boot.s

   1         bits 16 ; boot sectors run in real mode
   2         org 7c00h ; boot sector is loaded at 0x7c00
   3
   4 _start
   5         mov [boot_drive], dl ; BIOS sets dl to number of boot drive, we don't want to forget this
   6
   7         mov bp, 8000h ; Move the stack away
   8         mov sp, bp
   9
  10         push cs
  11         pop es
  12         mov bx, 9000h ; Load at 0000:9000
  13         mov dh, 2 ; Load 5 sectors
  14         mov dl, [boot_drive] ; Load from boot drive
  15         call read_sectors
  16
  17         mov dx, [9000h] ; Print value at 9000h, should be 0xDEAD
  18         call print_word
  19         mov dx, [9000h + 512] ; Print value at 9200h, should be 0xBEEF
  20         call print_word
  21
  22         jmp $ ; Endless loop
  23
  24 %include "print.s"
  25 %include "disk.s"
  26
  27 boot_drive resb 1 ; Reserve 1 byte for boot drive number
  28
  29 times 510-($-$$) db 0 ; fill with 0 until 0x1fc is reached
  30 dw 0AA55h ; BIOS MBR signature
  31
  32 times 256 dw 0DEADh ; Fill in 2nd sector with 0xDEAD
  33 times 256 dw 0BEEFh ; Fill in 3rd sector with 0xBEEF