Examples

Linux - 32 bit example

section .text
	global _start	; must be declared for linker (ld)

_start:				; tells linker entry point
	mov edx,len		; message length
	mov ecx,msg		; message to write
	mov	ebx,1       ; file descriptor (stdout)
   	mov	eax,4       ; system call number (sys_write)
   	int	0x80        ; call kernel
	
   	mov	eax,1       ; system call number (sys_exit)
   	int	0x80        ; call kernel

section	.data
msg db 'Hello, world!', 0xa  ; string to be printed
len equ $ - msg     		 ; length of the string

when the above code is “assembled” and executed, it produces the following result

Hello, world!

Compiling and Linking Assembly Program in NASM

• to assemble program (an object file named ./hello.o will be created)
• nasm -f elf hello.asm
• link the object file and create an executable file named ./hello)
• ld -m elf_i386 -s -o hello hello.o
• execute the program
• ./hello

Linux - 64 bit example

global _start
section .text
_start:
  mov rax, 1        ; write(
  mov rdi, 1        ;   STDOUT_FILENO,
  mov rsi, msg      ;   "Hello, world!\n",
  mov rdx, msglen   ;   sizeof("Hello, world!\n")
  syscall           ; );

  mov rax, 60       ; exit(
  mov rdi, 0        ;   EXIT_SUCCESS
  syscall           ; );

section .rodata
  msg: db "Hello, world!", 10
  msglen: equ $ - msg

$ nasm -f elf64 -o hello.o hello.s
$ ld -o hello hello.o
$ ./hello
Hello, world!

macOS - 32 bit example

; /usr/local/bin/nasm -f macho 32.asm && ld -macosx_version_min 10.7.0 -o 32 32.o && ./32

section .data
msg:    db      "Hello, world!", 10
.len:   equ     $ - msg

global start
section .text
start:
    push    dword msg.len
    push    dword msg
    push    dword 1
    mov     eax, 4
    sub     esp, 4
    int     0x80
    add     esp, 16

    push    dword 0
    mov     eax, 1
    sub     esp, 12
    int     0x80

macOS - 64 bit example

; /usr/local/bin/nasm -f macho64 64.asm && ld -macosx_version_min 10.7.0 -lSystem -o 64 64.o && ./64

section .data
msg:    db      "Hello, world!", 10
.len:   equ     $ - msg

global start
section .text
start:
    mov     rax, 0x2000004  ; write(
    mov     rdi, 1          ;   STDOUT_FILENO,
    mov     rsi, msg        ;   "Hello, world!\n",
    mov     rdx, msg.len    ;   sizeof("Hello, world!\n")
    syscall                 ; );

    mov     rax, 0x2000001  ; exit(
    mov     rdi, 0          ;   EXIT_SUCCESS
    syscall                 ; );