What is the difference between assembly on mac and assembly on linux?

Question

I've been trying to get familiar with assembly on mac, and from what I can tell, the documentation is really sparse, and most books on the subject are for windows or linux. I thought I would be able to translate from linux to mac pretty easily, however this (linux)

.file   "simple.c"
.text
.globl simple
.type   simple, @function
simple:
      pushl   %ebp
      movl    %esp, %ebp
      movl    8(%ebp), %edx
      movl    12(%ebp), %eax
      addl    (%edx), %eax
      movl    %eax, (%edx)
      popl    %ebp
      ret
.size   simple, .-simple
.ident  "GCC: (Ubuntu 4.3.2-1ubuntu11) 4.3.2"
.section        .note.GNU-stack,"",@progbits

seems pretty different from this (mac)

.section    __TEXT,__text,regular,pure_instructions
.globl  _simple
.align  4, 0x90
_simple:                                ## @simple
    .cfi_startproc
## BB#0:
pushq   %rbp
Ltmp2:
    .cfi_def_cfa_offset 16
Ltmp3:
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
Ltmp4:
    .cfi_def_cfa_register %rbp
    addl    (%rdi), %esi
    movl    %esi, (%rdi)
    movl    %esi, %eax
    popq    %rbp
    ret
    .cfi_endproc


.subsections_via_symbols

The "normal" (for lack of a better word) instructions and registers such as pushq %rbp don't worry me. But the "weird" ones like .cfi_startproc and Ltmp2: which are smack dab in the middle of the machine instructions don't make any sense.

I have no idea where to go to find out what these are and what they mean. I'm about to pull my hair out as I've been trying to find a good resource for beginners for months. Any suggestions?

Answer 1

To begin with, you're comparing 32-bit x86 assembly with 64-bit x86-64. While the OS X Mach-O ABI supports 32-bit IA32, I suspect you want the x86-64 SysV ABI. (Thankfully, the x86-64.org site seems to be up again). The Mach-O x86-64 model is essentially a variant of the ELF / SysV ABI, so the differences are relatively minor for user-space code, even with different assemblers.

The .cfi directives are DWARF debugging directives that you don't strictly need for assembly - they are used for call frame information, etc. Here are some minimal examples:

ELF x64-64 assembler:

    .text
    .p2align 4

    .globl  my_function
    .type   my_function,@function

my_function:
    ...
.L__some_address:

    .size    my_function,[.-my_function]

Mach-O x86-64 assembler:

    .text
    .p2align 4

    .globl  _my_function

_my_function:
    ...
L__some_address:

Short of writing an asm tutorial, the main differences between the assemblers are: leading underscores for Mach-O functions names, .L vs L for labels (destinations). The assembler with OS X understands the '.p2align' directive. .align 4, 0x90 essentially does the same thing.

Not all the directives in compiler-generated code are essential for the assembler to generate valid object code. They are required to generate stack frame (debugging) and exception handling data. Refer to the links for more information.