I wanted to write something basic in assembly under Windows. I'm using NASM, but I can't get anything working.
How do I write and compile a hello world program without the help of C functions on Windows?
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), &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
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.
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.
To get an .exe with NASM as the assembler and Visual Studio's linker this code works fine:
default rel ; Use RIP-relative addressing like [rel msg] by default
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 ; reserve shadow space and make RSP%16 == 0
mov rcx, 0 ; hWnd = HWND_DESKTOP
lea rdx,[msg] ; LPCSTR lpText
lea r8,[title] ; LPCSTR lpCaption
mov r9d, 0 ; uType = MB_OK
call MessageBoxA
mov ecx,eax ; exit status = return value of MessageBoxA
call ExitProcess
add rsp, 28h ; if you were going to ret, restore RSP
hlt ; privileged instruction that crashes if ever reached.
If this code is saved as test64.asm, then to assemble:
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).
(Using default rel makes the lea instructions work from anywhere, including outside the low 2GiB of virtual address space. But the call MessageBoxA is still a direct call rel32 that can only reach instructions +-2GiB away from itself.)
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.
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.
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 "Hello world" 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 "hello world" 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.
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.
For ARM Windows:
AREA data, DATA
Text DCB "Hello world(text)", 0x0
Caption DCB "Hello world(caption)", 0x0
EXPORT WinMainCRTStartup
IMPORT __imp_MessageBoxA
IMPORT __imp_ExitProcess
AREA text, CODE
WinMainCRTStartup PROC
movs r3,#0
ldr r2,Caption_ptr
ldr r1,Text_ptr
movs r0,#0
ldr r4,MessageBoxA_ptr # nearby, reachable with PC-relative
ldr r4,[r4]
blx r4
movs r0,#0
ldr r4,ExitProcess_ptr
ldr r4,[r4]
blx r4
MessageBoxA_ptr DCD __imp_MessageBoxA # literal pool (constants near code)
ExitProcess_ptr DCD __imp_ExitProcess
Text_ptr DCD Text
Caption_ptr DCD Caption
ENDP
END
This question already has answers here:
How To Properly call 64 Bit Windows API In Assembly
(1 answer)
Hello world in NASM with LINK.EXE and WinAPI
(1 answer)
Setting up an assembler on 64-bit Windows [closed]
(2 answers)
Is it possible to output a string to the console in C without including the standard library?
(2 answers)
Closed 2 years ago.
I am trying to write a 64-bit NASM assembly program in Windows to simply print some output to the console. I would like to do this with the most pure assembly possible, so that excludes any standard C libraries. I'm also trying to automate the assembly and linking process, so opening Visual Studio and doing it manually is not an option.
I know that I have to use the Windows APIs like _GetStdHandle and _WriteFile, but I can't figure out how to link those libraries, or how to use them in a 64-bit program.
This is the "Hello World" assembly code I am trying to run:
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), &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:
But I'm not sure how to link the Windows 64-bit APIs to the program. I have also been getting this error: "instruction not supported in 64-bit mode" for the instruction "push eax". I'm just not sure how to re-write this code for 64-bit.
This is the batch program I am using to assemble and link the program:
nasm -f win64 HelloWorld.asm -o HelloWorld.obj &&^
"C:\Program Files\GoLink\GoLink.exe" /console /entry _main HelloWorld.obj /fo HelloWorld.exe &&^
HelloWorld.exe
Can anyone walk me through the steps required to get this program to run with the given constraints?
What would the equivalent nasm would be in windows without using any libraries?
section .text
global _start
_start:
mov edx,len
mov ecx,msg
mov ebx,1
mov eax,4
int 0x80
mov eax,1
int 0x80
section .data
msg db 'Hello, world!', 0xa
len equ $ - msg
I read that _start is replaced with _main in windows but what about the system call? I found this table but im not sure which register gets what argument:
http://j00ru.vexillium.org/syscalls/nt/32/
Im especially confused about how to understand the system calls in windows because in linux its really clear what to do:
https://syscalls.kernelgrok.com/
The documentation for windows assembly, especially the system calls seems really really poor.
I have a simple Hello World program for Windows in pure x86 assembly code that I have compiled and linked with nasm and ld. The problem I am running into is that I can't get DWARF debugging to work. I am using gdb from Mingw64 (i686-posix-dwarf-rev1). This same problem happens if I use gcc to link instead of ld. But, the program builds fine, and if I use STABS debugging, then everything is fine and dandy.
EDIT: Oops, I completely forgot to give the error that gdb shows.
...Dwarf Error: bad offset (0x407000) in compilation unit header (offset 0x0
+ 6) [in module C:\Projects\AsmProjects\HelloWorldWin32\bin\x86\hello32.exe]
(no debugging symbols found)...done
The versions of each program are:
gdb 7.10.1
nasm 2.12.02
ld 2.25
gcc 6.2.0
These are the flags I'm sending to nasm: -f elf32 -Fdwarf -g
These are the flags for gcc link: -o $(BDIR)/x86/$#.exe $^ -L$(Mingw64-x86libs) -lkernel32 -luser32
And these are from ld link:
-mi386pe -o $(BDIR)/x86/$#.exe $^ -L$(Mingw64-x86libs) -lkernel32 -luser32
I have a pretty big makefile, so I'm trying to give the least information that is absolutely neccessary.
Here is the source code for the program:
global _main
extern _GetStdHandle#4
extern _WriteFile#20
extern _ExitProcess#4
section .text
_main:
push ebp
mov ebp,esp
; GetstdHandle( STD_OUTPUT_HANDLE)
push -11
call _GetStdHandle#4
mov ebx, eax
; WriteFile( hstdOut, message, length(message), &bytes, 0);
push 0
push esp
push message_end
push message
push ebx
call _WriteFile#20
; ExitProcess(0)
push 0
call _ExitProcess#4
section .data
message db 'Hello, World',10
message_end equ $ - message
This is not a proper answer but was too long for the comment section.
I compiled on Ubuntu and then ran dwarfdump
It gave an error that may be related to the offset error.
dwarfdump ERROR: dwarf_get_globals: DW_DLE_PUBNAMES_VERSION_ERROR (123)
From a similar error on LLVM, I conclude that the dwarf version information is possibly corrupt or unsupported.
This post indicates that the dwarf information is sensitive to the proper section names. The example appears to have the section names right however.
Have you tried a 64-bit version? Perhaps a clue will appear.
This program appears to work fine Ubuntu. Can you try it on Mingw64?
section .text
global _start ;must be declared for linker (ld)
_start: ;tell linker entry point
mov edx,len ;message length
mov ecx,msg ;message to write
mov ebx,1 ;file descriptor (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
section .data
msg db 'Hello, world!',0xa ;our dear string
len equ $ - msg ;length of our dear string
This question already has an answer here:
Mach-O 64-bit format does not support 32-bit absolute addresses. NASM Accessing Array
(1 answer)
Closed 4 years ago.
My code:
section .data
res db 0
section .text
global _main
extern _printf
extern _scanf
_main
..blablabla.....
mov rax,[res]
..blablbabla....
ret
And error: Mach-O 64-bit format does not support 32-bit absolute addresses on mov rax,[res].
So, in macho64, I can't link res, in [res]?
What should I do?
I don't know macho64 format, but there are alternatives to mov rax,[res]:
mov rax,res
mov rax,[rax]
Or using RIP-relative addressing:
mov rax,[rel res]