NASM (hello.asm)

section .data                           ;section declaration

msg     db      "Hello, world!",0xa     ;our dear string
len     equ     $ - msg                 ;length of our dear string

section .text                           ;section declaration

                        ;we must export the entry point to the ELF linker or
    global _start       ;loader. They conventionally recognize _start as their
                        ;entry point. Use ld -e foo to override the default.

_start:

;write our string to stdout

        mov     edx,len ;third argument: message length
        mov     ecx,msg ;second argument: pointer to message to write
        mov     ebx,1   ;first argument: file handle (stdout)
        mov     eax,4   ;system call number (sys_write)
        int     0x80    ;call kernel

;and exit

        mov     ebx,0   ;first syscall argument: exit code
        mov     eax,1   ;system call number (sys_exit)
        int     0x80    ;call kernel

GAS (hello.s)

.data                                   # section declaration

msg:
        .ascii  "Hello, world!\n"       # our dear string
        len = . - msg                   # length of our dear string

.text                                   # section declaration

                        # we must export the entry point to the ELF linker or
    .global _start      # loader. They conventionally recognize _start as their
                        # entry point. Use ld -e foo to override the default.

_start:

# write our string to stdout

        movl    $len,%edx       # third argument: message length
        movl    $msg,%ecx       # second argument: pointer to message to write
        movl    $1,%ebx         # first argument: file handle (stdout)
        movl    $4,%eax         # system call number (sys_write)
        int     $0x80           # call kernel

# and exit

        movl    $0,%ebx         # first argument: exit code
        movl    $1,%eax         # system call number (sys_exit)
        int     $0x80           # call kernel