First ASM Project

This is my first ASM project. It is a simple program that prints "Hello, World!" to the console.

How to run

1. Download or Clone using Github Desktop or Git
2. Open in vscode or whatever editor you use
3. Open folder in terminal/cmd prompt
4. Run the following commands:

> nasm -f win32 index.asm
> gcc index.obj
> a.exe

5. You should see "Hello, World!" in the console

How it works

This program uses the Windows API to print to the console. It uses the following instructions:

default rel     ; use relative addressing
global main     ; make main function visible to linker
extern printf   ; import printf function from msvcrt.dll
extern exit     ; import exit function from msvcrt.dll

section .data
    message: db "Hello, World!", 0 ; the format string for printf

section .text
    main:
        sub rsp, 28h ; allocate space for printf arguments
        lea rcx, [message] ; load the address of the format string into rcx
        call printf ; call printf(message)

        xor edi, edi ; set exit code to 0
        call exit ; call exit(0)

        add rsp, 28h ; deallocate space for printf arguments
        hlt ; halt the program

Instructions breakdown

• default rel - This tells the assembler to use relative addressing by default. This is important because the Windows API uses relative addressing.
• global main - This makes the main function visible to the linker. This is required for the program to run.
• extern printf - This imports the printf function from msvcrt.dll. This is the function we will use to print to the console.
• extern exit - This imports the exit function from msvcrt.dll. This is the function we will use to exit the program.
• section .data - This is the data section where we define our variables.
• message: db "Hello, World!", 0 - This defines a null-terminated string message with the value "Hello, World!". The 0 at the end is the null terminator required by printf.
• section .text - This is the code section where we define our instructions.
• main: - This is the entry point of our program.
• sub rsp, 28h - This allocates space on the stack for the arguments to printf. The 28h is the hexadecimal value for 28, which is the number of bytes we need to allocate.
• lea rcx, [message] - This loads the address of the message string into the rcx register. This is the first argument to printf.
• call printf - This calls the printf function with the message string as the argument.
• xor edi, edi - This sets the exit code to 0. This is the return value of the program.
• call exit - This calls the exit function with the exit code 0.
• add rsp, 28h - This deallocates the space on the stack that we allocated earlier.
• hlt - This halts the program. This isn't needed, but it's good practice.

References

• NASM Tutorial
• x64 Assembly Programming
• Windows API
• NASM Manual