x86 Assembly on a Mac


Does anyone know of any good tools (i'm looking for IDEs, primarily) to write assembly on the Mac... Xcode is a little cumbersome to me.

Also, on the Intel Macs, can I use generic x86 asm? or is there a modified instruction set? Any information about post Intel

Also: I know that on windows, asm can run in an emulated environment created by the OS to let the code think it's running on its own dedicated machine... does OSX provide the same thing?


After installing any version of Xcode targeting Intel-based Macs, you should be able to write assembly code. Xcode is a suite of tools, only one of which is the IDE, so you don't have to use it if you don't want to. (That said, if there are specific things you find clunky, please file a bug at Apple's bug reporter - every bug goes to engineering.) Furthermore, installing Xcode will install both the Netwide Assembler (NASM) and the GNU Assembler (GAS); that will let you use whatever assembly syntax you're most comfortable with.

You'll also want to take a look at the Compiler & Debugging Guides, because those document the calling conventions used for the various architectures that Mac OS X runs on, as well as how the binary format and the loader work. The IA-32 (x86-32) calling conventions in particular may be slightly different from what you're used to.

Another thing to keep in mind is that the system call interface on Mac OS X is different from what you might be used to on DOS/Windows, Linux, or the other BSD flavors. System calls aren't considered a stable API on Mac OS X; instead, you always go through libSystem. That will ensure you're writing code that's portable from one release of the OS to the next.

Finally, keep in mind that Mac OS X runs across a pretty wide array of hardware - everything from the 32-bit Core Single through the high-end quad-core Xeon. By coding in assembly you might not be optimizing as much as you think; what's optimal on one machine may be pessimal on another. Apple regularly measures its compilers and tunes their output with the "-Os" optimization flag to be decent across its line, and there are extensive vector/matrix-processing libraries that you can use to get high performance with hand-tuned CPU-specific implementations.

Going to assembly for fun is great. Going to assembly for speed is not for the faint of heart these days.

As stated before, don't use syscall. You can use standard C library calls though, but be aware that the stack MUST be 16 byte aligned per Apple's IA32 function call ABI.

If you don't align the stack, your program will crash in __dyld_misaligned_stack_error when you make a call into any of the libraries or frameworks.

The following snippet assembles and runs on my system:

; File: hello.asm
; Build: nasm -f macho hello.asm && gcc -o hello hello.o

SECTION .rodata
hello.msg db 'Hello, World!',0x0a,0x00


extern _printf ; could also use _puts...
GLOBAL _main

; aligns esp to 16 bytes in preparation for calling a C library function
; arg is number of bytes to pad for function arguments, this should be a multiple of 16
; unless you are using push/pop to load args
%macro clib_prolog 1
    mov ebx, esp        ; remember current esp
    and esp, 0xFFFFFFF0 ; align to next 16 byte boundary (could be zero offset!)
    sub esp, 12         ; skip ahead 12 so we can store original esp
    push ebx            ; store esp (16 bytes aligned again)
    sub esp, %1         ; pad for arguments (make conditional?)

; arg must match most recent call to clib_prolog
%macro clib_epilog 1
    add esp, %1         ; remove arg padding
    pop ebx             ; get original esp
    mov esp, ebx        ; restore

    ; set up stack frame
    push ebp
    mov ebp, esp
    push ebx

    clib_prolog 16
    mov dword [esp], hello.msg
    call _printf
    ; can make more clib calls here...
    clib_epilog 16

    ; tear down stack frame
    pop ebx
    mov esp, ebp
    pop ebp
    mov eax, 0          ; set return code

Recently I wanted to learn how to compile Intel x86 on Mac OS X:

For nasm:

-o hello.tmp - outfile
-f macho - specify format
Linux - elf or elf64
Mac OSX - macho

For ld:

-arch i386 - specify architecture (32 bit assembly)
-macosx_version_min 10.6 (Mac OSX - complains about default specification)
-no_pie (Mac OSX - removes ld warning)
-e main - specify main symbol name (Mac OSX - default is start)
-o hello.o - outfile

For Shell:

./hello.o - execution


nasm -o hello.tmp -f macho hello.s && ld -arch i386 -macosx_version_min 10.6 -no_pie -e _main -o hello.o hello.tmp && ./hello.o

Let me know if this helps!

I wrote how to do it on my blog here:


For a more verbose explanation, I explained on my Github here:


This video can help you solving your question :)
By: admin