Thursday 29 November 2012

[ASM] How to use the windows API in NASM

Hellooow everybody! ;-)

Today I will give you a short guide on how to use windows API in ASM programming! (at least in programs compiled with NASM because this is the one I'm using and as I'm quite noob about the different versions of asm, I prefer not risk to tell wrong things) This is because I wanted to learn some asm stuff and make little programs on windows, and quickly realized that windows was too protected to use pure asm and that we need to use the windows C++ API, including console I/O.

 What is the windows API?
It's a library of C++ functions. You have to use it if you want to make any program on windows. So that means you will have to call these functions in your asm code.
It's called kernel32.dll for 32bits systems, and normally it's in the path :-)
Here is the official documentation:
Here is the documentation for console stuff:

How to call a windows API function in asm?
1) Don't forget the header!
You have to declare any function you want to use in your code by specifying them as external functions. Basically at this point all you have to do is to put a line like this in the begining of your file:
  extern    Function1Name, Function2Name, FunctionNName
example (console I/O):
  extern    ExitProcess, GetStdHandle, WriteConsoleA, ReadConsoleInputA

/!\ If you want to use any windows constant in the function calls, you also have to declare those.
For example for the GetStdHandle, these can come in handy :
  STD_OUTPUT_HANDLE    equ -11
  STD_INPUT_HANDLE     equ -10
But of course you can directly pass the constant value as an argument (if you wanna do it in the hardcore way :p) 

2) Function calls in asm
Function calls in asm aren't really more complicated than in any other language. You just have to pass the arguments the function needs and call the function's name.
In asm your pass the arguments by pushing them on top of the stack:
  push    arg

/!\ Always remember that as the stack goes decreasing, (unlike a stack of papers) you have to pass the arguments in the reverse order:
  push    arg3
  push    arg2
  push    arg1

Once all the arguments are sent to the stack, you can call the function:
  call    function

Example: WriteConsole call
  push    NULL
  push    buffer_out
  push    msg.len
  push    msg
  push    dword[handleOut]
  call    WriteConsoleA

How to compile and link a program that uses win API functions? (using nasm and golink)
This is the easy part! Just open a console and eventually cd in the right directory.
To compile, type something like:
>> nasm -f win32 yourProgram.asm -o yourProgram.obj
and to link the library using golink:
>> GoLink.exe /console /entry starting_point_of_your_asm_program yourProgram.obj kernell32.dll

Or, you can do a makefile:
  yourProgram: yourProgram.obj
  GoLink.exe /console /entry starting_point_of_your_asm_program yourProgram.obj kernell32.dll
  yourProgram.obj: yourProgram.asm 
  nasm -f win32 yourProgram.asm -o yourProgram.obj

NB: 
If you don't already know GoLink, it's a linker, which means it's able to link your file with the libraries you want. (in this case the kernel32.dll file)
You can download it here: (direct download of last version)
Manual:

NB2:
In order to use these commands, the source folders for nasm and GoLink.exe have to be in the path. So when you install them don't forget to add these at the end of the path variable. (right click on computer => advanced system parameters => environment variables... => find "path" in the system variables table, hit "modify" and add your new paths at the end, separated by a ';')

Working example: system("pause")
  STD_OUTPUT_HANDLE   equ -11
  STD_INPUT_HANDLE    equ -10
  NULL                equ 0  global start
  extern ExitProcess, GetStdHandle, WriteConsoleA, ReadConsoleInputA

  section .data    ;message we want to display on console
  msg                 db "Press a key to continue...", 13, 10, 0
  msg.len             equ $ - msg
  consoleInHandle     dd 1

  section .bss     ;buffers declaration
  buffer_out          resd 2
  buffer_in           resb 32

  section .text
    start:       ;starting point of our program
        push    STD_OUTPUT_HANDLE
        call    GetStdHandle   ;call to get a handle to the
        push    NULL           ;specified mode (input, output...)
        push    buffer_out
        push    msg.len
        push    msg
        push    eax            ;contains the GetStdHandle result
        call    WriteConsoleA  ;call to print our msg in console

    read:
        push    STD_INPUT_HANDLE
        call    GetStdHandle        ;new call to get input handle
        push    NULL
        push    1
        push    buffer_in
        push    eax
        call    ReadConsoleInputA   ;call to detect user input
                                    ;this function will wait til
    exit:                           ;it detects enough keypresses
        push    NULL                ;(in this case, 1)
        call    ExitProcess

Manual pages for the functions used in this code:
- ExitProcess
- GetStdHandle
- WriteConsole (A stands for ANSI version)
- ReadConsoleInput (A stands for ANSI version)


I hope this helped some of you! See ya around and have fun coding ;-)


Additional source for this article:
This thread I posted on stackoverflow.com when I was having a hard time understanding all this: ;-p

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