Studio Command Prompt.

Now if you want all information you would use dumpbin /all bla.exe. This gives you all the information on the structure of the file, but it also print all the ‘raw’ content: the CPU instructions. This will leave you with a dump that is very hard to read. To omit the raw content use the following command:

dumpbin /all /rawdata:none bla.exe

This will print ALL the information about the file, except the raw data. You can also save the output to a file.

dumpbin /all /rawdata:none bla.exe > bla.exe.txt

This will save the dump to bla.exe.txt in the same directory. Running this command on C:\Windows\notepad.exe generates this output:

Microsoft (R) COFF/PE Dumper Version 10.00.30319.01
Copyright (C) Microsoft Corporation.  All rights reserved.
 
Dump of file notepad.exe
 
PE signature found
 
File Type: EXECUTABLE IMAGE
 
FILE