6.2. Hello, world!

6.2.1. Program layout

Linux is 32-bit, runs in protected mode, has flat memory model, and uses the ELF format for binaries.

A program can be divided into sections: .text for your code (read-only), .data for your data (read-write), .bss for uninitialized data (read-write); there can actually be a few other standard sections, as well as some user-defined sections, but there's rare need to use them and they are out of our interest here. A program must have at least .text section.

Now we will write our first program. Here is sample code:

6.2.2. 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

6.2.3. 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