Table of Contents

  • 1. Introduction
  • 2. Hello in Mach-O

1. Introduction

As now I'm getting into calculator programming, I wish I could have a cross
assembler on desktop computer so I can write programs for them.

I though once that macro assemblers on modern computers are all dead. It is
proved that I was wrong, by presenting Flat Assembler G. This is a macro
assembler down to the level of available instructions and output formats.

Note that fasmg itself is implemented in x86 assembly, that doesn't mean it
will expire soon as Apple has moved a step forward to ARM, as there is still
x86 emulation with macOS, and since fasmg only used a small subset of x86
writing a emulator is not a difficult task, besides the community of fasmg is
also thriving.

2. Hello in Mach-O

There are not as much x86 assembly programming tutorial for OS X compared to
Linux or even Windows. Most of the existing tutorials are dated, using the nasm
instead of the default available gas, and they didn't even bother to explain
the differences in system call and Mach-O format.

So, this is an example of the standard "Hail the world" for 64bit macOS.

   |include 'selfhost.inc'
   | 
   |format MachO64 executable
   |entry start
 5 | 
   |interpreter '/usr/lib/dyld'
   |uses '/usr/lib/libSystem.B.dylib'
   | 
   |import exit,'_exit'
10 | 
   |segment '__TEXT' readable executable
   |section '__text' align 16
   |start:
   |        mov rax,02000004h
15 |        mov rdi,1
   |        mov rsi,message
   |        mov rdx,14
   |        syscall
   |        mov rax,02000001h
20 |        xor rdi,rdi
   |        syscall
   |segment '__DATA' readable writable
   |section '__data' align 4
   |message:  
25 |        db  "Hello, World!", 10

The standard distribution has the selfhost.inc file that can be almost directly
used for x86_64 macOS assembly programming. It is just the file path needs to
be modified to work properly. The line that has import is not used but it is
important to have it sit there to generated the __LINKEDIT segment otherwise it
won't be treated as valid Mach-O executable. You do can build the __LINKEDIT
segment by self.

System calls has offset 2000000h, and 1 exit, 4 is write. They are available
from <sys/syscall.h> if you have installed macOS SDK.

Directly call fasmg on the file will produce the Mach-O binary. Just remember
to chmod it and you are done.