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 ...
Tagged . All content copyright James Fisher 2017. This post is not associated with my employer.