Develop a Bootloader In Assembly

Question

I've already done a part of my OS in Assembly, but now I want to build a own bootloader for it too instead of using GRUB. When I was developing my test OS in Assembly I remember that I boot it like this:

org 0x7c00
bits 16

; OS Kernel Here

times 510 - ($-$$) db 0
dw 0xAA55

This I've already know. Now I want to use this and execute the "real" OS that will be a *.bin file written to the 2nd sector of the floppy. Then I want to know somethings

• How can I do a bootloader in Assembly to execute what will be starting on the 2nd sector of the floppy?
• I need to add anything to the Assembly source that will be placed on the 2nd sector of the floppy?

Answer 1

You use int 0x13 to load in the required number of sectors and jump to the location you have place the new code. There is nothing you need to do in the second stage, but you will want to make sure that you set DS to be valid for wherever you load the code.

Example from my little OS archive:

  /* BIOS loads the sectors into es:bx */
  pushw    $STAGE1_WORKSEG
  popw     %es
  movw     $STAGE1_OFFSET, %bx

read_stage1:

  /* Try to read in a few sectors */
  movb     $0x2, %cl      /* Sector */
  movb     $0x0, %ch      /* Cylinder */
  movb     $0x0, %dh      /* Head */
  movb     $0x0, %dl      /* Drive */
  movb     $0x2, %ah      /* BIOS read function */

  /* How many sectors to load */
  movb     $STAGE1_SIZE, %al 
  int      $0x13
  jnc      read_stage1_done

  /* Reset drive */
  xorw     %ax, %ax
  int      $0x13
  jmp      read_stage1


read_stage1_done:

  /* Perform a long jump into stage1 */
  ljmp     $STAGE1_WORKSEG, $STAGE1_OFFSET

  call     halt

halt:
    /*
    * Function: halt
    * Synopsis: Sends the processor into a permanent halted status
    * Notes:
    *    The only way out of this is to manually reboot
    */
    hlt                     /* Halt the processor */
    jmp      halt

That's in GAS format so you'll want to reverse the operand order because it looks like you're using NASM from the times instruction. The variable names should be self-explanatory.

If you're developing a hobby OS then http://forum.osdev.org/ is a good place to get support from others doing the same thing. It's a bit more specialised than stackoverflow and a lot of OS stuff can be quite esoteric.