PC Boot Sector

When booting a PC the BIOS (real mode 16-bits) routine reads sector 0 from the booted media.  If the sector ends with the sinature AA55h, the pc recogizes it as a boot sector.

The content of the boot sector is loaded into memory at address 7C00h. This small program that fits on the bootsector prints the string 'Hello World!' on the screen of the pc and then halts execution of the code.
The program is written in NASM on a Linux system. Then written to a floppy disk at sector 0.

[org 0x7c00]
[bits 16]

start:

  cli
  xor ax, ax
  mov ss, ax
  mov ds, ax
  mov es, ax
  mov sp, 0x7c00
  sti

  mov si, msg
  call print
  hlt

print:
  lodsb
  cmp al, 0
  je p_end
  mov ah, 0x0e
  int 0x10
  jmp print
p_end:
  ret

msg db 'Hello World!', 0
times 510 - ($-$$) db 0
dw 0xaa55

To assemble the program into a binary file execute the following line: 

$ nasm -f bin boot.asm -o boot.bin

Then create an empty floppy image:

$ dd if=/dev/zero of=floppy.img bs=512 count=2880

Then copy the boot sector onto the floppy image:

$ dd if=boot.bin of=floppy.img conv=notrunc