How to write hello world in assembler under Windows

Question

I wanted to write something basic in assembly under Windows  I m using NASM  but I can t get anything working    How to write and compile hello world without the help of C functions on Windows

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NASM examples   Calling libc stdio printf  implementing int main    return printf message        ----------------------------------------------------------------------------   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   Then run  nasm -fwin32 helloworld asm gcc helloworld obj a     There s also The Clueless Newbies Guide to Hello World in Nasm without the use of a C library  Then the code would look like this   16-bit code with MS-DOS system calls  works in DOS emulators or in 32-bit Windows with NTVDM support   Can t be run  directly   transparently  under any 64-bit Windows  because an x86-64 kernel can t use vm86 mode   org 100h mov dx msg mov ah 9 int 21h mov ah 4Ch int 21h msg db  Hello  World   0Dh 0Ah       Build this into a  com executable so it will be loaded at cs 100h with all segment registers equal to each other  tiny memory model    Good luck

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This example shows how to go directly to the Windows API and not link in the C Standard Library        global  main     extern   GetStdHandle 4     extern   WriteFile 20     extern   ExitProcess 4      section  text  main        DWORD  bytes          mov     ebp  esp     sub     esp  4        hStdOut   GetstdHandle  STD OUTPUT HANDLE      push    -11     call     GetStdHandle 4     mov     ebx  eax            WriteFile  hstdOut  message  length message    amp bytes  0       push    0     lea     eax   ebp-4      push    eax     push     message end - message      push    message     push    ebx     call     WriteFile 20        ExitProcess 0      push    0     call     ExitProcess 4        never here     hlt message      db       Hello  World   10 message end    To compile  you ll need NASM and LINK EXE  from Visual studio Standard Edition        nasm -fwin32 hello asm    link  subsystem console  nodefaultlib  entry main hello obj

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If you want to use NASM and Visual Studio s linker  link exe  with anderstornvig s Hello World example you will have to manually link with the C Runtime Libary that contains the printf   function   nasm -fwin32 helloworld asm link exe helloworld obj libcmt lib   Hope this helps someone

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These are Win32 and Win64 examples using Windows API calls  They are for MASM rather than NASM  but have a look at them  You can find more details in this article   This uses MessageBox instead of printing to stdout   Win32 MASM   ---ASM Hello World Win32 MessageBox   386  model flat  stdcall include kernel32 inc includelib kernel32 lib include user32 inc includelib user32 lib   data title db  Win32   0 msg db  Hello World   0   code  Main  push 0              uType   MB OK push offset title   LPCSTR lpCaption push offset msg     LPCSTR lpText push 0              hWnd   HWND DESKTOP call MessageBoxA push eax            uExitCode   MessageBox      call ExitProcess  End Main     Win64 MASM   ---ASM Hello World Win64 MessageBox  extrn MessageBoxA  PROC extrn ExitProcess  PROC   data title db  Win64   0 msg db  Hello World    0   code main proc   sub rsp  28h     mov rcx  0         hWnd   HWND DESKTOP   lea rdx  msg       LPCSTR lpText   lea r8   title     LPCSTR lpCaption   mov r9d  0         uType   MB OK   call MessageBoxA   add rsp  28h     mov ecx  eax       uExitCode   MessageBox        call ExitProcess main endp  End     To assemble and link these using MASM  use this for 32-bit executable    ml exe  filename   link  subsystem windows   defaultlib kernel32 lib  defaultlib user32 lib  entry Main   or this for 64-bit executable   ml64 exe  filename   link  subsystem windows   defaultlib kernel32 lib  defaultlib user32 lib  entry main     Why does x64 Windows need to reserve 28h bytes of stack space before a call   That s 32 bytes  0x20  of shadow space aka home space  as required by the calling convention   And another 8 bytes to re-align the stack by 16  because the calling convention requires RSP be 16-byte aligned before a call    Our main s caller  in the CRT startup code  did that   The 8-byte return address means that RSP is 8 bytes away from a 16-byte boundary on entry to a function    Shadow space can be used by a function to dump its register args next to where any stack args  if any  would be   A system call requires 30h  48 bytes  to also reserve space for r10 and r11 in addition to the previously mentioned 4 registers   But DLL calls are just function calls  even if they re wrappers around syscall instructions   Fun fact  non-Windows  i e  the x86-64 System V calling convention  e g  on Linux  doesn t use shadow space at all  and uses up to 6 integer pointer register args  and up to 8 FP args in XMM registers     Using MASM s invoke directive  which knows the calling convention   you can use one ifdef to make a version of this which can be built as 32-bit or 64-bit   ifdef rax     extrn MessageBoxA  PROC     extrn ExitProcess  PROC else      386      model flat  stdcall     include kernel32 inc     includelib kernel32 lib     include user32 inc     includelib user32 lib endif  data caption db  WinAPI   0 text    db  Hello World   0  code main proc     invoke MessageBoxA  0  offset text  offset caption  0     invoke ExitProcess  eax main endp end   The macro variant is the same for both  but you won t learn assembly this way  You ll learn C-style asm instead  invoke is for stdcall or fastcall while cinvoke is for cdecl or variable argument fastcall  The assembler knows which to use   You can disassemble the output to see how invoke expanded

