How to write an assembly ‘hello world’ on macOS
I started with this gist, creating this file:
global start section .text start: mov rax, 0x2000004 ; write mov rdi, 1 ; stdout mov rsi, msg mov rdx, msg.len syscall mov rax, 0x2000001 ; exit mov rdi, 0 syscall section .data msg: db "Hello, world!", 10 .len: equ $ - msg
This is “assembly”, but this term is vague. Getting specific, this file:
- is written in “Intel” assembly syntax (not the AT&T syntax)
- is written for an Intel x86-64 machine (it uses x86-64 instructions)
- is written for macOS (it uses macOS system calls)
- is intended to generate a “Mach-O 64” object file, the format used by macOS on 64-bit machines
To compile it, we need tools which understand all of these. One of these is
nasm, the “Netwide Assembler”. It seems to be the most popular assembler. I ran the program with:
$ brew install nasm $ nasm -version NASM version 2.12.02 compiled on Sep 14 2016 $ nasm -f macho64 hello.s $ ld -macosx_version_min 10.7.0 -lSystem -o hello hello.o $ ./hello Hello, world!
There are lots of unknown bits in here. I’ll cover them in future posts …
I wrote this because I felt like it. This post is not associated with my employer.