.ASM File Format

Introduction

Assembly language is a text form of a processor’s machine code, with one line for each instruction. An assembler turns it into an object file or ELF object, which a linker makes into a program. There is no single assembly language. Each processor family has its own instructions, and each assembler has its own syntax and directives.

The extension mostly says which tools were used. .asm is the usual name for NASM, MASM and other DOS and Windows assemblers, .nasm marks source for NASM, and .s is the Unix convention, used by the GNU assembler and the output of C compilers. GCC and Clang pass a .S file, with a capital, through the C preprocessor first.

Structure

Assembly source is plain text, one statement per line: an optional label ending in a colon, an instruction or directive, its operands and a comment. Directives instruct the assembler rather than the processor, and set sections, define data and export symbols.

On x86 there are two main syntaxes. Intel syntax, used by NASM and MASM, puts the destination first and leaves registers and numbers unmarked. MASM and NASM differ in their directives and in details such as memory operands, which MASM may write as dword ptr [eax] where NASM writes dword [eax]. AT&T syntax, the default of the GNU assembler on x86, puts the destination last, prefixes registers with % and immediate values with $, and adds a size suffix to the instruction, as in movq. The GNU assembler also accepts Intel syntax after .intel_syntax noprefix.

Comments start with ; in NASM and MASM. In the GNU assembler the comment character depends on the processor: # on x86 and @ on 32-bit Arm. On AArch64 it uses //.

; NASM, Intel syntax: exit with status 0 on 64-bit Linux
section .text
global _start

_start:
    mov rax, 60        ; system call number for exit
    xor rdi, rdi       ; status 0
    syscall

# GNU assembler, AT&T syntax: the same program
    .text
    .globl _start

_start:
    movq $60, %rax       # system call number for exit
    xorq %rdi, %rdi      # status 0
    syscall

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