; hello.asm - a "hello, world" program using NASM

; $ nasm -f macho hello.asm
; $ ld hello.o -o hello -static -macosx_version_min 10.7 -pagezero_size 0 -no_uuid
; $ ./hello
; hello, world

SECTION .text

    mymsg db "Hello, world!", 0xA ; string with a carriage-return
    mylen equ $-mymsg             ; string length

global start

start:

; print "hello, world"

    ; syscall write arguments
    push dword mylen          ; length of message                           
    push dword mymsg          ; address of message
    push dword 1              ; write to standard output

    ; syscall write
    mov eax, 0x4              ; system call number for 'write'
    sub esp, 4                ; OS X (and BSD) system calls needs "extra space" on stack
    int 0x80                  ; make the actual system call

    ; clean up the stack
    add esp, 16               ; 3 args * 4 bytes/arg + 4 bytes extra space = 16 bytes
    
; exit the program

    ; syscall exit arguments
    push dword 0              ; exit status returned to the operating system

    ; syscall exit
    mov eax, 0x1              ; system call number for exit
    sub esp, 4                ; OS X (and BSD) system calls needs "extra space" on stack
    int 0x80                  ; make the system call