Nasm vs as

nasm (Netwide Assembler) and as (GNU Assembler) are two different assemblers for x86 and x86-64 assembly language, and they have different syntax and conventions.

Key Differences:

Section and Global Directives:
- nasm uses .section .text and .global _start.
- as uses section .text and global _start.
Register and Instruction Syntax:
- nasm uses % prefix for registers and $ for immediate values.
- as does not use % for registers and uses $ for immediate values.
Comment Syntax:
- nasm uses ; for comments.
- as uses # for comments.

Flags:

`nasm` Flags:

elf32 or elf64: Specifies the output format as 32-bit or 64-bit ELF.

Example:

`nasm` Syntax:

section .text
global _start

_start:
    mov rax, 60         ; Syscall number for exit
    xor rdi, rdi        ; Exit code 0
    syscall             ; Invoke the syscall

`as` Syntax:

.section .text
.global _start

_start:
    mov $60, %rax       # Syscall number for exit
    xor %rdi, %rdi      # Exit code 0
    syscall             # Invoke the syscall

These differences are due to the design choices and conventions adopted by the developers of each assembler.

Last modified: 14 January 2025