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assembly - Hello world in NASM with LINK.EXE and WinAPI

I'm trying to get a simple Hello world program in NASM to run. I want to print to the console without using C-Libraries, interfacing directly with WinAPI.

I am using the Visual Studio provided LINK.EXE for linking.

Here's my code so far:

section .data
    message:     db 'Hello world!',10    ; 'Hello world!' plus a linefeed character
    messageLen:  db $-message        ; Length of the 'Hello world!' string

    global _start
    extern  GetStdHandle
    extern  WriteConsoleW
    extern  ExitProcess

section .text

_start:
    ; DWORD  bytes;    
    mov     rbp, rsp
    sub     rsp, byte 8

    ; hStdOut = GetStdHandle(STD_OUTPUT_HANDLE)
    mov     ecx, -11
    call    GetStdHandle

    ; WriteFile(hstdOut, message, length(message), &bytes, 0);
    mov     rcx, rax
    mov     rdx, message
    mov     r8,  messageLen
    lea     r9,  [rsp-4]
    push    0
    call    WriteConsoleW

    ; ExitProcess(0)
    mov     rcx, 0
    call    ExitProcess

    ret

Which I assemble and link like this:

nasm -f win64 .ASM.ASM
link /entry:_start /nodefaultlib /subsystem:console .ASM.obj "C:Program Files (x86)Windows Kits10Lib10.0.18362.0umx64kernel32.lib" "C:Program Files (x86)Windows Kits10Lib10.0.18362.0umx64user32.lib"

However when I run the resulting .exe file, I get nothing.

Some things I tried so far are

  • Using the decorated names (like [email protected]), which resulted in the linker complaining about unresolved references
  • Not trying to print anything and calling Sleep, which resulted in the process sleeping indefinitely
  • Exiting with a different return code, which once again did nothing

What am I doing wrong?

EDIT: Fixed calling convention

See Question&Answers more detail:os

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1 Answer

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by (71.8m points)

There are three problems with your revised code. The first is:

message:     db 'Hello world!',10    ; 'Hello world!' plus a linefeed character
messageLen:  db $-message            ; Length of the 'Hello world!' string

You defined messageLen to be a byte containing the length of the message and storing that value at the address of messageLen. You then do this:

mov     r8,  messageLen

That would move the address of label messageLen to r8. What you really should have done is define messageLen as an assembly time constant like this:

messageLen equ $-message             ; Length of the 'Hello world!' string

The second problem is that you define the the string as a sequence of single byte characters:

message:     db 'Hello world!',10    ; 'Hello world!' plus a linefeed character

There is nothing wrong with this, but to print them out you need to use the Ansi version of the function WriteConsole which is WriteConsoleA. Using WriteConsoleW printed the string as Unicode (UTF-16 on Windows 2000 and later, UTS-2 on NT4 and earlier versions of Windows).


The third problem is with regards to a mandatory 32 bytes of shadow space before the stack based parameter(s) are placed on the stack before making a function call. You also need to make sure the stack (RSP) is a 16-byte aligned value at the point of making a function call. These requirement can be found in the Microsoft 64-bit calling convention.

Code that would take this into account would look like this:

section .data
    message:     db 'Hello world!',10    ; 'Hello world!' plus a linefeed character
    messageLen equ $-message      ; Length of the 'Hello world!' string

    global _start
    extern  GetStdHandle
    extern  WriteConsoleA
    extern  ExitProcess

section .text

_start:
    ; At _start the stack is 8 bytes misaligned because there is a return
    ; address to the MSVCRT runtime library on the stack.
    ; 8 bytes of temporary storage for `bytes`.
    ; allocate 32 bytes of stack for shadow space.
    ; 8 bytes for the 5th parameter of WriteConsole.
    ; An additional 8 bytes for padding to make RSP 16 byte aligned.
    sub     rsp, 8+8+8+32
    ; At this point RSP is aligned on a 16 byte boundary and all necessary
    ; space has been allocated.

    ; hStdOut = GetStdHandle(STD_OUTPUT_HANDLE)
    mov     ecx, -11
    call    GetStdHandle

    ; WriteFile(hstdOut, message, length(message), &bytes, 0);
    mov     rcx, rax
    mov     rdx, message
    mov     r8,  messageLen
    lea     r9,  [rsp-16]         ; Address for `bytes`
    ; RSP-17 through RSP-48 are the 32 bytes of shadow space
    mov     qword [rsp-56], 0     ; First stack parameter of WriteConsoleA function
    call    WriteConsoleA

    ; ExitProcess(0)
    ;    mov     rcx, 0
    ;    call    ExitProcess

    ; alternatively you can exit by setting RAX to 0
    ; and doing a ret

    add rsp, 8+8+32+8             ; Restore the stack pointer.
    xor eax, eax                  ; RAX = return value = 0
    ret

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