.section .text directive on Linux vs. Mac? What information to retrieve on my computer that completely characterizes the assembly syntax?

roi_saumon 0
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I would like to start learning assembler. The first question I ask myself is this : when I sit in front of a computer, how to find out which assembly syntax I have to use?
I read many terms on internet as "ABI", "computer architecture", "processor", "compiler" but ultimately I didn't understand what exactly determines the syntax of the assembly language I have to use.

For instance I have a Mac M1 and I installed a Linux virtual machine. I checked my architecture which is AArch64 so I wrote a very simple assembly program :

.global _start
.section .text

_start:
        mov x8, #0x5d
        mov x0, #0x41

.section .data

For example, this works (compiled with gcc) on my Linux virtual machine but not on the mac directly (also compiled with gcc) because apparently I have to replace .section .data by .data or .section .text by .text. So here I have the same AArch64 architecture and the same compiler, yet the assembly syntax is different... weird.

In short, I would like to know what exact information do I have to look for on a computer (ABI? Architecture? Something else?) in order to know for sure which assembly language syntax to use.

arm64 assembly clang macos
1 Answers

In this case, MacOS uses different section names than other platforms (like ELF on Linux, or COFF/PE on Windows)

.text is an alias for .section __TEXT,__text,regular,pure_instructions when targeting MachO64, unlike .section .text when targeting an ELF object file. At least that's what I see in the asm output from clang -target arm64-macos -S hello.c on my x86-64 Linux desktop for a simple program.

AFAIK, only Clang can target Mach-O 64-bit object files; not sure if GNU Binutils ever got that support. gcc on a Mac is normally actually Apple's version of clang (with some differences from mainline clang like I have on my Linux desktop). Running it doesn't involve the GNU assembler. as on a Mac is also clang; it's a compiler and an assembler.

In general - target OS (or object-file format) and assembler matter, not just CPU architecture. Since x86 was mentioned in comments, that's a good example of an ISA where many different asm source syntaxes exist, and also different OSes with incompatible ABIs and object file formats.

Different x86 assemblers use different syntax for the same machine code. For example, AT&T movl $1234, (%rdi) is the same instruction as NASM mov dword [rdi], 1234. Or GAS .intel_syntax noprefix mov dword ptr [rdi], 1234 (also MASM).

So no, the ISA doesn't uniquely determine the asm text syntax for instructions. Only the machine code. It does narrow down the asm choices to a handful.

Quite a few ISAs only have one syntax for instructions themselves, although 32-bit ARM has some syntax variations, like whether ldreqb or ldrbeq is right. (Predicate as infix before a load-size suffix, or as a suffix). And there's Keil's ARMASM vs. GNU Assembler syntax.

Peter Cordes0
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