gistfile1.asm

# http://en.wikipedia.org/wiki/INT_13H#INT_13h_AH.3D42h:_Extended_Read_Sectors_From_Drive
# http://www.ctyme.com/intr/rb-0708.htm
# http://stackoverflow.com/questions/19866549/int-13h-not-reading-sectors-from-virtual-disk


  # Load the second boot stage from the first HDD,
  # using the BIOS routine "INT 13h/AH=42h: Extended Read Sectors From Drive".
  mov $0x42, %ah
  mov $0x80, %dl # drive index (0x80 is first drive)

  mov $0, %ax
  mov %ax, %ds

  movw $(dap - _start + 0x7c00), %si

  int $0x13
  jmp skip

  .align 4
dap:
  .byte 0x10 # size
  .byte 0
  # number of sectors to read
  .word 1

   # target position for read, offset then segment because of little endian
  .word 0x7e00
  .word 0x0000

  .long 0 # first sector to read
  .long 0 # high address

gistfile2.diff

$ diff memory_before_int0x13 memory_after_int0x13
123,124c123
< 0006f00: 0000 0008 0706 0008 0307 1900 0241 0000  .............A..
< 0006f10: 0230 0000 0000 0f7c 0000 0202 4fd3 00f0  .0.....|....O...
---
> 0006f10: 0000 0000 0000 1d7c 0000 0202 4fd3 00f0  .......|....O...