ASM helloworld (nasm)

helloworld.asm

	;32bit x86 code
	[bits 32]
	[section .text]

	global _start

_start:

	;linux specific
	;interrupt 80hex for system calls
	;4 = write /usr/include/x86_64-linux-gnu/asm/unistd_32.h
	;1 = file descriptor of STDOUT
	;13 = number of characters

	;calc string size
	mov eax, hellostr
	call strlen
	mov edx, eax

	mov eax, 4
	mov ebx, 1
	mov ecx, hellostr
	int 0x80

	;print reversed
	mov eax, hellostr
	call printrev

	;print newline
	push byte 10
	mov eax, 4
	mov ebx, 1
	;stack pointer esp has a newline
	mov ecx, esp
	mov edx, 1
	int 0x80
	;remove the newline from the stack
	inc esp

	;1 = exit system call
	;0 = status (return 0 in c)
	mov eax, 1
	mov ebx, 0
	int 0x80

strlen:
	;argument in eax (ptr to string)
	;string in edx
	mov edx, eax
	;dec eax value -1
	dec eax
count_loop:
	inc eax
	;[] dereference (read eax data)
	;1 byte bl
	mov bl, [eax]
	;compare (= subtract but not write)
	cmp bl, 0
	;if 0 flag not set (jump not zero)
	jnz count_loop

	;eax = eax - edx
	sub eax, edx
	;pops return address from the stack and jumps there
	ret

printrev:
	;arg = ptr to str
	mov esi, eax
	call strlen
	;eax has the length
	add esi, eax

print_loop:
	dec esi
	push eax

	;call write for the character
	mov eax, 4
	mov ebx, 1
	mov ecx, esi
	mov edx, 1
	int 0x80

	pop eax
	dec eax
	jnz print_loop
	ret

	[section .data]
	;10 = newline ASCII on nasm
	;0 = 0 terminated string
hellostr: db "hello world!", 10, 0