Very simple booting code (http://blog.ghedini.me/post/6175473948/x86-booting-code)

boot.asm

; Very simple booting code
;
; Copyright (C) 2011 Alessandro Ghedini <[email protected]>
; --------------------------------------------------------------
; "THE BEER-WARE LICENSE" (Revision 42):
; Alessandro Ghedini wrote this file. As long as you retain this
; notice you can do whatever you want with this stuff. If we
; meet some day, and you think this stuff is worth it, you can
; buy me a beer in return.
; --------------------------------------------------------------

org 7C00h			; where BIOS wants me to be loaded

section .data
	msg db "Fuck yeah!", 0	; string to be printed

section .text
	global start

start:
	cli			; disable interrupts (not really needed)
	mov SI, msg		; setup SI for our string (used by lodsb)
	mov AH, 0Eh		; set BIOS int to print a char

print:
	lodsb			; load the next byte to be printed
	cmp AL, 0		; if AL is 0 we are at the end of the string
	je $			; loop to infinity and beyond if AL is 0
	int 10h			; BIOS interrupt call (print byte)
	jmp print		; re-do

Makefile

ASM=nasm
ISO=genisoimage -input-charset utf-8 -boot-load-size 4 -no-emul-boot -r

all: boot.iso

boot.bin: boot.asm
	$(ASM) -o boot.bin boot.asm

boot.iso: boot.bin
	mkdir iso/
	cp boot.bin iso/boot
	$(ISO) -b boot -no-emul-boot -o boot.iso iso 2> /dev/null

run: boot.iso
	qemu -boot once=d -cdrom boot.iso

clean:
	rm -rf *.iso *.bin iso/