global _start

; constants
SYS_WRITE   equ 1
SYS_EXIT    equ 60
STDOUT      equ 1

SECTION .data
hello           db  "Hello world!", 0x0a
hello_len       equ $ - hello

SECTION .text

_start:
    ; syscall(SYS_WRITE, STDOUT, hello, hello_len);
    mov     rax, SYS_WRITE
    mov     rdi, STDOUT
    mov     rsi, hello
    mov     rdx, hello_len
    syscall
    push    rax

    ; syscall(SYS_EXIT, <sys_write return value> - hello_len);
    mov     rax, SYS_EXIT
    pop     rdi
    sub     rdi, hello_len
    syscall