Hello World in MASM

[Gemini]

Microsoft Macro Assembler (MASM) is the standard for writing assembly code on Windows. Modern MASM usually targets x64 (64-bit), which uses a specific calling convention and 64-bit registers.

Below is a classic “Hello World” example written for MASM x64, utilizing the Windows API (MessageBoxA).

1. MASM x64 “Hello World” Example

; --- External Windows API functions ---
extern MessageBoxA : proc
extern ExitProcess : proc

.data
    window_title db "MASM x64", 0
    message      db "Hello from Microsoft Assembler!", 0

.code
main proc
    ; --- Shadow Space ---
    ; Windows x64 ABI requires 32 bytes of "shadow space" on the stack 
    ; before calling a function.
    sub rsp, 28h             

    ; --- Setup Arguments for MessageBoxA ---
    ; RCX = hWnd (NULL = 0)
    ; RDX = lpText (Address of message)
    ; R8  = lpCaption (Address of title)
    ; R9  = uType (MB_OK = 0)
    
    xor rcx, rcx             ; Set RCX to 0
    lea rdx, message         ; Load effective address of message
    lea r8, window_title     ; Load effective address of title
    xor r9, r9               ; Set R9 to 0
    
    call MessageBoxA         ; Call the API

    ; --- Cleanup and Exit ---
    xor rcx, rcx             ; Exit code 0
    call ExitProcess
main endp

end

; --- External Windows API functions ---
extern MessageBoxA : proc
extern ExitProcess : proc

.data
    window_title db "MASM x64", 0
    message      db "Hello from Microsoft Assembler!", 0

.code
main proc
    ; --- Shadow Space ---
    ; Windows x64 ABI requires 32 bytes of "shadow space" on the stack 
    ; before calling a function.
    sub rsp, 28h             

    ; --- Setup Arguments for MessageBoxA ---
    ; RCX = hWnd (NULL = 0)
    ; RDX = lpText (Address of message)
    ; R8  = lpCaption (Address of title)
    ; R9  = uType (MB_OK = 0)
    
    xor rcx, rcx             ; Set RCX to 0
    lea rdx, message         ; Load effective address of message
    lea r8, window_title     ; Load effective address of title
    xor r9, r9               ; Set R9 to 0
    
    call MessageBoxA         ; Call the API

    ; --- Cleanup and Exit ---
    xor rcx, rcx             ; Exit code 0
    call ExitProcess
main endp

end

2. Key Components Explained

The Registers (x64)

In the example above, we use 64-bit registers. The Windows calling convention uses specific registers for passing arguments:

1. RCX: 1st Argument
2. RDX: 2nd Argument
3. R8: 3rd Argument
4. R9: 4th Argument

Shadow Space (sub rsp, 28h)

One of the most common pitfalls in MASM for Windows is forgetting the Shadow Space. Even if a function takes fewer than 4 arguments, the caller must reserve 32 bytes ($20h$) plus an extra 8 bytes ($8h$) to maintain 16-byte stack alignment.

Directives

• .data: Defines the section for initialized data (strings, constants).
• .code: Defines the section for executable instructions.
• proc / endp: Marks the beginning and end of a procedure (function).

3. How to Assemble and Link

To run this, you typically use the tools provided with Visual Studio (ml64.exe for the assembler and link.exe for the linker).

1. Assemble: ml64.exe /c my_file.asm
2. Link: link.exe my_file.obj /subsystem:windows /defaultlib:user32.lib /defaultlib:kernel32.lib /entry:main

Would you like me to show you how to perform a simple mathematical calculation (like a Fibonacci sequence) using MASM instead?