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Unless you call some function this is not at all trivial   And  seriously  there s no real difference in complexity between calling printf and calling a win32 api function   Even DOS int 21h is really just a function call  even if its a different API  If you want to do it without help you need to talk to your video hardware directly  likely writing bitmaps of the letters of  quot Hello world quot  into a framebuffer  Even then the video card is doing the work of translating those memory values into DisplayPort HDMI DVI VGA signals  Note that  really  none of this stuff all the way down to the hardware is any more interesting in ASM than in C  A  quot hello world quot  program boils down to a function call  One nice thing about ASM is that you can use any ABI you want fairly easily  you just need to know what that ABI is

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To get an  exe with NASM compiler and Visual Studio s linker this code works fine   global WinMain extern ExitProcess    external functions in system libraries  extern MessageBoxA  section  data  title   db  Win64   0 msg     db  Hello world    0  section  text WinMain      sub rsp  28h       mov rcx  0         hWnd   HWND DESKTOP     lea rdx  msg       LPCSTR lpText     lea r8  title      LPCSTR lpCaption     mov r9d  0         uType   MB OK     call MessageBoxA     add rsp  28h        mov  ecx eax     call ExitProcess      hlt       never here   If this code is saved on e g   test64 asm   then to compile   nasm -f win64 test64 asm   Produces  test64 obj  Then to link from command prompt   path to link link exe test64 obj  subsystem windows  entry WinMain   libpath path to libs  nodefaultlib kernel32 lib user32 lib  largeaddressaware no   where path to link could be C  Program Files  x86  Microsoft Visual Studio 10 0 VC bin or wherever is your link exe program in your machine  path to libs could be C  Program Files  x86  Windows Kits 8 1 Lib winv6 3 um x64 or wherever are your libraries  in this case both kernel32 lib and user32 lib are on the same place  otherwise use one option for each path you need  and the  largeaddressaware no option is necessary to avoid linker s complain about addresses to long  for user32 lib in this case   Also  as it is done here  if Visual s linker is invoked from command prompt  it is necessary to setup the environment previously  run once vcvarsall bat and or see MS C   2010 and mspdb100 dll

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Flat Assembler does not need an extra linker  This makes assembler programming quite easy  It is also available for Linux   This is hello asm from the Fasm examples   include  win32ax inc    code    start      invoke  MessageBox HWND DESKTOP  Hi  I m the example program   invoke GetCommandLine MB OK     invoke  ExitProcess 0   end start   Fasm creates an executable     fasm hello asm flat assembler  version 1 70 03   1048575 kilobytes memory  4 passes  1536 bytes    And this is the program in IDA     You can see the three calls  GetCommandLine  MessageBox and ExitProcess

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The best examples are those with fasm  because fasm doesn t use a linker  which hides the complexity of windows programming by another opaque layer of complexity  If you re content with a program that writes into a gui window  then there is an example for that in fasm s example directory  If you want a console program  that allows redirection of standard in and standard out that is also possible  There is a  helas highly non-trivial  example program available that doesn t use a gui  and works strictly with the console  that is fasm itself  This can be thinned out to the essentials   I ve written a forth compiler  which is another non-gui example  but it is also non-trivial   Such a program  has the following command to generate a proper header for 32-bit executable  normally done by a linker  FORMAT PE CONSOLE   A section called   idata  contains a table that helps windows during startup to couple names of functions to the runtimes addresses  It also contains a reference to KERNEL DLL which is the Windows Operating System   section   idata  import data readable writeable     dd 0 0 0 rva kernel name rva kernel table     dd 0 0 0 0 0    kernel table       ExitProcess 4    DD rva  ExitProcess     CreateFile        DD rva  CreateFileA                              GetStdHandle 4   DD rva  GetStdHandle                       DD 0  The table format is imposed by windows and contains names that are looked up in system files  when the program is started  FASM hides some of the complexity behind the rva keyword  So  ExitProcess 4 is a fasm label and  exitProcess is a string that is looked up by Windows  Your program is in section   text   If you declare that section readable writeable and executable  it is the only section you need to add      section   text  code executable readable writable  You can call all the facilities you declared in the  idata section  For a console program you need  GetStdHandle to find he filedescriptors for standard in and standardout  using symbolic names like STD INPUT HANDLE which fasm finds in the include file win32a inc   Once you have the file descriptors you can do WriteFile and ReadFile  All functions are described in the kernel32 documentation  You are probably aware of that or you wouldn t try assembler programming  In summary  There is a table with asci names that couple to the windows OS  During startup this is transformed into a table of callable addresses  which you use in your program

[winapi] How to write hello world in assembler under Windows?

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