Win32 Assembly language programming
(using NASM)

Required Tools:

• NASM - Assembler
  
  
• Alink - Linker
  
  
• Gorc - Resource compiler
  
  
• Any text editor.

What Is NASM?

       The Netwide Assembler, NASM, is an 80x86 assembler designed for portability and modularity. It supports a range of object file formats, including Linux and NetBSD/FreeBSD a.out, ELF, COFF, Microsoft 16-bit OBJ and Win32. It can also output plain binary files. Its syntax is simple and easy to understand, similar to Intel's but less complex. It supports Pentium, P6, MMX, 3DNow!, SSE and SSE2 opcodes, and has macro capability.

Hello World Program:

      Lets start with a hello world example and compare both C version and NASM version.
This program just displays a message box and on clicking ok, the program exits.

-----------------------------------hello.cc------------------------------

#include <windows.h>

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInst, PSTR szCmdLine, int iCmdShow)
{
	char *title="hello";
	char *message="hello world";
	MessageBoxA(0,message,title,MB_OK);
	return 0;
}

; This line is a comment.

%include "win32n.inc"

extern MessageBoxA
import MessageBoxA user32.dll
extern ExitProcess
import ExitProcess kernel32.dll
segment .data USE32

title db "hello",0
message db "hello world",0

segment .bss USE32

var1 resb 32

segment .code USE32

..start:

push dword MB_OK
push dword title
push dword message
push dword 0
call [MessageBoxA]

push dword 0
call [ExitProcess]

Here 'dword' indicates that all are 32 bit values.
All details about the windows api functions can be found in msdn collection
or any winapi help file.
The constant MB_OK indicates that only OK button will be displayed in the message box.
The constants MB_OKCANCEL, MB_YESNO can also be used in place of MB_OK..
MB_OKCANCEL -- displays OK and CANCEL buttons.
MB_YESNO -- displays YES and NO buttons.

All these constants are defined in the include file "win32n.inc"

push dword 0
call [ExitProcess]
Since ExitProcess function needs just one parameter(error code), only one value is pushed.
This function exits the program.This is similar to exit(0);   in C

Assembling and Linking

Visual C vs NASM:

Other Features:

%include "nagoa+.inc"
;; our API macro (we can include this in our nagoa+.inc ,
;; so that we need not type this macro in all pgms)
%macro API 2
	import %1 %2
	extern %1
%endmacro

API MessageBoxA,user32.dll
API ExitProcess,kernel32.dll

segment .data USE32
title db "hello",0
message db "hello world",0

segment .bss USE32
var1 resb 32

segment .code USE32
..start:
call MessageBoxA,0,message,title,MB_OK
call ExitProcess,0

call MessageBoxA,0,"hello world","hello",0    ; in nasm

MessageBoxA(0,"hello world","hello",0);  // in C

%include "nagoa+.inc"  ;; now we have added API macro in nagoa+.inc :)
API MessageBoxA,user32.dll
API ExitProcess,kernel32.dll
segment .data USE32

segment .bss USE32
var1 resb 32
segment .code USE32
..start:
call MessageBoxA,0,"hello world","hello",MB_OK
call ExitProcess,0